Mineral formation in stellar winds

Free access article

Issue		A&A Volume 371, Number 1, May III 2001


Page(s)		133 - 151
Section		Formation, structure and evolution of stars
DOI		http://dx.doi.org/10.1051/0004-6361:20010316

A&A 371, 133-151 (2001)
DOI: 10.1051/0004-6361:20010316

II. Effects of Mg/Si abundance variations on dust composition in AGB stars

A. S. Ferrarotti and H.-P. Gail

Institut für Theoretische Astrophysik, Universität Heidelberg, Tiergartenstraße 15, 69121 Heidelberg, Germany

(Received 1 December 2000 / Accepted 27 February 2001 )

Abstract
Galactic F and G dwarf stars show a considerable star-to-star scatter of their individual abundance ratios of Mg to Si. We study the consequences of such abundance variations on the composition of the dust mixture formed in the circumstellar dust shells if the stars enter the AGB stage of stellar evolution. From thermodynamic equilibrium condensation calculations for the abundant dust components we find: (i) For $1\lesssim\epsilon_{\rm Mg}/\epsilon_{\rm Si} \lesssim 2$ a mixture of forsterite (Mg₂SiO₄) and enstatite (MgSiO₃) is formed which consumes all of the available Mg and Si, (ii) for $\epsilon_{\rm Mg}/\epsilon_{\rm Si} \lesssim 1$ enstatite and quartz (SiO₂) are formed, while (iii) for $\epsilon_{\rm Mg}/\epsilon_{\rm Si}\gtrsim2$ forsterite and solid MgO are formed. According to observed Mg/Si abundance ratios, about 90% of all stars fall into the first category, about 10% of all stars seem to fall into the second category, and virtually no stars fall into the third category. For the range of observed Mg/Si abundance ratios $0.6\lesssim\epsilon_{\rm Mg}/ \epsilon_{\rm Si}\lesssim 1.5$ we have studied the formation of a multicomponent mixture of dust in circumstellar dust shells by calculating simple stationary wind models including the non-equilibrium condensation of enstatite, forsterite, quartz, solid iron, and solid MgO. The variation of the composition of the non-equilibrium mineral mixture with varying Mg/Si abundance ratio is studied. The mineral mixture found to be formed under non-equilibrium conditions is somewhat different from the chemical equilibrium composition, but follows the general trends found in equilibrium calculations. Particular emphasis is laid on stars with a low Mg/Si abundance ratio ( $\la$ 1) where quartz grains are shown to form a significant dust component which should also be detectable by the absorption bands of quartz in the IR.

Key words: circumstellar matter -- dust -- stars: mass-loss -- stars: winds -- stars: AGB and post-AGB

Offprint request: H.-P. Gail, gail@ita.uni-heidelberg.de

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.