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  • 1. Adén, D.
    et al.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Feltzing, S.
    Grebel, E. K.
    Koch, A.
    Wilkinson, M. I.
    An abundance study of red-giant-branch stars in the Hercules dwarf spheroidal galaxy2011In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 525, p. A153-Article in journal (Refereed)
    Abstract [en]

    Context. Dwarf spheroidal galaxies are some of the most metal-poor, and least luminous objects known. Detailed elemental abundance analysis of stars in these faint objects is key to our understanding of star formation and chemical enrichment in the early universe, and may provide useful information on how larger galaxies form. Aims. Our aim is to provide a determination of [Fe/H] and [Ca/H] for confirmed red-giant branch member stars of the Hercules dwarf spheroidal galaxy. Based on this we explore the ages of the prevailing stellar populations in Hercules, and the enrichment history from supernovae. Additionally, we aim to provide a new simple metallicity calibration for Stromgren photometry for metal-poor, red giant branch stars. Methods. High-resolution, multi-fibre spectroscopy and Stromgren photometry are combined to provide as much information on the stars as possible. From this we derive abundances by solving the radiative transfer equations through marcs model atmospheres. Results. We find that the red-giant branch stars of the Hercules dSph galaxy are more metal-poor than estimated in our previous study that was based on photometry alone. From this, we derive a new metallicity calibration for the Stromgren photometry. Additionally, we find an abundance trend such that [Ca/Fe] is higher for more metal-poor stars, and lower for more metal-rich stars, with a spread of about 0.8 dex. The [Ca/Fe] trend suggests an early rapid chemical enrichment through supernovae of type II, followed by a phase of slow star formation dominated by enrichment through supernovae of type Ia. A comparison with isochrones indicates that the red giants in Hercules are older than 10 Gyr.

  • 2. Aoki, W
    et al.
    Frebel, A
    Christlieb, Norbert
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Observational Astronomy. Theoretical Astrophysics.
    Norris, J
    Beers, T
    Minezaki, T
    Barklem, Paul
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Observational Astronomy. Theoretical Astrophysics.
    Honda, S
    Takada-Hidai, M
    Asplund, M
    Ryan, S
    Tsangarides, S
    Eriksson, Kjell
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Observational Astronomy. Theoretical Astrophysics.
    HE 1327-2326, an Unevolved Star with [Fe/H] < -5.0.: I. A Comprehensive Abundance Analysis2006In: The Astrophysical Journal, Vol. 639, p. 897-917Article in journal (Refereed)
  • 3.
    Bladh, Sara
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Aringer, Bernhard
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Exploring Mass-Loss in M-type AGB Stars2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 345-350Conference paper (Refereed)
    Abstract [en]

    Stellar winds observed in asymptotic giant branch (AGB) stars are usually attributed to a combination of stellar pulsations and radiation pressure on dust. Strong candidates for wind-driving dust species in M-type AGB stars are magnesium silicates (Mg2SiO4 and MgSiO3). Such grains can form close to the stellar surface; they consist of abundant materials and, if they grow to sizes comparable to the wavelength of the stellar flux maximum, they experience strong acceleration by photon scattering. Here we present results from an extensive set of time-dependent wind models for M-type AGB stars with a detailed description for the growth of Mg2SiO4 grains. We show that these models reproduce observed mass-loss rates and wind velocities, as well as visual and near-IR photometry. However, the current models do not show the characteristic silicate features at 10 and 18 mu m, due to a rapidly falling temperature of Mg2SiO4 grains in the wind. Including a small amount of Fe in the grains further out in the circumstellar envelope will increase the grain temperature and result in pronounced silicate features, without significantly affecting the photometry in the visual and near-IR.

  • 4.
    Bladh, Sara
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Nowotny, W.
    Aringer, B.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Exploring wind-driving dust species in cool luminous giants II. Constraints from photometry of M-type AGB stars2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 553, p. A20-Article in journal (Refereed)
    Abstract [en]

    Context. The heavy mass loss observed in evolved asymptotic giant branch (AGB) stars is usually attributed to a two-stage process: atmospheric levitation by pulsation-induced shock waves, followed by radiative acceleration of newly formed dust grains. The dust transfers momentum to the surrounding gas through collisions and thereby triggers a general outflow. Radiation-hydrodynamical models of M-type AGB stars suggest that these winds can be driven by photon scattering - in contrast to absorption - on Fe-free silicate grains of sizes 0.1-1 mu m. Aims. In this paper we study photometric constraints for wind-driving dust species in M-type AGB stars, as part of an ongoing effort to identify likely candidates among the grain materials observed in circumstellar envelopes. Methods. To investigate the scenario of stellar winds driven by photon scattering on dust, and to explore how different optical and chemical properties of wind-driving dust species affect photometry we focus on two sets of dynamical models atmospheres: (i) models using a detailed description for the growth of Mg2SiO4 grains, taking into account both scattering and absorption cross-sections when calculating the radiative acceleration; and (ii) models using a parameterized dust description, constructed to represent different chemical and optical dust properties. By comparing synthetic photometry from these two sets of models to observations of M-type AGB stars we can provide constraints on the properties of wind-driving dust species. Results. Photometry from wind models with a detailed description for the growth of Mg2SiO4 grains reproduces well both the values and the time-dependent behavior of observations of M-type AGB stars, providing further support for the scenario of winds driven by photon scattering on dust. The photometry from the models with a parameterized dust description suggests that wind-drivers need to have a low absorption cross-section in the visual and near-IR to reproduce the time-dependent behavior, i. e. small variations in (J-K) and spanning a larger range in (V-K). This places constraints on the optical and chemical properties of the wind-driving dust species. Conclusions. To reproduce the observed photometric variations in (V-K) and (J-K) both detailed and parameterized models suggest that the wind-driving dust materials have to be quite transparent in the visual and near-IR. Consequently, strong candidates for outflows driven by photon scattering on dust grains are Mg2SiO4, MgSiO3, and potentially SiO2.

  • 5.
    Bladh, Sara
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics, Theoretical Astrophysics.
    Susanne, Höfner
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, Bernhard
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Exploring wind-driving dust species in cool luminous giants III: Wind models for M-type AGB stars2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 575, article id A105Article in journal (Refereed)
  • 6. Chiar, J. E.
    et al.
    Pendleton, Y. J.
    Allamandola, L. J.
    Boogert, A. C. A.
    Ennico, K.
    Greene, T. P.
    Geballe, T. R.
    Keane, J. V.
    Lada, C. J.
    Mason, R. E.
    Roellig, T. L.
    Sandford, S. A.
    Tielens, A. G. G. M.
    Werner, M. W.
    Whittet, D. C. B.
    Decin, L.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Ices in the Quiescent IC 5146 Dense Cloud2011In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 731, no 1, p. 9-Article in journal (Refereed)
    Abstract [en]

    This paper presents spectra in the 2 to 20 mu m range of quiescent cloud material located in the IC 5146 cloud complex. The spectra were obtained with NASA's Infrared Telescope Facility SpeX instrument and the Spitzer Space Telescope's Infrared Spectrometer. We use these spectra to investigate dust and ice absorption features in pristine regions of the cloud that are unaltered by embedded stars. We find that the H2O-ice threshold extinction is 4.03 +/- 0.05 mag. Once foreground extinction is taken into account, however, the threshold drops to 3.2 mag, equivalent to that found for the Taurus dark cloud, generally assumed to be the touchstone quiescent cloud against which all other dense cloud and embedded young stellar object observations are compared. Substructure in the trough of the silicate band for two sources is attributed to CH3OH and NH3 in the ices, present at the similar to 2% and similar to 5% levels, respectively, relative to H2O-ice. The correlation of the silicate feature with the E(J-K) color excess is found to follow a much shallower slope relative to lines of sight that probe diffuse clouds, supporting the previous results by Chiar et al.

  • 7. Cruzalebes, P.
    et al.
    Jorissen, A.
    Rabbia, Y.
    Sacuto, S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Chiavassa, A.
    Pasquato, E.
    Plez, B.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Spang, A.
    Chesneau, O.
    Fundamental parameters of 16 late-type stars derived from their angular diameter measured with VLTI/AMBER(star)2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 434, no 1, p. 437-450Article in journal (Refereed)
    Abstract [en]

    Thanks to their large angular dimension and brightness, red giants and supergiants are privileged targets for optical long-baseline interferometers. 16 red giants and supergiants have been observed with the VLTI/AMBER facility over a 2-year period, at medium spectral resolution (R = 1500) in the K band. The limb-darkened angular diameters are derived from fits of stellar atmospheric models on the visibility and the triple product data. The angular diameters do not show any significant temporal variation, except for one target: TX Psc, which shows a variation of 4 per cent using visibility data. For the eight targets previously measured by long-baseline interferometry (LBI) in the same spectral range, the difference between our diameters and the literature values is less than 5 per cent, except for TX Psc, which shows a difference of 11 per cent. For the eight other targets, the present angular diameters are the first measured from LBI. Angular diameters are then used to determine several fundamental stellar parameters, and to locate these targets in the Hertzsprung-Russell diagram (HRD). Except for the enigmatic Tc-poor low-mass carbon star W Ori, the location of Tc-rich stars in the HRD matches remarkably well the thermally-pulsating asymptotic giant branch, as it is predicted by the stellar evolution models. For pulsating stars with periods available, we compute the pulsation constant and locate the stars along the various sequences in the period-luminosity diagram. We confirm the increase in mass along the pulsation sequences, as predicted by theory, except for W Ori which, despite being less massive, appears to have a longer period than T Cet along the first-overtone sequence.

  • 8. Dehaes, S.
    et al.
    Bauwens, E.
    Decin, L.
    Eriksson, K.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Raskin, G.
    Butler, B.
    Dowell, C. D.
    Ali, B.
    Blommaert, J. A. D. L.
    Structure of the outer layers of cool standard stars2011In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 533, p. A107-Article in journal (Refereed)
    Abstract [en]

    Context. Among late-type red giants, an interesting change occurs in the structure of the outer atmospheric layers as one moves to later spectral types in the Hertzsprung-Russell diagram: a chromosphere is always present, but the coronal emission diminishes and a cool massive wind steps in.

    Aims. Where most studies have focussed on short-wavelength observations, this article explores the influence of the chromosphere and the wind on long-wavelength photometric measurements. The goal of this study is to assess wether a set of standard near-infrared calibration sources are fiducial calibrators in the far-infrared, beyond 50 mu m.

    Methods. The observational spectral energy distributions were compared with the theoretical model predictions for a sample of nine K- and M-giants. The discrepancies found are explained using basic models for flux emission originating in a chromosphere or an ionised wind.

    Results. For seven out of nine sample stars, a clear flux excess is detected at (sub)millimetre and/or centimetre wavelengths, while only observational upper limits are obtained for the other two. The precise start of the excess depends upon the star under consideration. For six sources the flux excess starts beyond 210 mu m and they can be considered as fiducial calibrators for Herschel/PACS (60-210 mu m). Out of this sample, four sources show no flux excess in the Herschel/SPIRE wavelength range (200-670 mu m) and are good calibration sources for this instrument as well. The flux at wavelengths shorter than similar to 1 mm is most likely dominated by an optically thick chromosphere, where an optically thick ionised wind is the main flux contributor at longer wavelengths.

    Conclusions. Although the optical to mid-infrared spectrum of the studied K- and M-type infrared standard stars is represented well by a radiative equilibrium atmospheric model, a chromosphere and/or ionised stellar wind at higher altitudes dominates the spectrum in the (sub)millimetre and centimetre wavelength ranges. The presence of a flux excess has implications on the role of the stars as fiducial spectrophotometric calibrators in these wavelength ranges.

  • 9. Dehaes, S.
    et al.
    Bauwens, E.
    Decin, L.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Raskin, G.
    Butler, B.
    Dowell, C. D.
    Ali, B.
    Blommaert, J. A. D. L.
    SED for 7 stellar calibrators (Dehaes+, 2011)2011In: VizieR Online Data Catalog, Vol. 353Article in journal (Refereed)
    Abstract [en]

    This catalogue presents the theoretical spectral energy distributions for 7 stellar calibrators of the ESA Herschel satellite in the wavelength range from 2 to 200 micron. The stellar atmosphere model and theoretical spectrum are generated using the MARCS theoretical stellar atmosphere code (Gustafsson et al., 1975A\ampA....42..407G and further updates; Gustafsson et al. 2003A\ampA...400..709D) and the TURBOSPECTRUM synthetic spectrum code (Plez et al., 1992A\ampA...256..551P). Stellar parameters (and uncertainties thereon) have been derived by Decin et al. (2003A\ampA...400..709D) and are described in the text files added to this catalogue. The line lists used in the spectrum calculation are discussed in Decin (2000, PhD Thesis, Leuven University) and Decin and Eriksson (2007A\ampA...472.1041D). The outermost depth point of the theoretical atmosphere model was taken at log(tau$_lambda$)=-7.2 with lambda being 2.2 micron. The atmosphere model was calculated with a spherically symmetric (parallel for Sirius) geometry, under the assumption of radiative and hydrostatic equilibrium, local thermodynamic equilibrium (LTE) and homogeneous layers. The original theoretical spectrum was calculated at a resolution of Δλ=0.5Å, and then degraded to a resolution of λ/Δλ=5000 applying a gaussian convolution. The wavelength coverage is from 2 to 200 micron. Uncertainties on the theoretical spectrum predictions are discussed in depth in Decin \amp Eriksson (2007A\ampA...472.1041D). Absolute flux calibration is based on Selby (1988A\ampAS...74..127S) K-band (TCS for Sirius) photometry Zero-point is determined on the basis of an ideal ’Vega’, i.e. the K-band photometry of Vega is corrected for a flux excess of 1.29% (cf. Absil et al. 2006A\ampA...452..237A). The determined Selby K-band zeropoint is 4.0517E-10 W/m2/um. The determined TCS K-band zeropoint is 4.4506E-10W/m2/um (for Sirius) The estimated absolute flux uncertainty is 1% These theoretical spectra should be referenced as Decin \amp Eriksson (2007A\ampA...472.1041D). (3 data files).

  • 10. Distefano, E.
    et al.
    Lanzafame, A. C.
    Lanza, A. F.
    Messina, S.
    Korn, Andreas J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Cuypers, J.
    Determination of rotation periods in solar-like stars with irregular sampling: the Gaia case2012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 421, no 4, p. 2774-2785Article in journal (Refereed)
    Abstract [en]

    We present a study on the determination of rotation periods (P) of solar-like stars from the photometric irregular time sampling of the European Space Agency Gaia mission, currently scheduled for launch in 2013, taking into account its dependence on ecliptic coordinates. We examine the case of solar twins as well as thousands of synthetic time series of solar-like stars rotating faster than the Sun. In the case of solar twins, we assume that the Gaia unfiltered photometric passband G will mimic the variability of the total solar irradiance as measured by the Variability of solar IRradiance and Gravity Oscillations (VIRGO) experiment. For stars rotating faster than the Sun, light curves are simulated using synthetic spectra for the quiet atmosphere, the spots and the faculae combined by applying semi-empirical relationships relating the level of photospheric magnetic activity to the stellar rotation and the Gaia instrumental response. The capabilities of the Deeming, LombScargle and phase dispersion minimization methods in recovering the correct rotation periods are tested and compared. The false alarm probability is computed using Monte Carlo simulations and compared with analytical formulae. The Gaia scanning law makes the rate of correct detection of rotation periods strongly dependent on the ecliptic latitude (beta). We find that for P? 1 d, the rate of correct detection increases with beta from 2030 per cent at beta? 0 to a peak of 70 per cent at beta= 45 degrees; then it abruptly falls below 10 per cent at beta > 45 degrees. For P > 5 d, the rate of correct detection is quite low and for solar twins is only 5 per cent on average.

  • 11.
    Eriksson, Kjell
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Nowotny, W.
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    A New Library of Synthetic Spectra and Photometry for Evolved C Stars2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 111-112Conference paper (Refereed)
    Abstract [en]

    We present a library of synthetic spectra and photometry calculated on the basis of a grid of atmosphere and wind models for carbon-rich, pulsating AGB stars. The spectra cover the range between 0.35 and 25 mu m with a spectral resolution of R=200. The corresponding photometric variations during several pulsation periods were computed for standard broadband filters in the visual and near-infrared.

  • 12.
    Eriksson, Kjell
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Nowotny, W.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Wachter, A.
    Synthetic photometry for carbon-rich giants IV: An extensive grid of dynamic atmosphere and wind models2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 566, p. A95-Article in journal (Refereed)
    Abstract [en]

    Context. The evolution and spectral properties of stars on the asymptotic giant branch (AGB) are significantly affected by mass loss through dusty stellar winds. Dynamic atmosphere and wind models are an essential tool for studying these evolved stars, both individually and as members of stellar populations, to understand their contribution to the integrated light and chemical evolution of galaxies. Aims. This paper is part of a series with the purpose of testing state-of-the-art atmosphere and wind models of C-type AGB stars against observations, and making them available to the community for use in various theoretical and observational studies. Methods. We have computed low-resolution spectra and photometry (in the wavelength range 0.35-25 mu m) for a grid of 540 dynamic models with stellar parameters typical of solar-metallicity C-rich AGB stars and with a range of pulsation amplitudes. The models cover the dynamic atmosphere and dusty outflow (if present), assuming spherical symmetry, and taking opacities of gas-phase species and dust grains consistently into account. To characterize the time-dependent dynamic and photometric behaviour of the models in a concise way we defined a number of classes for models with and without winds. Results. Comparisons with observed data in general show a quite satisfactory agreement for example regarding mass-loss rates vs. (J - K) colours or K magnitudes vs. (J - K) colours. Some exceptions from the good overall agreement, however, are found and attributed to the range of input parameters (e.g. relatively high carbon excesses) or intrinsic model assumptions (e.g. small particle limit for grain opacities). Conclusions. While current results indicate that some changes in model assumptions and parameter ranges should be made in the future to bring certain synthetic observables into better agreement with observations, it seems unlikely that these pending improvements will significantly affect the mass-loss rates of the models.

  • 13.
    Eriksson, Kjell
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Wachter, A.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Nowotny, W.
    Synthetic Colors for Dynamic C-Star Models2011In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper (Refereed)
    Abstract [en]

    Dust is formed in the outer atmospheres and winds of AGB stars and severely affects their observed spectra and colours. Synthetic JHK colours from a grid of time-dependent C-star models are presented and compared to observed colours. The combined effects of pulsation and dust are significant, especially for the coolest and most luminous models. We are now extending the calculations with respect to effective temperature, luminosity, stellar mass, and pulsation amplitude, as well as the carbon excess.

  • 14.
    Eriksson, Kjell
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Wachter, A.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Nowotny, W.
    Synthetic Colors for Dynamic C-Star Models2011In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper (Refereed)
    Abstract [en]

    Dust is formed in the outer atmospheres and winds of AGB stars and severely affects their observed spectra and colours. Synthetic JHK colours from a grid of time-dependent C-star models are presented and compared to observed colours. The combined effects of pulsation and dust are significant, especially for the coolest and most luminous models. We are now extending the calculations with respect to effective temperature, luminosity, stellar mass, and pulsation amplitude, as well as the carbon excess.

  • 15. Frebel, A.
    et al.
    Aoki, W.
    Christlieb, N.
    Ando, H.
    Asplund, M.
    Barklem, Paul
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Observational Astronomy. Theoretical Astrophysics.
    Beers, T.C.
    Eriksson, Kjell
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Observational Astronomy. Theoretical Astrophysics.
    Nucleosynthetic signatures of the first stars2005In: Nature, Vol. 434, p. 871-873Article in journal (Refereed)
  • 16. Frebel, A
    et al.
    Aoki, W
    Christlieb, N
    Ando, H
    Asplund, M
    Barklem, Paul
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Observational Astronomy. Theoretical Astrophysics.
    Beers, T.C.
    Eriksson, Kjell
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Observational Astronomy. Theoretical Astrophysics.
    Fechner, C.
    Fujimoto, M.Y.
    The new record holder for the most iron-poor star: HE 1327 2326: A dwarf or subgiant with Fe/H = -5.42005In: From Lithium to Uranium: Elemental Tracers of Early Cosmic Evolution: IAU Symposium 228, 2005, p. p. 207-212Conference paper (Other scientific)
  • 17.
    Gustafsson, Bengt
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Theoretical Astrophysics.
    Edvardsson, Bengt
    Department of Physics and Astronomy, Theoretical Astrophysics.
    Eriksson, Kjell
    Department of Physics and Astronomy, Theoretical Astrophysics.
    Gråe-Jørgensen, Uffe
    Mizuno-Wiedner, Michelle
    Department of Physics and Astronomy, Theoretical Astrophysics.
    Plez, Bertrand
    A new MARCS Grid2002In: International Astronomical Union Symposium: Modelling of Stellar Atmospheres, 2002, p. A2-Conference paper (Other scientific)
    Abstract [en]

    An extensive grid of spherically symmetric model atmospheres of stars with 1. 2500 K <= Teff <= 8000 K, 2. -1.0 <= log g (= log GM/R2) <= 5.0 (cgs units), 3. different combinations of M and R, 4. -5 <= [A/H] <= 1, and 5. a number of CNO abundance combinat

  • 18.
    Heiter, Ulrike
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Observational Astronomy. Theoretical Astrophysics.
    Eriksson, Kjell
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Observational Astronomy. Theoretical Astrophysics.
    Geometry of giant star model atmospheres: A consistency test2006In: Astronomy & Astrophysics, Vol. 452, p. 1039-Article in journal (Refereed)
  • 19. Jonsson, H.
    et al.
    Ryde, N.
    Harper, G. M.
    Cunha, K.
    Schultheis, M.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Kobayashi, C.
    Smith, V. V.
    Zoccali, M.
    Chemical evolution of fluorine in the bulge High-resolution K-band spectra of giants in three fields2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 564, p. A122-Article in journal (Refereed)
    Abstract [en]

    Context. Possible main formation sites of fluorine in the Universe include asymptotic giant branch (AGB) stars, the v-process in Type II supernova, and/or Wolf-Rayet stars. The importance of the Wolf-Rayet stars has theoretically been questioned and they are probably not needed in modeling the chemical evolution of fluorine in the solar neighborhood. It has, however, been suggested that Wolf-Rayet stars are indeed needed to explain the chemical evolution of fluorine in the bulge. The molecular spectral data, needed to determine the fluorine abundance, of the often used HF-molecule has not been presented in a complete and consistent way and has recently been debated in the literature. Aims. We intend to determine the trend of the fluorine-oxygen abundance ratio as a function of a metallicity indicator in the bulge to investigate the possible contribution from Wolf-Rayet stars. Additionally, we present here a consistent HF line list for the K- and L-bands including the often used 23 358.33 angstrom line. Methods. High-resolution near-infrared spectra of eight K giants were recorded using the spectrograph CRIRES mounted at the VLT. A standard setting was used that covered the HF molecular line at 23 358.33 angstrom. The fluorine abundances were determined using spectral fitting. We also re-analyzed five previously published bulge giants observed with the Phoenix spectrograph on Gemini using our new HF molecular data. Results. We find that the fluorine-oxygen abundance in the bulge probably cannot be explained with chemical evolution models that only include AGB stars and the v-process in supernovae Type II, that is a significant amount of fluorine production in Wolf-Rayet stars is most likely needed to explain the fluorine abundance in the bulge. For the HF line data, we find that a possible reason for the inconsistencies in the literature, where two different excitation energies were used, is two different definitions of the zero-point energy for the HF molecule and therefore also two accompanying different dissociation energies. Both line lists are correct as long as the corresponding consistent partition function is used in the spectral synthesis. However, we suspect this has not been the case in several earlier works, which led to fluorine abundances similar to 0.3 dex too high. We present a line list for the K- and L-bands and an accompanying partition function.

  • 20.
    Jonsson, H.
    et al.
    Lund Univ, Dept Astron & Theoret Phys, Lund Observ, Box 43, S-22100 Lund, Sweden.;IAC, Tenerife 38205, Spain.;Univ La Laguna, Dept Astrofis, E-38206 Tenerife, Spain..
    Ryde, N.
    Lund Univ, Dept Astron & Theoret Phys, Lund Observ, Box 43, S-22100 Lund, Sweden..
    Nordlander, Thomas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Rhodin, A. Pehlivan
    Lund Univ, Dept Astron & Theoret Phys, Lund Observ, Box 43, S-22100 Lund, Sweden.;Malmo Univ, Mat Sci & Appl Math, S-20506 Malmo, Sweden..
    Hartman, H.
    Lund Univ, Dept Astron & Theoret Phys, Lund Observ, Box 43, S-22100 Lund, Sweden.;Malmo Univ, Mat Sci & Appl Math, S-20506 Malmo, Sweden..
    Jonsson, P.
    Malmo Univ, Mat Sci & Appl Math, S-20506 Malmo, Sweden..
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Abundances of disk and bulge giants from high-resolution optical spectra I. O, Mg, Ca, and Ti in the solar neighborhood and Kepler field samples2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 598, article id A100Article in journal (Refereed)
    Abstract [en]

    Context. The Galactic bulge is an intriguing and significant part of our Galaxy, but it is hard to observe because it is both distant and covered by dust in the disk. Therefore, there are not many high-resolution optical spectra of bulge stars with large wavelength coverage, whose determined abundances can be compared with nearby, similarly analyzed stellar samples. Aims. We aim to determine the diagnostically important alpha elements of a sample of bulge giants using high-resolution optical spectra with large wavelength coverage. The abundances found are compared to similarly derived abundances from similar spectra of similar stars in the local thin and thick disks. In this first paper we focus on the solar neighborhood reference sample. Methods. We used spectral synthesis to derive the stellar parameters as well as the elemental abundances of both the local and bulge samples of giants. We took special care to benchmark our method of determining stellar parameters against independent measurements of effective temperatures from angular diameter measurements and surface gravities from asteroseismology. Results. In this first paper we present the method used to determine the stellar parameters and elemental abundances, evaluate them, and present the results for our local disk sample of 291 giants. Conclusions. When comparing our determined spectroscopic temperatures to those derived from angular diameter measurements, we reproduce these with a systematic difference of + 10 K and a standard deviation of 53 K. The spectroscopic gravities reproduce those determined from asteroseismology with a systematic off set of + 0.10 dex and a standard deviation of 0.12 dex. When it comes to the abundance trends, our sample of local disk giants closely follows trends found in other works analyzing solar neighborhood dwarfs, showing that the much brighter giant stars are as good abundance probes as the often used dwarfs.

  • 21. Jönsson, H.
    et al.
    Ryde, N.
    Nissen, P. E.
    Collet, R.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Asplund, M.
    Gustafsson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Sulphur abundances in halo giants from the [S I] line at 1082 nm and the [S I] triplet around 1045 nm2011In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 530, p. A144-Article in journal (Refereed)
    Abstract [en]

    Context. It is still debated whether or not the Galactic chemical evolution of sulphur in the halo follows the flat trend with [Fe/H] that is ascribed to the result of explosive nucleosynthesis in type II SNe. It has been suggested that the disagreement between different investigations of sulphur abundances in halo stars might be owing to problems with the diagnostics used, that a new production source of sulphur might be needed in the early Universe, like hypernovae, or that the deposition of supernova ejecta into the interstellar medium is time-delayed. Aims. The aim of this study is to try to clarify this situation by measuring the sulphur abundance in a sample of halo giants using two diagnostics: the S I triplet around 1045 nm and the [S I] line at 1082 nm. The latter of the two is not believed to be sensitive to non-LTE effects. We can thereby minimize the uncertainties in the diagnostic used and estimate the usefulness of the triplet for the sulphur determination in halo K giants. We will also be able to compare our sulphur abundance differences from the two diagnostics with the expected non-LTE effects in the 1045 nm triplet previously calculated by others. Methods. High-resolution near-infrared spectra of ten K giants were recorded using the spectrometer CRIRES mounted at VLT. Two standard settings were used, one covering the S I triplet and one covering the [S I] line. The sulphur abundances were individually determined with equivalent widths and synthetic spectra for the two diagnostics using tailored 1D model atmospheres and relying on non-LTE corrections from the litterature. Effects of convective inhomogeneities in the stellar atmospheres are investigated. Results. The sulphur abundances derived from both the [S I] line and the non-LTE corrected 1045 nm triplet favor a flat trend for the evolution of sulphur. In contrast to some previous studies, we saw no "high" values of [S/Fe] in our sample. Conclusions. We corroborate the flat trend in the [S/Fe] vs. [Fe/H] plot for halo stars found in some previous studies but do not find a scatter or a rise in [S/Fe] as obtained in other works. We find the sulphur abundances deduced from the non-LTE corrected triplet to be somewhat lower than the abundances from the [S I] line, possibly indicating too large non-LTE corrections. Considering 3D modeling, however, they might instead be too small. Moreover, we show that the [S I] line can be used as a sulphur diagnostic down to [Fe/H] similar to - 2.3 in giants.

  • 22. Lacour, S.
    et al.
    Meimon, S.
    Thiebaut, E.
    Perrin, G.
    Verhoelst, T.
    Pedretti, E.
    Schuller, P. A.
    Mugnier, L.
    Monnier, J.
    Berger, J. P.
    Haubois, X.
    Poncelet, A.
    Le Besnerais, G.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
    Millan-Gabet, R.
    Ragland, S.
    Lacasse, M.
    Traub, W.
    The limb-darkened Arcturus: imaging with the IOTA/IONIC interferometer2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 485, no 2, p. 561-570Article in journal (Refereed)
    Abstract [en]

    Aims. We undertook an H band interferometric examination of Arcturus, a star frequently used as a spatial and spectral calibrator. Methods. Using the IOTA 3 telescope interferometer, we performed spectro-interferometric observations (R approximate to 35) of Arcturus. Atmospheric models and prescriptions were fitted to the data to derive the brightness distribution of the photosphere. Image reconstruction was performed using two software algorithms: WISARD and MIRA. Results. An achromatic power law proved to be a good model of the brightness distribution, with a limb darkening compatible with the one derived from atmospheric model simulations using our mARCS model. A Rosseland diameter of 21.05 +/- 0.21 was derived, corresponding to an effective temperature of T-eff = 4295 +/- 26 K. No companion was detected from the closure phases, with an upper limit on the brightness ratio of 8 x 10(-4) at 1 AU. The dynamic range at such distance from the photosphere was established as 1.5 x 10(-4) (1 sigma rms). An upper limit of 1.7 x 10(-3) was also derived for the level of brightness asymmetries present in the photosphere.

  • 23. Lebzelter, T.
    et al.
    Heiter, Ulrike
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Abia, C.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Ireland, M.
    Neilson, H.
    Nowotny, W.
    Maldonado, J.
    Merle, T.
    Peterson, R.
    Plez, B.
    Short, C. I.
    Wahlgren, G. M.
    Worley, C.
    Aringer, B.
    Bladh, Sara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    De Laverny, P.
    Goswami, A.
    Mora, A.
    Norris, R. P.
    Recio-Blanco, A.
    Scholz, M.
    Thévenin, F.
    Tsuji, T.
    Kordopatis, G.
    Montesinos, B.
    Wing, R. F.
    Comparative modelling of the spectra of cool giants2012In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 547, p. A108-Article in journal (Refereed)
    Abstract [en]

    Context. Our ability to extract information from the spectra of stars depends on reliable models of stellar atmospheres and appropriate techniques for spectral synthesis. Various model codes and strategies for the analysis of stellar spectra are available today. Aims. We aim to compare the results of deriving stellar parameters using different atmosphere models and different analysis strategies. The focus is set on high-resolution spectroscopy of cool giant stars. Methods. Spectra representing four cool giant stars were made available to various groups and individuals working in the area of spectral synthesis, asking them to derive stellar parameters from the data provided. The results were discussed at a workshop in Vienna in 2010. Most of the major codes currently used in the astronomical community for analyses of stellar spectra were included in this experiment. Results. We present the results from the different groups, as well as an additional experiment comparing the synthetic spectra produced by various codes for a given set of stellar parameters. Similarities and differences of the results are discussed. Conclusions. Several valid approaches to analyze a given spectrum of a star result in quite a wide range of solutions. The main causes for the differences in parameters derived by different groups seem to lie in the physical input data and in the details of the analysis method. This clearly shows how far from a definitive abundance analysis we still are.

  • 24.
    Liljegren, Sofie
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Winds of AGB Stars - The Role of Stellar Pulsation2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 127-128Conference paper (Refereed)
    Abstract [en]

    Changing the stellar pulsation properties has large impact on the behavior of the atmosphere of C-type AGB stars. This relationship is examined.

  • 25.
    Liljegren, Soofie
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Nowotny, W.
    University of Vienna, Department of Astrophysics.
    Pulsation-induced atmospheric dynamics in M-type AGB stars: Effects on wind properties, photometric variations and near-IR CO line profiles2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 606, article id A6Article in journal (Refereed)
    Abstract [en]

    Context. Wind-driving in asymptotic giant branch (AGB) stars is commonly attributed to a two-step process. First, matter in the stellar atmosphere is levitated by shock waves, induced by stellar pulsation, and second, this matter is accelerated by radiation pressure on dust, resulting in a wind. In dynamical atmosphere and wind models the effects of the stellar pulsation are often simulated by a simplistic prescription at the inner boundary.

    Aims. We test a sample of dynamical models for M-type AGB stars, for which we kept the stellar parameters fixed to values characteristic of a typical Mira variable but varied the inner boundary condition. The aim was to evaluate the effect on the resulting atmosphere structure and wind properties. The results of the models are compared to observed mass-loss rates and wind velocities, photometry, and radial velocity curves, and to results from 1D radial pulsation models. The goal is to find boundary conditions which give realistic atmosphere and wind properties.

    Methods. Dynamical atmosphere models are calculated, using the DARWIN code for different combinations of photospheric velocities and luminosity variations. The inner boundary is changed by introducing an offset between maximum expansion of the stellar surface and the luminosity and/or by using an asymmetric shape for the luminosity variation. Ninety-nine different combinations of theses two changes are tested.

    Results. The model atmospheres are very sensitive to the inner boundary. Models that resulted in realistic wind velocities and mass-loss rates, when compared to observations, also produced realistic photometric variations. For the models to also reproduce the characteristic radial velocity curve present in Mira stars (derived from CO Delta v = 3 lines), an overall phase shift of 0.2 between the maxima of the luminosity and radial variation had to be introduced. This is a larger phase shift than is found by 1D radial pulsation models.

    Conclusions. We find that a group of models with different boundary conditions (29 models, including the model with standard boundary conditions) results in realistic velocities and mass-loss rates, and in photometric variations. To achieve the correct line splitting time variation a phase shift is needed.

  • 26.
    Liljegren, Soofie
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Nowotny, W.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Dust-driven winds of AGB stars: The critical interplay of atmospheric shocks and luminosity variations2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 589, article id A130Article in journal (Refereed)
    Abstract [en]

    Context. Winds of AGB stars are thought to be driven by a combination of pulsation-induced shock waves and radiation pressure on dust. In dynamic atmosphere and wind models, the stellar pulsation is often simulated by prescribing a simple sinusoidal variation in velocity and luminosity at the inner boundary of the model atmosphere.

    Aims. We experiment with different forms of the luminosity variation in order to assess the effects on the wind velocity and mass-loss rate, when progressing from the simple sinusoidal recipe towards more realistic descriptions. This will also give an indication of how robust the wind properties derived from the dynamic atmosphere models are.

    Methods. Using state-of-the-art dynamical models of C-rich AGB stars, a range of different asymmetric shapes of the luminosity variation and a range of phase shifts of the luminosity variation relative to the radial variation are tested. These tests are performed on two stellar atmosphere models. The first model has dust condensation and, as a consequence, a stellar wind is triggered, while the second model lacks both dust and wind.

    Results. The first model with dust and stellar wind is very sensitive to moderate changes in the luminosity variation. There is a complex relationship between the luminosity minimum, and dust condensation: changing the phase corresponding to minimum luminosity can either increase or decrease mass-loss rate and wind velocity. The luminosity maximum dominates the radiative pressure on the dust, which in turn, is important for driving the wind. An earlier occurrence of the maximum, with respect to the propagation of the pulsation-induced shock wave, then increases the wind velocity, while a later occurrence leads to a decrease. These effects of changed luminosity variation are coupled with the dust formation. In contrast there is very little change to the structure of the model without dust.

    Conclusions. Changing the luminosity variation, both by introducing a phase shift and by modifying the shape, influences wind velocity and the mass-loss rate. To improve wind models it would probably be desirable to extract boundary conditions from 3D dynamical interior models or stellar pulsation models.

  • 27. Maercker, M.
    et al.
    Olofsson, H.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Gustafsson, B.
    Schöier, F. L.
    Detached Shells of Dust and Gas around Carbon Stars2011In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper (Refereed)
    Abstract [en]

    We present observations of dust-scattered light of the carbon stars U Ant, R Scl, and U Cam taken with the EFOSC2 camera on the ESO 3.6-m telescope and the ACS on the Hubble Space Telescope. The observations show the detached shells around these stars in unprecedented detail, revealing a distinctively clumpy structure in the HST images of R Scl, and a separation of the dust and gas in the ground-based data for U Ant. This allows us to investigate the detached shells and their origin with exceptional precision.

  • 28. Maercker, M.
    et al.
    Olofsson, H.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Gustafsson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Schöier, F. L.
    Detached Shells of Dust and Gas around Carbon Stars2011In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper (Refereed)
    Abstract [en]

    We present observations of dust-scattered light of the carbon stars U Ant, R Scl, and U Cam taken with the EFOSC2 camera on the ESO 3.6-m telescope and the ACS on the Hubble Space Telescope. The observations show the detached shells around these stars in unprecedented detail, revealing a distinctively clumpy structure in the HST images of R Scl, and a separation of the dust and gas in the ground-based data for U Ant. This allows us to investigate the detached shells and their origin with exceptional precision.

  • 29. Mashonkina, L.
    et al.
    Christlieb, N.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    The Hamburg/ESO R-process Enhanced Star survey (HERES) X. HE 2252-4225, one more r-process enhanced and actinide-boost halo star2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 569, p. A43-Article in journal (Refereed)
    Abstract [en]

    Context. Studies of the r-process enhanced stars are important for understanding the nature and origin of the r-process better. Aims. We present a detailed abundance analysis of a very metal-poor giant star discovered in the HERES project, HE 2252-4225, which exhibits overabundances of the r-process elements with [r/Fe] = +0.80. Methods. We determined the stellar atmosphere parameters, T-eff = 4710 K, log g = 1.65, and [Fe/H] = -2.63, and chemical abundances by analysing the high-quality VLT/UVES spectra. The surface gravity was calculated from the non-local thermodynamic equilibrium (NLTE) ionisation balance between Fe i and Fe ii. Results. Accurate abundances for a total of 38 elements, including 22 neutron-capture elements beyond Sr and up to Th, were determined in HE 2252-4225. For every chemical species, the dispersion in the single line measurements around the mean does not exceed 0.12 dex. This object is deficient in carbon, as expected for a giant star with T-eff < 4800 K. The stellar Na-Zn abundances are well fitted by the yields of a single supernova of 14.4 M-circle dot. For the neutron-capture elements in the Sr-Ru, Ba-Yb, and Os-Ir regions, the abundance pattern of HE 2252-4225 is in excellent agreement with the average abundance pattern of the strongly r-process enhanced stars CS 22892-052, CS 31082-001, HE 1219-0312, and HE 1523-091. This suggests a common origin of the first, second, and third r-process peak elements in HE 2252-4225 in the classical r-process. We tested the solar r-process pattern based on the most recent s-process calculations of Bisterzo, Travaglio, Gallino, Wiescher, and Kappeler and found that elements in the range from Ba to Ir match it very well. No firm conclusion can be drawn about the relationship between the first neutron-capture peak elements, Sr to Ru, in HE 2252-4225 and the solar r-process, owing to the uncertainty in the solar r-process. The investigated star has an anomalously high Th/Eu abundance ratio, so that radioactive dating results in a stellar age of tau = 1.5 +/- 1.5 Gyr that is not expected for a very metal-poor halo star.

  • 30. Norris, John E.
    et al.
    Christlieb, N.
    Bessell, M. S.
    Asplund, M.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Korn, Andreas J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    The oxygen abundance of the ultra-metal-poor star HE 0557-48402012In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 753, no 2, p. 150-Article in journal (Refereed)
    Abstract [en]

    We present a high-resolution ultraviolet (UV) spectrum of the ultra-metal-poor (UMP) carbon-enhanced red giant HE 0557-4840 (T-eff/log g/[Fe/H] = 4900/2.2/-4.8). Combining these data with earlier observations, the radial velocity is 212.0 +/- 0.4 km s(-1), with no evidence of variability during 2006 February to 2007 December. One-dimensional (1D) LTE model-atmosphere analysis of UV Fe and CH lines confirms the iron and carbon abundances obtained previously ([Fe/H] = -4.8 and [C/Fe](1D) = +1.7), and places a more stringent limit on nitrogen abundance of [N/Fe](1D) < +1.0. Analysis of the UV OH lines yields [O/Fe](1D) = +2.3 +/- 0.4. When corrections are made for three-dimensional (3D) effects we obtain [C/Fe](3D) = +1.1, [N/Fe](3D) < +0.1, and [O/Fe](3D) = +1.4. Comparison of the abundances of HE 0557-4840 with those of supernova models of Nomoto et al. and Joggerst et al. suggests that none is able to explain fully the observed abundance pattern. For HE 0557-4840, the Frebel et al. transition discriminant D-trans(= log(10([C/H]) + 0.3 x 10([O/H])) = -3.4 +/- 0.2, consistent with fine-structure transitions of C II and O I being a major cooling mechanism of star-forming regions at the earliest times. Of the four stars known to have [Fe/H] less than or similar to -4.3, three are strongly carbon and oxygen enhanced. If the suggestion by Caffau et al. that SDSS J102915+172927 ([Fe/H] = -4.7) does not belong to the class of C-rich, O-rich, UMP stars is supported by future similar discoveries, one will need to consider multiple channels for the production of stars having [Fe/H] less than or similar to -4.3.

  • 31. Nowotny, W.
    et al.
    Aringer, B.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Synthetic photometry for carbon-rich giants III. Tracing the sequence of mass-losing galactic C-type Miras2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 552, p. A20-Article in journal (Refereed)
    Abstract [en]

    Late-type giant stars in the evolutionary stage of the asymptotic giant branch increasingly lose mass via comparatively slow but dense stellar winds. Not only do these evolved red giants contribute in this way to the enrichment of the surrounding interstellar medium, but the outflows also have a substantial influence on the spectro-photometric appearance of such objects. In the case of carbon-rich atmospheric chemistries, the developing cool circumstellar envelopes contain dust grains mainly composed of amorphous carbon. With increasing mass-loss rates, this leads to more and more pronounced circumstellar reddening. With the help of model calculations we aim at reproducing the observational photometric findings for a large sample of well-characterised galactic C-type Mira variables losing mass at different rates. We used dynamic model atmospheres, describing the outer layers of C-rich Miras, which are severly affected by dynamic effects. Based on the resulting structures and under the assumptions of chemical equilibrium as well as LTE, we computed synthetic spectra and synthetic broad-band photometry (Johnson-Cousins-Glass BVRIJHKL'M). A set of five representative models with different stellar parameters describes a sequence from less to more evolved objects with steadily increasing mass-loss rates. This allowed us to study the significant influence of circumstellar dust on the spectral energy distributions and the (amplitudes of) lightcurves in different filters. We tested the photometric properties (mean NIR magnitudes, colours, and amplitudes) and other characteristics of the models (mass-loss rates, periods, and bolometric corrections) by comparing these with the corresponding observational data adopted from the literature. Using different kinds of diagrams we illustrate where the models are located in a supposed evolutionary sequence defined by observed C-type Mira samples. Based on comparisons of galactic targets with empirical relations derived for C stars in the Large Magellanic Cloud we discuss the relevance of metallicity and excess carbon (C-O) for the development of dust-driven winds. Having investigated the dynamic model atmospheres from different (mainly photometric) perspectives, we conclude that our modelling approach (meaning the combination of numerical method and a suitable choice of model parameters) is able to describe C-rich long-period variables over a wide range of mass-loss rates, i.e., from moderately pulsating objects without any dusty wind to highly dust-enshrouded Carbon Miras. Thus, we can trace the observed sequence of C-type Miras, which is mainly determined by the mass loss.

  • 32. Plez, Bertrand
    et al.
    van Eck, S.
    Jorissen, A.
    Edvardsson, Bengt
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Faculty of Science and Technology, Astronomical observatory. Department of Physics and Astronomy, Theoretical Astrophysics.
    Eriksson, Kjell
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Faculty of Science and Technology, Astronomical observatory. Department of Physics and Astronomy, Theoretical Astrophysics.
    Gustafsson, Bengt
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Faculty of Science and Technology, Astronomical observatory. Department of Physics and Astronomy, Theoretical Astrophysics.
    A MARCS Grid of S-type Star Atmospheres2003In: International Astronomical Unium Symposium: Modelling of Stellar Atmospheres, 2003, p. A2-Conference paper (Other (popular scientific, debate etc.))
  • 33.
    Rau, G.
    et al.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    Hron, J.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    Paladini, C.
    Univ Libre Bruxelles, Inst Astron & Astrophys, Blvd Triomphe CP 226, B-1050 Brussels, Belgium..
    Aringer, B.
    Astron Observ Padova INAF, Vicolo Osservatorio 5, I-35122 Padua, Italy..
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Marigo, P.
    Astron Observ Padova INAF, Vicolo Osservatorio 5, I-35122 Padua, Italy..
    Nowotny, W.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    Grellmann, R.
    Univ Cologne, Phys Inst, Zulpicher Str 77, D-50397 Cologne, Germany..
    The adventure of carbon stars: Observations and modeling of a set of C-rich AGB stars2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 600, article id A92Article in journal (Refereed)
    Abstract [en]

    Context. Modeling stellar atmospheres is a complex and intriguing task in modern astronomy. A systematic comparison of models with multi-technique observations is the only efficient way to constrain the models. Aims. We intend to perform self-consistent modeling of the atmospheres of six carbon-rich AGB stars (R Lep, R Vol, Y Pav, AQ Sgr, U Hya, and X TrA) with the aim of enlarging the knowledge of the dynamic processes occurring in their atmospheres. Methods. We used VLTI/MIDI interferometric observations, in combination with spectro-photometric data, and compared them with self-consistent, dynamic model atmospheres. Results. We found that the models can reproduce spectral energy distribution (SED) data well at wavelengths longer than 1 mu m, and the interferometric observations between 8 mu m and 10 mu m. Discrepancies observed at wavelengths shorter than 1 mu m in the SED, and longer than 10 mu m in the visibilities, could be due to a combination of data- and model-related effects. The models best fitting the Miras are significantly extended, and have a prominent shell-like structure. On the contrary, the models best fitting the non-Miras are more compact, showing lower average mass loss. The mass loss is of episodic or multi-periodic nature but causes the visual amplitudes to be notably larger than the observed ones. A number of stellar parameters were derived from the model fitting: T-Ross, L-Ross, M, C/O, and. M. Our findings agree well with literature values within the uncertainties. TRoss, and LRoss are also in good agreement with the temperature derived from the angular diameter T(theta((V-K))) and the bolometric luminosity from the SED fitting L-bol, except for AQ Sgr. The possible reasons are discussed in the text. Finally, theta(Ross) and theta((V-K)) agree with one another better for the Miras than for the non-Miras targets, which is probably connected to the episodic nature of the latter models. We also located the stars in the H-R diagram, comparing them with evolutionary tracks. We found that the main derived properties (L, T-eff, C/O ratios and stellar masses) from the model fitting are in good agreement with TP-AGB evolutionary calculations for carbon stars carried out with the COLIBRI code.

  • 34.
    Rau, G.
    et al.
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    Hron, J.
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    Paladini, C.
    ULB Brussels, Inst Astron & Astrophys, Brussels, Belgium..
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics. Uppsala Univ, Dept Phys & Astron, Uppsala, Sweden..
    Aringer, B.
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    Groenewegen, M. A. T.
    Royal Observ Belgium, Brussels, Belgium..
    Mecina, M.
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    Into the Modelling of RU Vir2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 137-138Conference paper (Refereed)
    Abstract [en]

    We present an attempt to model the atmosphere of the carbon-rich Mira star RU Vir, using different techniques including spectroscopy, photometry, and interferometry. A radiative transfer code and hydrostatic model atmospheres were used for a preliminary study. To investigate the dynamic processes happening in RU Vir, dynamic model atmospheres were compared to new MIDI/VLTI observations obtained in April 2014, and SiC opacities were added.

  • 35. Ryde, N.
    et al.
    Lambert, J.
    Farzone, M.
    Richter, M. J.
    Josselin, E.
    Harper, G. M.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Greathouse, T. K.
    Systematic trend of water vapour absorption in red giant atmospheres revealed by high resolution TEXES 12 mu m spectra2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 573, article id A28Article in journal (Refereed)
    Abstract [en]

    Context. The structures of the outer atmospheres of red giants are very complex. Recent interpretations of a range of different observations have led to contradictory views of these regions. It is clear, however, that classical model photospheres are inadequate to describe the nature of the outer atmospheres. The notion of large optically thick molecular spheres around the stars (MOLspheres) has been invoked in order to explain spectro-interferometric observations and low-and high-resolution spectra. On the other hand high-resolution spectra in the mid-IR do not easily fit into this picture because they rule out any large sphere of water vapour in LTE surrounding red giants. Aims. In order to approach a unified scenario for these outer regions of red giants, more empirical evidence from different diagnostics are needed. Our aim here is to investigate high-resolution, mid-IR spectra for a range of red giants, spanning spectral types from early K to mid M. We want to study how the pure rotational lines of water vapour change with effective temperature, and whether we can find common properties that can put new constraints on the modelling of these regions, so that we can gain new insights. Methods. We have recorded mid-IR spectra at 12.2-12.4 mu m at high spectral resolution of ten well-studied bright red giants, with TEXES mounted on the IRTF on Mauna Kea. These stars span effective temperatures from 3450 K to 4850 K. Results. We find that all red giants in our study cooler than 4300 K, spanning a wide range of effective temperatures (down to 3450 K), show water absorption lines stronger than expected and none are detected in emission, in line with what has been previously observed for a few stars. The strengths of the lines vary smoothly with spectral type. We identify several spectral features in the wavelength region that are undoubtedly formed in the photosphere. From a study of water-line ratios of the stars, we find that the excitation temperatures, in the line-forming regions, are several hundred Kelvin lower than expected from a classical photospheric model. Conclusions. All stars in our sample show several photospheric features in their 12 mu m spectra, which can be modelled with a classical model photosphere. However, in all stars showing water-vapour lines (stars cooler than similar to 4300 K), the water lines are found to be much deeper than expected. The line ratios of these pure-rotational lines reveal low excitation temperatures. This could either be due to lower temperatures than expected in the outer regions of the photospheres caused by for example extra cooling, or due to non-LTE level populations, affecting the source function and line opacities, but this needs further investigation. We have demonstrated that these diagnostically interesting water lines are a general feature of red giants across spectral types, and we argue for a general explanation of their formation rather than explanations requiring specific properties, such as dust. Since the water lines are neither weak (filled in by emission) nor do they appear in emission, as predicted by LTE MOLsphere models in their simplest forms, the evidence of the existence of such large optically-thick, molecular spheres enshrouding the stars is weakened. It is still a challenge to find a unifying picture of the outer regions of the atmospheres of red giants, but we have presented new empirical evidence that needs to be taken into account and explained in any model of these regions.

  • 36.
    Ryde, N.
    et al.
    Lund Univ, Lund Observ, Dept Astron & Theoret Phys, Lund, Sweden..
    Lambert, J.
    Lund Univ, Lund Observ, Dept Astron & Theoret Phys, Lund, Sweden..
    Richter, M. J.
    Univ Calif Davis, Dept Phys, Davis, CA 95616 USA..
    Josselin, E.
    Univ Montpellier 2, UPM, Montpellier, France..
    Harper, G. M.
    Univ Dublin Trinity Coll, Astrophys Res Grp, Dublin 2, Ireland..
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Boogert, A.
    NASA, Ames Res Ctr, USRA SOFIA Sci Ctr, Moffett Field, CA 94035 USA..
    DeWitt, C.
    Univ Calif Davis, Dept Phys, Davis, CA 95616 USA..
    Encrenaz, T.
    Univ Paris 07, UPMC, CNRS, LESIA,Observ Paris, Meudon, France..
    Greathouse, T.
    SW Res Inst, San Antonio, TX USA..
    Jaffe, D.
    Univ Texas Austin, Austin, TX 78712 USA..
    Kulas, K.
    NASA, Ames Res Ctr, Moffett Field, CA 94035 USA..
    McKelvey, M.
    NASA, Ames Res Ctr, Moffett Field, CA 94035 USA..
    Najita, J.
    Natl Opt Astron Observ, Tucson, AZ USA..
    Vacca, W.
    NASA, Ames Res Ctr, USRA SOFIA Sci Ctr, Moffett Field, CA 94035 USA..
    Evolved Stars with Complex Atmospheres - the High Spectral Resolution, mid-IR View2015In: Why galaxies care about AGB stars III: a closer look in space and time, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 67-71Conference paper (Refereed)
    Abstract [en]

    The physical structures of the outer atmospheres of red giants are not known. They are certainly complex, and a range of recent observations are showing that we need to embrace non-classical model atmospheres to interpret these regions. This region's properties are of importance, not the least for an understanding of the mass-loss mechanism for these stars. Here we present observational constraints on the outer regions of red giants, based on mid-IR, high spectral resolution spectra. We also discuss possible non-LTE effects and highlight a new non-LTE code that will be used to analyse the spectra of these atmospheric layers. We conclude by mentioning our new SOFIA/EXES observations of red giants at 67 mu m, where the vibration-rotation lines of water vapour can be detected and spectrally resolved for the first time.

  • 37.
    Ryde, Nils
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Faculty of Science and Technology, Astronomical observatory.
    Eriksson, Kjell
    Faculty of Science and Technology, Astronomical observatory. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics.
    The 3 micron spectrum of R Doradus observed with the ISO-SWS2002In: Astronomy and Astrophysics, Vol. 386, p. 874-Article in journal (Refereed)
  • 38.
    Ryde, Nils
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Faculty of Science and Technology, Astronomical observatory. Physics, Department of Astronomy and Space Physics.
    Eriksson, Kjell
    Gustafsson, Bengt
    The nature of the circumstellar CO2 emission from M giants1999In: ASTRONOMY AND ASTROPHYSICS, ISSN 0004-6361, Vol. 341, no 2, p. 579-586Article in journal (Refereed)
    Abstract [en]

    The 13-16 mu m region observed by the Infrared Space Observatory (ISO) of several ABG stars are discussed. We present and analyse spectra of seven M giants which show carbon dioxide features. To explain the features of the bands, we suggest they originate

  • 39.
    Ryde, Nils
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Faculty of Science and Technology, Astronomical observatory. Physics, Department of Astronomy and Space Physics.
    Eriksson, Kjell
    Gustafsson, Bengt
    Hinkle, K. H.
    Lambert, D. L.
    Olofsson, H.
    Schoier, F. L.
    Resolved CO 4.6 um Line Emission from Inner Circumstellar Envelopes -- the Case of Mira1999In: 2nd Austrian ISO Workshop, 1999, p. 66-Conference paper (Other scientific)
    Abstract [en]

    The 13-16 mu m region observed by the Infrared Space Observatory (ISO) of several ABG stars are discussed. We present and analyse spectra of seven M giants which show carbon dioxide features. To explain the features of the bands, we suggest they originate

  • 40.
    Ryde, Nils
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Faculty of Science and Technology, Astronomical observatory. Physics, Department of Astronomy and Space Physics.
    Eriksson, Kjell
    Gustafsson, Bengt
    Jorgensen, U. G.
    Nyman, L.-A.
    Olofsson, H.
    Plez, Bertrand
    Wolstencroft, R.
    Mid and far-infrared spectra of the third brightest carbon star IRAS 15194-51151998In: Asymptotic Giant Branch Stars, IAU Symposium 191, 1998Conference paper (Other scientific)
    Abstract [en]

    ISO observations from 2 to 197 microns of the recently discovered, third brightest (at 12 microns) carbon star IRAS 15194-5115, are analysed.

  • 41.
    Ryde, Nils
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Astronomical observatory. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
    Eriksson, Kjell
    Gustafsson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Astronomical observatory.
    Lindqvist, M
    Olofsson, H
    Tentative detection of circumstellar CO2 from the AGB star R Crateris1998In: Astrophysics and Space Science, ISSN 0004-640X, E-ISSN 1572-946X, Vol. 255, no 1-2, p. 301-302Article in journal (Refereed)
    Abstract [en]

    We report on and discuss the detection of an emission feature at 14.98 mu m from the oxygen-rich, semi-regularly pulsating asymptotic Giant Branch star R Crateris, a feature which we suggest to be due to the 2(0)(1) Q-branch of circumstellar CO2. We also suggest a reasonable excitation mechanism, which could explain the height, the width and the asymmetry of the feature.

  • 42.
    Ryde, Nils
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Faculty of Science and Technology, Astronomical observatory. Physics, Department of Astronomy and Space Physics.
    Eriksson, Kjell
    Gustafsson, Bengt
    Olofsson, H.
    Plez, Bertrand
    Modelling of molecular bands of oxygen-rich AGB stars1998In: Asymptotic Giant Branch Stars, IAU Symposium 191, 1998Conference paper (Other scientific)
    Abstract [en]

    ISO observations of infrared molecular bands of oxygen-rich AGB stars are compared with synthetic spectra generated using model atmospheres

  • 43.
    Ryde, Nils
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Faculty of Science and Technology, Astronomical observatory.
    Gustafsson, B
    Eriksson, K
    Hinkle, KH
    MIRA's wind explored in scattering infrared CO lines2000In: ASTROPHYSICAL JOURNAL, ISSN 0004-637X, Vol. 545, no 2, p. 945-956Article in journal (Refereed)
    Abstract [en]

    We have observed the intermediate regions of the circumstellar envelope of Mira (o Ceti) in photospheric light scattered by three vibration-rotation transitions of the fundamental band of CO, from low-excited rotational levels of the ground vibrational st

  • 44.
    Ryde, Nils
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Faculty of Science and Technology, Astronomical observatory.
    Gustafsson, Bengt
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Faculty of Science and Technology, Astronomical observatory.
    Eriksson, Kjell
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Faculty of Science and Technology, Astronomical observatory.
    Wahlin, Rurik
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Faculty of Science and Technology, Astronomical observatory.
    The Abundance of Elements in Cool Stars, as Determined from High-Resolution, 1-5 Micron Spectroscopy2005In: High Resolution Infrared Spectroscopy in Astronomy: eds. H. U. Käufl, R. Siebenmorgen, & A. Moorwood, p. 365-Article, review/survey (Other (popular scientific, debate etc.))
  • 45.
    Ryde, Nils
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Faculty of Science and Technology, Astronomical observatory. Physics, Department of Astronomy and Space Physics.
    Gustafsson, Bengt
    Hinkle, KH
    Eriksson, Kjell
    Lambert, DL
    Olofsson, H
    Preliminary results on the circumstellar envelopes of alpha Ori and R Leo from CO 4.6 mu m line emission1999In: ASTRONOMY AND ASTROPHYSICS, ISSN 0004-6361, Vol. 347, no 3, p. L35-L38Article in journal (Refereed)
    Abstract [en]

    CO 4.6 mu m vibration-rotational lines are detected in fluorescent emission from the inner regions of the Betelgeuse (a Orionis) and R Leonis stellar winds. The spatially and spectrally resolved 1-0 R(1), R(2), and R(3) line profiles are found to be highl

  • 46.
    Ryde, Nils
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Astronomical observatory.
    Richter, M. J.
    Harper, G. M.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Astronomical observatory.
    Lambert, D. L.
    Water Vapor on Supergiants: The 12 μm TEXES Spectra of μ Cephei2006In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 645, no 1, p. 652-658Article in journal (Refereed)
    Abstract [en]

    Several recent papers have argued for warm, semidetached, molecular layers surrounding red giant and supergiant stars, a concept known as a MOLsphere. Spectroscopic and interferometric analyses have often corroborated this general picture. Here we present high-resolution spectroscopic data of pure rotational lines of water vapor at 12 mu m for the supergiant mu Cep. This star has often been used to test the concept of molecular layers around supergiants. Given the prediction of an isothermal, optically thick water vapor layer in local thermodynamic equilibrium around the star ( MOLsphere), we expected the 12 mu m lines to be in emission or at least in absorption but filled in by emission from the molecular layer around the star. Our data, however, show the contrary; we find definite absorption. Thus, our data do not easily fit into the suggested isothermal MOLsphere scenario. The 12 mu m lines, therefore, put new, strong constraints on the MOLsphere concept and on the nature of water seen in signatures across the spectra of early M supergiants. We also find that the absorption is even stronger than that calculated from a standard, spherically symmetric model photosphere without any surrounding layers. A cool model photosphere, representing cool outer layers, is, however, able to reproduce the lines, but this model does not account for water vapor emission at 6 mu m. Thus, a unified model for water vapor on mu Cep appears to be lacking. It does seem necessary to model the underlying photospheres of these supergiants in their whole complexity. The strong water vapor lines clearly reveal inadequacies of classical model atmospheres.

  • 47.
    Sacuto, Stéphane
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Olofsson, H.
    Bladh, Sara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Klotz, D.
    Maercker, M.
    The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 551, p. A72-Article in journal (Refereed)
    Abstract [en]

    Aims. We study the circumstellar environment of the M-type AGB star RT Vir using mid-infrared high spatial resolution observations from the ESO-VLTI focal instrument MIDI. The aim of this study is to provide observational constraints on theoretical prediction that the winds of M-type AGB objects can be driven by photon scattering on iron-free silicate grains located in the close environment (about 2 to 3 stellar radii) of the star. Methods. We interpreted spectro-interferometric data, first using wavelength-dependent geometric models. We then used a self-consistent dynamic model atmosphere containing a time-dependent description of grain growth for pure forsterite dust particles to reproduce the photometric, spectrometric, and interferometric measurements of RT Vir. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort was first made to determine these parameters. Results. MIDI differential phases reveal the presence of an asymmetry in the stellar vicinity. Results from the geometrical modeling give us clues to the presence of aluminum and silicate dust in the close circumstellar environment (<5 stellar radii). Comparison between spectro-interferometric data and a self-consistent dust-driven wind model reveals that silicate dust has to be present in the region between 2 to 3 stellar radii to reproduce the 59 and 63 m baseline visibility measurements around 9.8 mu m. This gives additional observational evidence in favor of winds driven by photon scattering on iron-free silicate grains located in the close vicinity of an M-type star. However, other sources of opacity are clearly missing to reproduce the 10-13 mu m visibility measurements for all baselines. Conclusions. This study is a first attempt to understand the wind mechanism of M-type AGB stars by comparing photometric, spectrometric, and interferometric measurements with state-of-the-art, self-consistent dust-driven wind models. The agreement of the dynamic model atmosphere with interferometric measurements in the 8-10 mu m spectral region gives additional observational evidence that the winds of M-type stars can be driven by photon scattering on iron-free silicate grains. Finally, a larger statistical study and progress in advanced self-consistent 3D modeling are still required to solve the remaining problems.

  • 48. Trevisan, M.
    et al.
    Barbuy, B.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Gustafsson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Grenon, M.
    Pompeia, L.
    Analysis of old very metal rich stars in the solar neighbourhood2011In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 535, p. A42-Article in journal (Refereed)
    Abstract [en]

    Context. A sample of mostly old metal-rich dwarf and turn-off stars with high eccentricity and low maximum height above the Galactic plane has been identified. From their kinematics, it was suggested that the inner disk is their most probable birthplace. Their chemical imprints may therefore reveal important information about the formation and evolution of the still poorly understood inner disk.

    Aims. To probe the formation history of these stellar populations, a detailed analysis of a sample of very metal-rich stars is carried out. We derive the metallicities, abundances of a elements, ages, and Galactic orbits.

    Methods. The analysis of 71 metal-rich stars is based on optical high-resolution echelle spectra obtained with the FEROS spectrograph at the ESO 1.52-m Telescope at La Silla, Chile. The metallicities and abundances of C, O, Mg, Si, Ca, and Ti were derived based on LTE detailed analysis, employing the MARCS model atmospheres.

    Results. We confirm the high metallicity of these stars reaching up to [Fe(I)/H] = 0.58, and the sample of metal-rich dwarfs can be kinematically subclassified in samples of thick disk, thin disk, and intermediate stellar populations. All sample stars show solar alpha-Fe ratios, and most of them are old and still quite metal rich. The orbits suggest that the thin disk, thick disk and intermediate populations were formed at Galactocentric distances of similar to 8 kpc, similar to 6 kpc, and similar to 7 kpc, respectively. The mean maximum height of the thick disk subsample of Z(max) similar to 380 pc, is lower than for typical thick disk stars. A comparison of a-element abundances of the sample stars with bulge stars shows that the oxygen is compatible with a bulge or inner thick disk origin. Our results suggest that models of radial mixing and dynamical effects of the bar and bar/spiral arms might explain the presence of these old metal-rich dwarf stars in the solar neighbourhood.

  • 49. Trevisan, M.
    et al.
    Barbuy, B.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Gustafsson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Grenon, M.
    Pompeia, L.
    Metal rich stars in solar neighbourhood (Trevisan+, 2011)2011In: VizieR Online Data Catalog, Vol. 353Article in journal (Refereed)
    Abstract [en]

    Spectroscopic stellar parameters and abundances of C, O, Mg, Si, Ca, Ti and Ni for 71 stars. U, V, W velocities and orbital parameters derived using the GRINTON integrator (Carraro et al. 2002MNRAS.332..705C, Bedin et al. 2006A\ampA...460L..27B). Atomic data for Fe I, Fe II, Si I, Ca I, and Ti I lines. (7 data files).

  • 50. Trevisan, M.
    et al.
    Barbuy, B.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Gustafsson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Grenon, M.
    Pompeia, L.
    Metal rich stars in solar neighbourhood (Trevisan+, 2011)2011In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, VizieR Online Data Catalog, Vol. 353Article in journal (Refereed)
    Abstract [en]

    Spectroscopic stellar parameters and abundances of C, O, Mg, Si, Ca, Ti and Ni for 71 stars. U, V, W velocities and orbital parameters derived using the GRINTON integrator (Carraro et al. 2002MNRAS.332..705C, Bedin et al. 2006A\ampA...460L..27B). Atomic data for Fe I, Fe II, Si I, Ca I, and Ti I lines. (7 data files).

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