6 research outputs found
Evolution of low-mass metal-free stars including effects of diffusion and external pollution
We investigate the evolution of low-mass metal-free Population III stars.
Emphasis is laid upon the question of internal and external sources for
CNO-elements, which - if present in sufficient amounts in the hydrogen-burning
regions - lead to a strong modification of the stars' evolutionary behavior.
For the production of carbon due to nuclear processes inside the stars, we use
an extended nuclear network, demonstrating that hot pp-chains do not suffice to
produce enough carbon or are less effective than the triple3-alpha-process. As
an external source of CNO-elements we test the efficiency of pollution by a
nearby massive star combined with particle diffusion. For all cases
investigated, the additional metals fail to reach nuclear burning regions
before deep convection on the Red Giant Branch obliterates the previous
evolution. The surface abundance history of the polluted Pop III stars is
presented. The possibilities to discriminate between a Pop II and a polluted
Pop III field star are also discussed.Comment: Accepted for publication in Ap
Evolution and Nucleosynthesis of Zero Metal Intermediate Mass Stars
New stellar models with mass ranging between 4 and 8 Mo, Z=0 and Y=0.23 are
presented. The models have been evolved from the pre Main Sequence up to the
Asymptotic Giant Branch (AGB). At variance with previous claims, we find that
these updated stellar models do experience thermal pulses in the AGB phase. In
particular we show that: a) in models with mass larger than 6 Mo, the second
dredge up is able to raise the CNO abundance in the envelope enough to allow a
"normal" AGB evolution, in the sense that the thermal pulses and the third
dredge up settle on; b) in models of lower mass, the efficiency of the CNO
cycle in the H-burning shell is controlled by the carbon produced locally via
the 3alpha reactions. Nevertheless the He-burning shell becomes thermally
unstable after the early AGB. The expansion of the overlying layers induced by
these weak He-shell flashes is not sufficient by itself to allow a deep
penetration of the convective envelope. However, immediately after that, the
maximum luminosity of the He flash is attained and a convective shell
systematically forms at the base of the H-rich envelope. The innermost part of
this convective shell probably overlaps the underlying C-rich region left by
the inter-shell convection during the thermal pulse, so that fresh carbon is
dredged up in a "hot" H-rich environment and a H flash occurs. This flash
favours the expansion of the outermost layers already started by the weak
thermal pulse and a deeper penetration of the convective envelope takes place.
Then, the carbon abundance in the envelope rises to a level high enough that
the further evolution of these models closely resembles that of more metal rich
AGB stars. These stars provide an important source of primary carbon and
nitrogen.Comment: 28 pages, 5 tables and 17 figures. Accepted for publication in Ap