4,167 research outputs found
The evolutionary time scale of Sakurai's object: A test of convection theory?
Sakurai's object (V4334 Sgr) is a born again AGB star following a very late
thermal pulse. So far no stellar evolution models have been able to explain the
extremely fast evolution of this star, which has taken it from the pre-white
dwarf stage to its current appearance as a giant within only a few years. A
very high stellar mass can be ruled out as the cause of the fast evolution.
Instead the evolution time scale is reproduced in stellar models by making the
assumption that the efficiency for element mixing in the He-flash convection
zone during the very late thermal pulse is smaller than predicted by the
mixing-length theory. As a result the main energy generation from fast proton
capture occurs closer to the surface and the expansion to the giant state is
accelerated to a few years. Assuming a mass of V4334 Sgr of 0.604Msun -- which
is consistent with a distance of 4kpc -- a reduction of the mixing length
theory mixing efficiency by a factor of ~ 100 is required to match its
evolutionary time scale. This value decreases if V4334 Sgr has a smaller mass
and accordingly a smaller distance. However, the effect does not disappear for
the smallest possible masses. These findings may present a semi-empirical
constraint on the element mixing in convective zones of the stellar interior.Comment: 16 pages, 3 figures, ApJ Letter, in press; some additional
information as well as modifications as a result of the refereeing process,
improved layout of prev. Fig.1 (now Fig.1 and Fig.2
Life Products of Stars
We attempt to document complete energetic transactions of stars in their
life. We calculate photon and neutrino energies that are produced from stars in
their each phase of evolution from 1 to 8 M_sun, using the state-of-the-art
stellar evolution code, tracing the evolution continuously from pre-main
sequence gravitational contraction to white dwarfs. We also catalogue
gravitational and thermal energies and helium, and heavier elements that are
stored in stars and those ejected into interstellar space in each evolutionary
phase.Comment: 26 pages, including 8 figures and 3 tables. Submitted to ApJ
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