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Mattsson, Lars
Publications (10 of 11) Show all publications
Mattsson, L., Wahlin, R. & Höfner, S. (2010). Dust driven mass loss from carbon stars as a function of stellar parameters I: A grid of solar-metallicity wind models. Astronomy and Astrophysics, 509(1), 13
Open this publication in new window or tab >>Dust driven mass loss from carbon stars as a function of stellar parameters I: A grid of solar-metallicity wind models
2010 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 509, no 1, p. 13-Article in journal (Refereed) Published
Abstract [en]

Context. Knowing how the mass loss of carbon-rich AGB stars depends on stellar parameters is crucial for stellar evolution modelling, as well as for the understanding of when and how circumstellar structures emerge around these stars, e.g., dust shells and so-called detached shells of expelled gas.

Aims. The purpose of this paper is to explore the stellar parameter space using a numerical radiation hydrodynamic (RHD) model of carbon-star atmospheres, including a detailed description of dust formation and frequency-dependent radiative transfer, in order to determine how the mass loss of carbon stars changes with stellar parameters.

Methods. We have computed a grid of 900 numeric dynamic model atmospheres (DMAs) using a well-tested computer code. This grid of models covers most of the expected combinations of stellar parameters, which are the stellar temperature, the stellar luminosity, the stellar mass, the abundance of condensible carbon, and the velocity amplitude of the pulsation.

Results. The resultant mass-loss rates and wind speeds are clearly affected by the choice of stellar temperature, mass, luminosity and the abundance of available carbon. In certain parts of the parameter space there is also an inevitable mass-loss threshold, below which a dust-driven wind is not possible. Contrary to some previous studies, we find a strong dependence on the abundance of free carbon, which turns out to be a critical parameter. Furthermore, we have found that the dust grains that form in the atmosphere may grow too large for the commonly-used small-particle approximation of the dust opacity to be strictly valid. This may have some bearing on the wind properties, although further study of this problem is needed before quantitative conclusions can be drawn.

Conclusions. The wind properties show relatively simple dependences on stellar parameters above the mass-loss threshold, while the threshold itself is of a more complicated nature. Hence, we chose not to derive any simplistic mass-loss formula, but rather provide a mass-loss prescription in the form of aeasy-to-use FORTRAN routine. Since this mass-loss routine is based on data coming from an essentially self-consistent model of mass loss, it may therefore serve as a better mass-loss prescription for stellar evolution calculations than empirical formulae. Furthermore, we conclude that there are still some issues that need to be investigated, such as the role of grain-sizes.

Keywords
stars: AGB and post-AGB; stars: atmospheres; stars: carbon; circumstellar matter; stars: evolution; stars: mass-loss
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy; Physics
Identifiers
urn:nbn:se:uu:diva-99592 (URN)10.1051/0004-6361/200912084 (DOI)000274159400026 ()
Available from: 2009-03-16 Created: 2009-03-16 Last updated: 2018-06-04Bibliographically approved
Mattsson, L. (2010). The origin of carbon: Low-mass stars and an evolving, initially top-heavy IMF?. Astronomy and Astrophysics, 515, A68
Open this publication in new window or tab >>The origin of carbon: Low-mass stars and an evolving, initially top-heavy IMF?
2010 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 515, p. A68-Article in journal (Refereed) Published
Abstract [en]

Multi-zone chemical evolution models (CEMs), differing in the nucleosynthesis prescriptions (yields) and prescriptions of star formation, have been computed for the Milky Way. All models fit the observed O/H and Fe/H gradients well and reproduce the main characteristics of the gas distribution, but they are also designed to do so. For the C/H gradient the results are inconclusive with regards to yields and star formation. The C/Fe and O/Fe vs. Fe/H, as well as C/O vs. O/H trends predicted by the models for the solar neighbourhood zone were compared with stellar abundances from the literature. For O/Fe vs. Fe/H all models fit the data, but for C/O vs. O/H, only models with increased carbon yields for zero-metallicity stars or an evolving initial mass function provide good fits. Furthermore, a steep star formation threshold in the disc can be ruled out since it predicts a steep fall-off in all abundance gradients beyond a certain galactocentric distance (similar to 13 kpc) and cannot explain the possible flattening of the C/H and Fe/H gradients in the outer disc seen in observations. Since in the best-fit models the enrichment scenario is such that carbon is primarily produced in low-mass stars, it is suggested that in every environment where the peak of star formation happened a few Gyr back in time, winds of carbon-stars are responsible for most of the carbon enrichment. However, a significant contribution by zero-metallicity stars, especially at very early stages, and by winds of high-mass stars, which are increasing in strength with metallicity, cannot be ruled out by the CEMs presented here. In the solar neighbourhood, as much as 80%, or as little as 40% of the carbon may have been injected to the interstellar medium by low- and intermediate-mass stars. The stellar origin of carbon remains an open question, although production in low-and intermediate-mass stars appears to be the simplest explanation of observed carbon abundance trends.

Keywords
Galaxy: abundances; Galaxy: evolution; Galaxy: formation; Galaxy: stellar content; Stars: carbon; Stars: mass-loss
National Category
Physical Sciences
Research subject
Astronomy; Physics
Identifiers
urn:nbn:se:uu:diva-99581 (URN)10.1051/0004-6361/200913315 (DOI)000280505000037 ()
Note
Uppdaterad från Manuskript till Artikel 20101208Available from: 2009-03-16 Created: 2009-03-16 Last updated: 2018-06-04Bibliographically approved
Mattsson, L. (2009). On the Winds of Carbon Stars and the Origin of Carbon: A Theoretical Study. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
Open this publication in new window or tab >>On the Winds of Carbon Stars and the Origin of Carbon: A Theoretical Study
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Carbon is the basis for life, as we know it, but its origin is still largely unclear. Carbon-rich Asymptotic Giant Branch (AGB) stars (carbon stars) play an important rôle in the cosmic matter cycle and may contribute most of the carbon in the Galaxy.

In this thesis it is explored how the dust-driven mass loss of these stars depends on the basic stellar parameters by computing a large grid of wind models. The existence of a critical wind regime and mass-loss thresholds for dust-driven winds are confirmed. Furthermore, a steep dependence of mass loss on carbon excess is found. Exploratory work on the effects of different stellar metallicities and the sizes of dust grains shows that strong dust-driven winds develop also at moderately low metallicities, and that typical sizes of dust grains affect the wind properties near a mass-loss threshold.

It is demonstrated that the mass-loss rates obtained with the wind models have dramatic consequences when used in models of carbon-star evolution. A pronounced superwind develops soon after the star becomes carbon rich, and it therefore experiences only a few thermal pulses as a carbon star before the envelope is lost. The number of dredge-up events and the thermal pulses is limited by a self-regulating mechanism: each thermal pulse dredges up carbon, which increases the carbon excess and hence also the mass-loss rate. In turn, this limits the number of thermal pulses.

The mass-loss evolution during a thermal pulse (He-shell flash) is considered as an explanation of the observations of so-called detached shells around carbon stars. By combining models of dust-driven winds with a stellar evolution model, and a simple hydrodynamic model of the circumstellar envelope, it is shown that wind properties change character during a He-shell flash such that a thin detached gas shell can form by wind-wind interaction.

Finally, it is suggested that carbon stars are responsible for much of the carbon in the interstellar medium, but a scenario where high-mass stars are major carbon producers cannot be excluded. In either case, however, the carbon abundances of the outer Galactic disc are relatively low, and most of the carbon has been released quite recently. Thus, there may neither be enough carbon, nor enough time, for more advanced carbon-based life to emerge in the outer Galaxy. This lends some support to the idea that only the mid-part of the Galactic disc can be a “Galactic habitable zone”, since the inner parts of the Galaxy are plagued by frequent supernova events that are presumably harmful to all forms of life.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2009. p. 105
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 625
Keywords
AGB stars, carbon stars, mass loss, stellar winds, circumstellar matter, cosmic dust, stellar evolution, nucleosynthesis, galactic chemical evolution
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy; Physics
Identifiers
urn:nbn:se:uu:diva-99593 (URN)978-91-554-7472-0 (ISBN)
Public defence
2009-04-29, Ångströmlaboratoriet, Sal 4001, Lägerhyddsvägen 1, Uppsala, 14:00 (English)
Opponent
Supervisors
Available from: 2009-04-08 Created: 2009-03-16 Last updated: 2018-06-04Bibliographically approved
Mattsson, L., Wahlin, R., Höfner, S. & Eriksson, K. (2008). Intense mass loss from C-rich AGB stars at low metallicity?. Astronomy and Astrophysics, 484(1), L5-L8
Open this publication in new window or tab >>Intense mass loss from C-rich AGB stars at low metallicity?
2008 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 484, no 1, p. L5-L8Article in journal (Refereed) Published
Abstract [en]

We argue that the energy injection of pulsations may be of greater importance to the mass-loss rate of AGB stars than metallicity, and that the mass-loss trend with metallicity is not as simple as sometimes assumed. Using our detailed radiation hydrodynamical models that include dust formation, we illustrate the effects of pulsation energy on wind properties. We find that the mass-loss rate scales with the kinetic energy input by pulsations as long as a dust-saturated wind does not occur, and all other stellar parameters are kept constant. This includes the absolute abundance of condensible carbon (not bound in CO), which is more relevant than keeping the C/O-ratio constant when comparing stars of different metallicity. The pressure and temperature gradients in the atmospheres of stars, become steeper and flatter, respectively, when the metallicity is reduced, while the radius where the atmosphere becomes opaque is typically associated with a higher gas pressure. This effect can be compensated for by adjusting the velocity amplitude of the variable inner boundary (piston), which is used to simulate the effects of pulsation, to obtain models with comparable kinetic-energy input. Hence, it is more relevant to compare models with similar energy-injections than of similar velocity amplitude. Since there is no evidence for weaker pulsations in low-metallicity AGB stars, we conclude that it is unlikely that low-metallicity C-stars have lower mass-loss rates, than their more metal-rich counterparts with similar stellar parameters, as long as they have a comparable amount of condensible carbon.

Keywords
stars: AGB and post-AGB, stars: atmospheres, stars: carbon, stars:, mass-loss, hydrodynamics, radiative transfer
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy; Physics
Identifiers
urn:nbn:se:uu:diva-99594 (URN)10.1051/0004-6361:200809689 (DOI)000256309400002 ()
Available from: 2009-03-16 Created: 2009-03-16 Last updated: 2018-06-04Bibliographically approved
Mattsson, L., Höfner, S. & Herwig, F. (2007). Mass loss evolution and the formation of detached shells around TP-AGB stars. Astronomy and Astrophysics, 470(1), 339-352
Open this publication in new window or tab >>Mass loss evolution and the formation of detached shells around TP-AGB stars
2007 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 470, no 1, p. 339-352Article in journal (Refereed) Published
Abstract [en]

Context: The origin of the so called “detached shells” around AGB stars is not fully understood, but two common hypotheses state that these shells form either through the interaction of distinct wind phases or an eruptive mass loss associated with a He-shell flash. We present a model of the formation of detached shells around thermal pulse asymptotic giant branch (TP-AGB) stars, based on detailed modelling of mass loss and stellar evolution, leading to a combination of eruptive mass loss and wind interaction. Aims: The purpose of this paper is first of all to connect stellar evolution with wind and mass loss evolution and demonstrate its consistency with observations, but also to show how thin detached shells around TP-AGB stars can be formed. Previous attempts to link mass loss evolution with the formation of detached shells were based on approximate prescriptions for the mass loss and have not included detailed modelling of the wind formation as we do here. Methods: Using stellar parameters sampled from an evolutionary track for a 2 ~M_ȯ star, we have computed the time evolution of the atmospheric layers and wind acceleration region during a typical thermal pulse with detailed radiation hydrodynamical models including dust formation. Based on these results, we simulate the subsequent circumstellar envelope (CSE) evolution using a spherical hydrodynamic model. Results: We find that existing simple mass loss prescriptions all suggest different mass loss evolutions and that they differ from our detailed wind modelling. The most important factor for the formation of a detached shell is the wind velocity evolution which has a strong impact on the wind interaction and the resulting pile-up of matter. Our CSE model shows that a thin shell structure may be formed as a consequence of a rather short phase of intense mass loss in combination with a significant variation in the wind velocity, as obtained by our wind models. This situation can only be obtained for a limited range of amplitudes for the piston boundary used in the dynamic atmosphere models. Conclusions: The combined mass loss eruption and wind interaction scenario for the formation of detached shells around AGB stars (suggested by previous work) is confirmed by the present modelling. Changes in mass loss rate and wind velocity due to a He-shell flash are adequate for creating distinct wind phases and a “snow plow effect” that is necessary to form a geometrically thin detached shell. The derived properties of the shell (i.e. radius, thickness and density) are more or less consistent with existing observational constraints.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-14027 (URN)10.1051/0004-6361:20066368 (DOI)000247977400034 ()
Note
Appendix A is only available in electronic form at http://www.aanda.orgAvailable from: 2008-01-29 Created: 2008-01-29 Last updated: 2017-12-11Bibliographically approved
Mattsson, L., Höfner, S., Wahlin, R. & Herwig, F. (2007). On the Connection between Mass Loss and Evolution of C-rich AGB Stars. In: Why Galaxies Care About AGB Stars: Their Importance as Actors and Probes (pp. 239-243).
Open this publication in new window or tab >>On the Connection between Mass Loss and Evolution of C-rich AGB Stars
2007 (English)In: Why Galaxies Care About AGB Stars: Their Importance as Actors and Probes, 2007, p. 239-243Conference paper, Published paper (Other scientific)
Abstract [en]

The mass-loss properties of carbon rich AGB stars are not very well constrained at present. A variety of empirical or theoretical formulae with different parameterisations are available in the literature and the agreement between them is anything but good. These simple mass-loss prescriptions are nonetheless used in many models of stellar evolution without much consideration of their applicability in various cases. We present here an on-going project aiming at a better description of the mass loss, that could be used to improve stellar evolution models -- especially the evolution during the TP-AGB phase. As a first step, we have considered the mass-loss evolution during a He-shell flash. Using stellar parameters sampled from a stellar evolutionary track, we have computed the time evolution of the atmospheric layers and wind acceleration region during a flash event with detailed frequency-dependent radiation-hydrodynamical models including dust formation. We find that existing simple mass-loss prescriptions imply mass-loss evolutions different than our model. Based on these results, we have also simulated the subsequent long-term dynamical evolution of the circumstellar envelope (CSE), including the formation of a detached shell. The second step of the project deals with the dependence of mass loss on the basic stellar parameters. At the moment we are computing a large grid of wind models for C-rich AGB stars. Preliminary results show that simple parameterisations are difficult to obtain in certain regions of the parameter space considered, due to strong non-linearities in the wind mechanism.

Identifiers
urn:nbn:se:uu:diva-14048 (URN)978-1-58381-318-8 (ISBN)
Available from: 2008-01-29 Created: 2008-01-29
Mattsson, L., Wahlin, R. & Höfner, S. (2007). Thresholds for the Dust Driven Mass Loss from C-rich AGB Stars. In: Stellar Populations as Building Blocks of Galaxies: Proceedings of IAU Symposium #241 (pp. 37-38).
Open this publication in new window or tab >>Thresholds for the Dust Driven Mass Loss from C-rich AGB Stars
2007 (English)In: Stellar Populations as Building Blocks of Galaxies: Proceedings of IAU Symposium #241, 2007, p. 37-38Conference paper, Published paper (Other scientific)
Abstract [en]

It is well established that mass loss from AGB stars due to dust driven winds cannot be arbitrarily low. We model the mass loss from carbon rich AGB stars using detailed frequency-dependent radiation hydrodynamics including dust formation. We present a study of the thresholds for the mass loss rate as a function of stellar parameters based on a subset of a larger grid of such models and compare these results to previous observational and theoretical work. Furthermore, we demonstrate the impact of the pulsation mechanism and dust formation for the creation of a stellar wind and how it affects these thresholds and briefly discuss the consequences for stellar evolution.

Identifiers
urn:nbn:se:uu:diva-14042 (URN)doi:10.1017/S1743921307007375 (DOI)
Available from: 2008-01-29 Created: 2008-01-29
Wahlin, R., Mattsson, L., Höfner, S. & Aringer, B. (2007). Towards a low metallicity carbon star spectral library. In: Stellar Populations as Building Blocks of Galaxies: Proceedings of IAU Symposium #241 (pp. 105-106).
Open this publication in new window or tab >>Towards a low metallicity carbon star spectral library
2007 (English)In: Stellar Populations as Building Blocks of Galaxies: Proceedings of IAU Symposium #241, 2007, p. 105-106Conference paper, Published paper (Other scientific)
Identifiers
urn:nbn:se:uu:diva-14044 (URN)doi:10.1017/S1743921307007570 (DOI)
Available from: 2008-01-29 Created: 2008-01-29
Zackrisson, E., Bergvall, N., Marquart, T., Mattsson, L. & Östlin, G. (2005). Modelling the red halos of blue compact galaxies. In: Starbursts - From 30 Doradus to Lyman Break Galaxies.
Open this publication in new window or tab >>Modelling the red halos of blue compact galaxies
Show others...
2005 (Swedish)In: Starbursts - From 30 Doradus to Lyman Break Galaxies, 2005Conference paper, Published paper (Other (popular scientific, debate etc.))
Identifiers
urn:nbn:se:uu:diva-71027 (URN)
Available from: 2005-04-29 Created: 2005-04-29
Mattsson, L. & Höfner, S.Dust Driven Mass Loss from Carbon Stars as Function of Stellar Parameters II: Effects of Relaxing the Small Particle Approximation.
Open this publication in new window or tab >>Dust Driven Mass Loss from Carbon Stars as Function of Stellar Parameters II: Effects of Relaxing the Small Particle Approximation
(English)Manuscript (Other academic)
Abstract [en]

Context. It is well-established that the winds of carbon-rich AGB stars (carbon stars) can be driven by radiation pressure on grains of amorphous carbon and collisional transfer of momentum to the gas. This has been demonstrated by different numerical wind models including time-dependent dust formation, where it has been assumed that the dust grains that form never grow to sizes comparable to wavelengths around the stellar flux maximum (or beyond), which simplifies the treatment of grain opacities considerably. It is not clear, however, if this small particle approximation (SPA) is always valid.

Aims. In this paper we explore the effects of relaxing the SPA by considering a few less severe approximations for the radiation pressure efficiency,  which include the effects of grain sizes. The purpose of the study is mainly to establish when the SPA can be applied and to quantify the possible errors that may occur when it does not hold.

Methods. We have computed wind models with time-dependent dust formation and grain-size dependent opacities, where (1) the radiation pressure efficiency is approximated using grain sizes based on various means of the grain size distribution, and (2) where the problem is simplified by assuming a single dust-grain size.

Results. It is shown that in critical cases, the effect of grain sizes can be significant. Mass-loss rates may increase by a factor of two, or more, and wind speeds may be an order of magnitude higher. Furthermore, all models with grain-size dependent opacities that have resultant winds appear to have much lower degrees of dust condensation, compared to corresponding SPA models. Consequently, the "dust-loss rates" are much lower in the new models. However, for well-developed dust-driven winds, where the dust formation has saturated, the effect of grain sizes on the mass-loss rate and wind speed is almost negligible.

Conclusions. We conclude that the SPA is, under many circumstances, a reasonable simplification in models of carbon star mass loss. However, critical cases do exist, where especially the effects of momentum transfer due to scattering become significant. It is therefore uncertain whether previous results for winds in the transition regions (between no dust-driven mass loss and well-developed winds) are quantitatively correct. However, we argue that the SPA is a reasonable simplification for strong dust-driven winds. Furthermore, we note that there are other effects (not yet included in the model) than those of grain-size dependent opacities, which may become important in the critical wind regime.

National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy; Physics
Identifiers
urn:nbn:se:uu:diva-99591 (URN)
Available from: 2009-03-16 Created: 2009-03-16 Last updated: 2018-06-04
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