Intense Mass Loss from C-rich AGB Stars at low Metallicity?
Apr, 2008Citations per year
Abstract: (arXiv)
In this letter we argue that the energy injection by the pulsations may be of greater importance for the mass-loss rate of AGB stars than the metallicity and that the trend with metallicity is not as simple as it is sometimes assumed to be. Using our detailed radiation hydrodynamical models including dust formation, we demonstrate the effects of pulsation energy on the wind properties. We find that the mass-loss rate scales with the kinetic energy input by pulsations as long as the dust-saturated wind regime is not reached and all other stellar parameters are kept constant -- including the absolute abundance of condensible carbon (not bound in CO), which is more relevant than keeping the -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 has to be compensated for by adjusting the velocity amplitude of the variable inner boundary (piston), which is used to simulate the effects of pulsation, in order to obtain models with comparable kinetic energy input. Hence, it is more relevant to compare models with similar energy-injections rather than the same velocity amplitude. Thus, as there is no evidence for weaker pulsations in low-metallicity AGB stars, we conclude that it is actually unlikely that low-metallicity C-stars have a lower mass-loss rate, than their more metal-rich counterparts with similar stellar parameters, as long as they have a comparable amount of condensible carbon.Note:
- 4 pages, 3 figures. Accepted for publication in A&A
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