Issue |
A&A
Volume 418, Number 1, April IV 2004
|
|
---|---|---|
Page(s) | 225 - 233 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20040023 | |
Published online | 02 April 2004 |
Blue loops of intermediate mass stars
II. Metallicity and blue loops
1
National Astronomical Observatories/Yunnan Observatory, PO Box 110, 650011 Kunming, PR China
2
Joint Laboratory for Optical Astronomy, Chinese Academy of Sciences, PR China
Corresponding author: H. Y. Xu, xuhuayin@sohu.com
Received:
12
May
2003
Accepted:
19
December
2003
Based on the results of the blue loop formation for
models of solar-like metallicity, we have explored the blue
loop evolution of metal-poor stars. Three series of models
with a wide range of metallicity and the initial helium abundance
were calculated. An important parameter, , defined as the
envelope convection mass divided by the total envelope mass at the
bottom of the RGB, was introduced as a criterion to determine the
formation of the blue loop. We have found that the low-Z models
will develop extensive blue loops when
is lower than a
critical value
. The physical explanation for this
result could be as follows. Lower Z reduces the envelope opacity
and leads to a hotter stellar envelope and a bluer RGB. Thus the
model will have a smaller
at the top and an even smaller
at the bottom of the RGB. When
is lower than
the critical value
, the envelope is
radiation-dominated. Under this condition and the constraint of
the virial theorem, the response of the star to the increase of
the stellar luminosity is to contract to increase the thermal
conductivity coefficient in the stellar envelope and to form a
blue loop. Compared with the high-Z models, we have confirmed that
the development of convection in the stellar envelope is a crucial
factor to determine the formation of the blue loop, but the low-Z
models reach low
values in a different way from the
high-Z models, in which the modulation of the nuclear reaction
rates by higher 14N abundance in the H-burning shell is
responsible for the stars to get small
values. It has
been found that
depends not only on the stellar
mass, but also on metallicity and the initial helium
abundance. Our numerical results show that
decreases with Z while slowly increases with Y.
Key words: stars: evolution / stars: interiors / stars: Hertzsprung-Russell (HR) and C-M diagrams
© ESO, 2004
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