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Dust Driven Mass Loss from Carbon Stars as Function of Stellar Parameters II: Effects of Relaxing the Small Particle Approximation
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
(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: urn:nbn:se:uu:diva-99591OAI: oai:DiVA.org:uu-99591DiVA: diva2:208256
Available from: 2009-03-16 Created: 2009-03-16 Last updated: 2010-01-14
In thesis
1. On the Winds of Carbon Stars and the Origin of Carbon: A Theoretical Study
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. 105 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 625
Keyword
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: 2010-12-08Bibliographically approved

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