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Gruyters, Pieter
Publications (10 of 11) Show all publications
Gruyters, P., Lind, K., Richard, O., Grundahl, F., Asplund, M., Casagrande, L., . . . Korn, A. J. (2016). Atomic diffusion and mixing in old stars VI. The lithium content of M30. Astronomy and Astrophysics, 589, Article ID A61.
Open this publication in new window or tab >>Atomic diffusion and mixing in old stars VI. The lithium content of M30
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2016 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 589, article id A61Article in journal (Refereed) Published
Abstract [en]

Context. The prediction of the Planck-constrained primordial lithium abundance in the Universe is in discordance with the observed Li abundances in warm Population II dwarf and subgiant stars. Among the physically best motivated ideas, it has been suggested that this discrepancy can be alleviated if the stars observed today had undergone photospheric depletion of lithium.

Aims. The cause of this depletion is investigated by accurately tracing the behaviour of the lithium abundances as a function of effective temperature. Globular clusters are ideal laboratories for such an abundance analysis as the relative stellar parameters of their stars can be precisely determined.

Methods. We performed a homogeneous chemical abundance analysis of 144 stars in the metal-poor globular cluster M30, ranging from the cluster turnoff point to the tip of the red giant branch. Non-local thermal equilibrium (NLTE) abundances for Li, Ca, and Fe were derived where possible by fitting spectra obtained with VLT/FLAMES-GIRAFFE using the quantitative-spectroscopy package SME. Stellar parameters were derived by matching isochrones to the observed V vs. V I colour-magnitude diagram. Independent effective temperatures were obtained from automated profile fitting of the Balmer lines and by applying colour-T-eff calibrations to the broadband photometry.

Results. Li abundances of the turno ff and early subgiant stars form a thin plateau that is broken off abruptly in the middle of the SGB as a result of the onset of Li dilution caused by the first dredge-up. Abundance trends with effective temperature for Fe and Ca are observed and compared to predictions from stellar structure models including atomic diffusion and ad hoc additional mixing below the surface convection zone. The comparison shows that the stars in M30 are affected by atomic diffusion and additional mixing, but we were unable to determine the efficiency of the additional mixing precisely. This is the fourth globular cluster (after NGC6397, NGC6752, and M4) in which atomic diffusion signatures are detected. After applying a conservative correction (T6.0 model) for atomic diffusion, we find an initial Li abundance of A(Li) = 2.48 +/- 0.10 for the globular cluster M30. We also detected a Li-rich SGB star with a Li abundance of A(Li) = 2.39. The finding makes Li-rich mass transfer a likely scenario for this star and rules out models in which its Li enhancement is created during the RGB bump phase.

Keywords
techniques: spectroscopic, stars: abundances, stars: atmospheres, globular clusters: individual: M 30, stars: Population II, stars: fundamental parameters
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-298681 (URN)10.1051/0004-6361/201527948 (DOI)000375318300073 ()
Funder
European Science Foundation (ESF)Swedish National Space Board
Available from: 2016-07-07 Created: 2016-07-06 Last updated: 2017-11-28Bibliographically approved
Lind, K., Koposov, S. E., Battistini, C., Marino, A. F., Ruchti, G., Serenelli, A., . . . Zaggia, S. (2015). The Gaia-ESO Survey: A globular cluster escapee in the Galactic halo. Astronomy and Astrophysics, 575, Article ID L12.
Open this publication in new window or tab >>The Gaia-ESO Survey: A globular cluster escapee in the Galactic halo
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2015 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 575, article id L12Article in journal (Refereed) Published
Abstract [en]

A small fraction of the halo field is made up of stars that share the light element (Z <= 13) anomalies characteristic of second generation globular cluster (GC) stars. The ejected stars shed light on the formation of the Galactic halo by tracing the dynamical history of the clusters, which are believed to have once been more massive. Some of these ejected stars are expected to show strong Al enhancement at the expense of shortage of Mg, but until now no such star has been found. We search for outliers in the Mg and Al abundances of the few hundreds of halo field stars observed in the first eighteen months of the Gaia-ESO public spectroscopic survey. One halo star at the base of the red giant branch, here referred to as 22593757-4648029 is found to have [Mg/Fe] = -0.36 +/- 0.04 and [Al/Fe] = 0.99 +/- 0.08, which is compatible with the most extreme ratios detected in GCs so far. We compare the orbit of 22593757-4648029 to GCs of similar metallicity and find it unlikely that this star has been tidally stripped with low ejection velocity from any of the clusters. However, both chemical and kinematic arguments render it plausible that the star has been ejected at high velocity from the anomalous GC omega Centauri within the last few billion years. We cannot rule out other progenitor GCs, because some may have disrupted fully, and the abundance and orbital data are inadequate for many of those that are still intact.

Keywords
stars: abundances, stars: Population II, techniques: spectroscopic, globular clusters: general, Galaxy: stellar content, Galaxy: halo
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-251446 (URN)10.1051/0004-6361/201425554 (DOI)000350249100111 ()
Available from: 2015-04-23 Created: 2015-04-17 Last updated: 2017-12-04Bibliographically approved
Guaita, L., Melinder, J., Hayes, M., Ostlin, G., Gonzalez, J. E., Micheva, G., . . . Pardy, S. A. (2015). The Lyman alpha reference sample IV. Morphology at low and high redshift. Astronomy and Astrophysics, 576, Article ID A51.
Open this publication in new window or tab >>The Lyman alpha reference sample IV. Morphology at low and high redshift
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2015 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 576, article id A51Article in journal (Refereed) Published
Abstract [en]

Context. The transport of Ly alpha photons in galaxies is a complex process and the conditions under which Ly alpha photons manage to escape from certain galaxies is still under investigation. The Lyman alpha reference sample (LARS) is a sample of 14 local star-forming galaxies, designed to study Ly alpha in detail and relate it to rest-frame UV and optical emission. Aims. With the aim of identifying rest-frame UV and optical properties, which are typical of Ly alpha emitters (LAEs, galaxies with EW(Ly alpha) > 20 angstrom) at both low and high redshift, we investigated the morphological properties of the LARS galaxies, in particular the ones that exhibit intense Ly alpha radiation. Methods. We measured sizes and morphological parameters in the continuum, Ly alpha, and Ha images. We studied morphology by using the Gini coefficient vs. M20 and asymmetry vs. concentration diagrams. We then simulated LARS galaxies at z similar to 2 and 5.7, performing the same morphological measurements. We also investigated the detectability of LARS galaxies in current deep field observations. The subsample of LAEs within LARS (LARS-LAEs) was stacked to provide a comparison to stacking studies performed at high redshift. Results. LARS galaxies have continuum size, stellar mass, and rest-frame absolute magnitude typical of Lyman break analogues in the local Universe and also similar to 2 < z < 3 star-forming galaxies and massive LAEs. LARS optical morphology is consistent with the one of merging systems, and irregular or starburst galaxies. For the first time we quantify the morphology in Ly alpha images: even if a variety of intrinsic conditions of the interstellar medium can favour the escape of Ly alpha photons, LARS-LAEs appear small in the continuum, and their Ly alpha is compact. LARS galaxies tend to be more extended in Ly alpha than in the rest-frame UV. It means that Ly alpha photons escape by forming haloes around HII regions of LARS galaxies. Conclusions. The stack of LARS-LAE Ly alpha images is peaked in the centre, indicating that the conditions, which make a galaxy an LAE, tend to produce a concentrated surface brightness profile. On the other hand, the stack of all LARS galaxies is shallower and more extended. This can be caused by the variety of dust and HI amount and distribution, which produces a more complex, patchy, and extended profile, like the one observed for Lyman break galaxies that can contribute to the stack. We cannot identify a single morphological property that controls whether a galaxy emits a net positive Ly alpha flux. However, the LARS-LAEs have continuum properties consistent with merging systems.

Keywords
techniques: imaging spectroscopy, galaxies: star formation, galaxies: starburst
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-251975 (URN)10.1051/0004-6361/201425053 (DOI)000351877600051 ()
Available from: 2015-05-18 Created: 2015-04-28 Last updated: 2017-12-04Bibliographically approved
Gruyters, P., Nordlander, T. & Korn, A. (2014). Atomic diffusion and mixing in old stars V: A deeper look into the globular cluster NGC6752. Astronomy and Astrophysics, 567, A72
Open this publication in new window or tab >>Atomic diffusion and mixing in old stars V: A deeper look into the globular cluster NGC6752
2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 567, p. A72-Article in journal (Refereed) Published
Abstract [en]

Context. Abundance trends in heavier elements with evolutionary phase have been shown to exist in the globular cluster NGC6752 ([Fe/H] = −1.6). These trends are a result of atomic diffusion and additional (non-convective) mixing. Studying such trends can provide us with important constraints on the extent to which diffusion modifies the internal structure and surface abundances of solartype, metal-poor stars.

Aims. Taking advantage of a larger data sample, we investigate the reality and the size of these abundance trends and address questions and potential biases associated with the various stellar populations that make up NGC6752.

Methods. We perform an abundance analysis by combining photometric and spectroscopic data of 194 stars located between the turnoff point and the base of the red giant branch. Stellar parameters are derived from ubvy Strömgren photometry. Using the quantitative-spectroscopy package SME, stellar surface abundances for light elements such as Li, Na, Mg, Al, and Si as well as heavier elements such as Ca, Ti, and Fe are derived in an automated way by fitting synthetic spectra to individual lines in the stellar spectra, obtained with the VLT/FLAMES-GIRAFFE spectrograph.

Results. Based on ubvy Strömgren photometry, we are able to separate three stellar populations in NGC 6752 along the evolutionary sequence from the base of the red giant branch down to the turnoff point. We find weak systematic abundance trends with evolutionary phase for Ca, Ti, and Fe which are best explained by stellar-structure models including atomic diffusion with efficient additional mixing. We derive a new value for the initial lithium abundance of NGC6752 after correcting for the effect of atomic diffusion and additional mixing which falls slightly below the predicted standard BBN value.

Conclusions. We find three stellar populations by combining photometric and spectroscopic data of 194 stars in the globular cluster NGC6752. Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the statistical significance of the individual trends is weak, they all support the notion that atomic diffusion is operational along the evolutionary sequence of NGC6752.

Keywords
stars: abundances – stars: atmospheres – stars: fundamental parameters – globular clusters: individual: NGC 6752 – techniques: spectroscopic
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics
Identifiers
urn:nbn:se:uu:diva-230180 (URN)10.1051/0004-6361/201423590 (DOI)000341185300086 ()
Available from: 2014-08-20 Created: 2014-08-20 Last updated: 2017-12-05Bibliographically approved
Gruyters, P. (2014). Exploring the Chemical Evolution of Globular Clusters and their Stars: Observational Constraints on Atomic Diffusion and Cluster Pollution in NGC 6752 and M4. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
Open this publication in new window or tab >>Exploring the Chemical Evolution of Globular Clusters and their Stars: Observational Constraints on Atomic Diffusion and Cluster Pollution in NGC 6752 and M4
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Through the cosmic matter cycle, the chemical evolution of the Milky Way is imprinted in the elemental abundance patterns of late-type stars (spectral types F to K). Due to their long lifetimes ( 1 Hubble time), these stars are of particular importance when it comes to studying the build-up of elements during the early times of our Galaxy. The chemical composition of the atmospheric layers of such stars is believed to resemble the gas from which they were formed. However, recent observations in globular clusters seem to contradict this assumption. The observations indicate that processes are at work that alter the surface compositions in these stars. The combined effect of processes responsible for an exchange of material between the stellar interior and atmosphere during the main sequence lifetime of the star, is referred to as atomic diffusion. Yet, the extent to which these processes alter surface abundances is still debated.

By comparing abundances in unevolved and evolved stars all drawn from the same stellar population, any surface abundance anomalies can be traced. The anomalies, if found, can be compared to theoretical predictions from stellar structure models including atomic diffusion. Globular clusters provide stellar populations suitable to conduct such a comparison. In this thesis, the results of three independent analyses of two globular clusters, NGC 6752 and M4, at different metallicities are presented. The comparison between observations and models yields constraints on the models and finally a better understanding of the physical processes at work inside stars.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. p. 91
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1168
Keywords
stars: abundances – stars: atmospheres – stars: fundamental parameters – globular clusters: individual: NGC 6752 and M4 – techniques: spectroscopic
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics
Identifiers
urn:nbn:se:uu:diva-230182 (URN)978-91-554-9008-9 (ISBN)
Public defence
2014-10-03, Å2001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2014-09-12 Created: 2014-08-20 Last updated: 2015-01-22
Bergemann, M., Ruchti, G. R., Serenelli, A., Feltzing, S., Alves-Brito, A., Asplund, M., . . . Tautvaisiene, G. (2014). The Gaia-ESO Survey: radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk. Astronomy and Astrophysics, 565, A89
Open this publication in new window or tab >>The Gaia-ESO Survey: radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk
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2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 565, p. A89-Article in journal (Refereed) Published
Abstract [en]

We study the relationship between age, metallicity, and alpha-enhancement of FGK stars in the Galactic disk. The results are based upon the analysis of high-resolution UVES spectra from the Gaia-ESO large stellar survey. We explore the limitations of the observed dataset, i.e. the accuracy of stellar parameters and the selection effects that are caused by the photometric target preselection. We find that the colour and magnitude cuts in the survey suppress old metal-rich stars and young metal-poor stars. This suppression may be as high as 97% in some regions of the age-metallicity relationship. The dataset consists of 144 stars with a wide range of ages from 0.5 Gyr to 13.5 Gyr, Galactocentric distances from 6 kpc to 9.5 kpc, and vertical distances from the plane 0 < vertical bar Z vertical bar < 1.5 kpc. On this basis, we find that i) the observed age-metallicity relation is nearly flat in the range of ages between 0 Gyr and 8 Gyr; ii) at ages older than 9 Gyr, we see a decrease in [Fe/H] and a clear absence of metal-rich stars; this cannot be explained by the survey selection functions; iii) there is a significant scatter of [Fe/H] at any age; and iv) [Mg/Fe] increases with age, but the dispersion of [Mg/Fe] at ages > 9 Gyr is not as small as advocated by some other studies. In agreement with earlier work, we find that radial abundance gradients change as a function of vertical distance from the plane. The [Mg/Fe] gradient steepens and becomes negative. In addition, we show that the inner disk is not only more alpha-rich compared to the outer disk, but also older, as traced independently by the ages and Mg abundances of stars.

Keywords
stars: abundances, stars: fundamental parameters, solar neighborhood, Galaxy: disk, Galaxy: formation, surveys
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-228484 (URN)10.1051/0004-6361/201423456 (DOI)000336730900089 ()
Available from: 2014-07-15 Created: 2014-07-15 Last updated: 2017-12-05Bibliographically approved
Smiljanic, R., Korn, A. J., Bergemann, M., Frasca, A., Magrini, L., Masseron, T., . . . Zaggia, S. (2014). The Gaia-ESO Survey: The analysis of high-resolution UVES spectra of FGK-type stars. Astronomy and Astrophysics, 570, A122
Open this publication in new window or tab >>The Gaia-ESO Survey: The analysis of high-resolution UVES spectra of FGK-type stars
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2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 570, p. A122-Article in journal (Refereed) Published
Abstract [en]

Context. The ongoing Gaia-ESO Public Spectroscopic Survey is using FLAMES at the VLT to obtain high-quality medium-resolution Giraffe spectra for about 10(5) stars and high-resolution UVES spectra for about 5000 stars. With UVES, the Survey has already observed 1447 FGK-type stars. Aims. These UVES spectra are analyzed in parallel by several state-of-the-art methodologies. Our aim is to present how these analyses were implemented, to discuss their results, and to describe how a final recommended parameter scale is defined. We also discuss the precision (method-to-method dispersion) and accuracy (biases with respect to the reference values) of the final parameters. These results are part of the Gaia-ESO second internal release and will be part of its first public release of advanced data products. Methods. The final parameter scale is tied to the scale defined by the Gaia benchmark stars, a set of stars with fundamental atmospheric parameters. In addition, a set of open and globular clusters is used to evaluate the physical soundness of the results. Each of the implemented methodologies is judged against the benchmark stars to define weights in three different regions of the parameter space. The final recommended results are the weighted medians of those from the individual methods. Results. The recommended results successfully reproduce the atmospheric parameters of the benchmark stars and the expected T-eff-log g relation of the calibrating clusters. Atmospheric parameters and abundances have been determined for 1301 FGK-type stars observed with UVES. The median of the method-to-method dispersion of the atmospheric parameters is 55K for T-eff, 0.13dex for log g and 0.07 dex for [Fe/H]. Systematic biases are estimated to be between 50-100 K for T-eff, 0.10-0.25 dex for log g and 0.05-0.10 dex for [Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and Eu. The typical method-to-method dispersion of the abundances varies between 0.10 and 0.20 dex. Conclusions. The Gaia-ESO sample of high-resolution spectra of FGK-type stars will be among the largest of its kind analyzed in a homogeneous way. The extensive list of elemental abundances derived in these stars will enable significant advances in the areas of stellar evolution and Milky Way formation and evolution.

Keywords
methods: data analysis, surveys, stars: abundances, stars: fundamental parameters, stars: late-type
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-238582 (URN)10.1051/0004-6361/201423937 (DOI)000344158500056 ()
Available from: 2014-12-16 Created: 2014-12-14 Last updated: 2017-12-05Bibliographically approved
Ostlin, G., Hayes, M., Duval, F., Sandberg, A., Rivera-Thorsen, T., Marquart, T., . . . Verhamme, A. (2014). The Ly alpha Reference Sample. I. Survey Outline and First Results for Markarian 259. Astrophysical Journal, 797(1), 11
Open this publication in new window or tab >>The Ly alpha Reference Sample. I. Survey Outline and First Results for Markarian 259
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2014 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 797, no 1, p. 11-Article in journal (Refereed) Published
Abstract [en]

The Ly alpha Reference Sample (LARS) is a substantial program with the Hubble Space Telescope (HST) that provides a sample of local universe laboratory galaxies in which to study the detailed astrophysics of the visibility and strength of the Ly alpha line of neutral hydrogen. Ly alpha is the dominant spectral line in use for characterizing high-redshift (z) galaxies. This paper presents an overview of the survey, its selection function, and HST imaging observations. The sample was selected from the combined GALEX+Sloan Digital Sky Survey catalog at z = 0.028-0.19, in order to allow Ly alpha to be captured with combinations of long-pass filters in the Solar Blind Channel (SBC) of the Advanced Camera for Surveys (ACS) onboard HST. In addition, LARS utilizes H alpha and H beta narrowband and u, b, i broadband imaging with ACS and the Wide Field Camera 3 (WFC3). In order to study galaxies in which large numbers of Ly alpha photons are produced (whether or not they escape), we demanded an H alpha equivalent width W(H alpha) >= 100 angstrom. The final sample of 14 galaxies covers far-UV (FUV, lambda similar to 1500 angstrom) luminosities that overlap with those of high-z Ly alpha emitters (LAEs) and Lyman break galaxies (LBGs), making LARS a valid comparison sample. We present the reduction steps used to obtain the Ly alpha images, including our LARS eXtraction software (LaXs), which utilizes pixel-by-pixel spectral synthesis fitting of the energy distribution to determine and subtract the continuum at Ly alpha. We demonstrate that the use of SBC long-pass-filter combinations increase the signal-to-noise ratio by an order of magnitude compared to the nominal Ly alpha filter available in SBC. To exemplify the science potential of LARS, we also present some first results for a single galaxy, Mrk 259 (LARS #1). This irregular galaxy shows bright and extended (indicative of resonance scattering) but strongly asymmetric Ly alpha emission. Spectroscopy from the Cosmic Origins Spectrograph on board HST centered on the brightest UV knot shows a moderate outflow in the neutral interstellar medium (probed by low ionization stage absorption features) and Ly alpha emission with an asymmetric profile. Radiative transfer modeling is able to reproduce the essential features of the Ly alpha line profile and confirms the presence of an outflow. From the integrated photometry we measure an Ly alpha luminosity of L-Ly alpha= 1.3x10(42) erg s(-1) an equivalent width W(Ly alpha) = 45 angstrom and an FUV absolute magnitude M-FUV = -19.2 (AB). Mrk 259 would hence be detectable in high-z Ly alpha and LBG surveys. The total Ly alpha escape fraction is 12%. This number is higher than the low-z average, but similar to that at z > 4, demonstrating that LARS provides a valid comparison sample for high-z galaxy studies.

Keywords
cosmology: observations, galaxies: evolution, galaxies: individual (Mrk 259), galaxies: starburst, radiative transfer, ultraviolet: galaxies
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-240202 (URN)10.1088/0004-637X/797/1/11 (DOI)000345915000011 ()
Available from: 2015-01-07 Created: 2015-01-06 Last updated: 2017-12-05Bibliographically approved
Gruyters, P., Korn, A. J., Richard, O., Grundahl, F., Collet, R., Mashonkina, L. I., . . . Barklem, P. S. (2013). Atomic diffusion and mixing in old stars IV. Weak abundance trends in the globular cluster NGC6752. Astronomy and Astrophysics, 555, A31
Open this publication in new window or tab >>Atomic diffusion and mixing in old stars IV. Weak abundance trends in the globular cluster NGC6752
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2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 555, p. A31-Article in journal (Refereed) Published
Abstract [en]

Context. Atomic diffusion in stars can create systematic trends of surface abundances with evolutionary stage. Globular clusters off er useful laboratories to put observational constraints on this theory as one needs to compare abundances in unevolved and evolved stars, all drawn from the same stellar population. Aims. Atomic diffusion and additional mixing has been shown to be at work in the globular cluster NGC6397 at a metallicity of [Fe/H] similar to -2.1. We investigate possible abundance trends in Li, Mg, Ca, Ti, Sc, and Fe with evolutionary stage in another globular cluster NGC6752 at a metallicity of [Fe/H] similar to -1.6. This in order to better constrain stellar structure models including atomic diffusion and additional mixing. Methods. We performed a differential abundance analysis on VLT/FLAMES-UVES data of 16 stars in four groups between the turnoff point and the red giant branch. Continuum normalisation of the stellar spectra was performed in an automated way using DAOSPEC. Differential abundances relative to the sun were derived by fitting synthetic spectra to individual lines in the stellar spectrum. Results. We find weak systematic abundance trends with evolutionary phase for Fe, Sc, Ti, and Ca. The individual trends are weaker than the trends in NGC6397 and only significant at the 1-sigma level. However, the combined trend shows a significance on the 2-sigma level. The trends are best explained by stellar-structure models including atomic diffusion with more efficient additional mixing than needed in NGC6397. The model allows to correct for sub-primordial stellar lithium abundances of the stars on the Spite plateau. Conclusions. Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the significance of the trends is weak, they all seem to indicate that atomic diffusion is operational along the evolutionary sequence of NGC6752. The trends are weaker than those observed in NGC6397, which is perhaps due to more efficient mixing. Using models of atomic diffusion including efficient additional mixing, we find a diffusion-corrected primordial lithium abundance of log epsilon(Li) = 2.58 +/- 0.10, in agreement with WMAP-calibrated Big-Bang nucleosynthesis predictions within the mutual 1-sigma uncertainties.

Keywords
stars: abundances, stars: atmospheres, stars: fundamental parameters, globular clusters: individual: NGC 6752, techniques: spectroscopic
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-207105 (URN)10.1051/0004-6361/201220821 (DOI)000322008600031 ()
Available from: 2013-09-10 Created: 2013-09-09 Last updated: 2017-12-06Bibliographically approved
Gruyters, P., Exter, K., Roberts, T. P. & Rappaport, S. (2012). A VLT VIMOS IFU study of the ionisation nebula surrounding the supersoft X-ray source CAL 83. Astronomy and Astrophysics, 544, A86
Open this publication in new window or tab >>A VLT VIMOS IFU study of the ionisation nebula surrounding the supersoft X-ray source CAL 83
2012 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 544, p. A86-Article in journal (Refereed) Published
Abstract [en]

Context. CAL 83 is a prototype of the class of Super Soft X-ray Sources (SXS). It is a binary consisting of a low mass secondary that is transferring mass onto a white dwarf primary and is the only known SXS surrounded by an ionisation nebula, made up of the interstellar medium (ISM) ionised by the source itself. We study this nebula using integral field spectroscopy.

Aims. The study of ionised material can inform us about the source that is responsible for the ionisation, in a way that is complementary to studying the source directly. Since CAL 83 is the only SXS known with an ionisation nebula, we have an opportunity to see if such studies are as useful for SXSs as they have been for other X-ray ionised nebulae. We can use these data to compare to models of how CAL83 should ionise its surroundings, based on what we know about the source emission spectrum and the physical conditions of the surrounding ISM.

Methods. With the VIMOS integral field spectrograph we obtained spectra over a 25x25 '' field of view, encompassing one quarter of the nebula. Emission line maps - HI, HeII lambda 4686, [O III] lambda lambda 4959, 5007, [N II] lambda lambda 6548, 5683, and [S II] lambda lambda 6716, 6731 - are produced in order to study the morphology of the ionised gas. We include CAL 83 on diagrams of various diagnostic ion ratios to compare it to other X-ray ionised sources. Finally we computed some simple models of the ionised gas around CAL 83 and compare the predicted to the observed spectra.

Results. CAL 83 appears to have a fairly standard ionisation nebula as far as the morphology goes: the edges where H is recombining are strong in the low stage ionisation lines and the central, clumpy regions are stronger in the higher stage ionisation lines. But the He II emission is unusual in being confined to one side of CAL 83 rather than being homogeneously distributed as with the other ions. We model the CAL 83 nebula with cloudy using model parameters for SXSs found in the literature. The He II emission does not fit in with model predictions; in fact none of the models is able to fit the observed spectrum very well.

Conclusions. The spectral line images of the region surrounding CAL 83 are revealing and instructive. However, more work on modelling the spectrum of the ionised gas is necessary, and especially for the high-ionisation level emission from CAL83. In particular, we wish to know if the He II emission and the other nebular lines are powered by the same ionising source.

Keywords
Magellanic Clouds, ISM: individual objects: CAL 83, X-rays: ISM, X-rays: stars, binaries: close
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-182539 (URN)10.1051/0004-6361/201219051 (DOI)000308290100086 ()
Available from: 2012-10-12 Created: 2012-10-11 Last updated: 2017-12-07Bibliographically approved
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