Issue |
A&A
Volume 393, Number 1, October I 2002
|
|
---|---|---|
Page(s) | 215 - 223 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20021001 | |
Published online | 18 September 2002 |
Yields from low metallicity, intermediate mass AGB stars:
Their role for the CNO and lithium abundances in Globular Cluster stars
1
Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio C. (RM), Italy
2
Istituto di Astrofisica Spaziale C.N.R., Via Fosso del Cavaliere, 00133 Rome, Italy
Corresponding author: P. Ventura, paolo@coma.mporzio.astro.it
Received:
7
May
2002
Accepted:
5
July
2002
We present the results of extensive computation of the Thermal Pulse phase
AGB evolution of stars of metallicities in mass fraction , for those masses in the range
, which suffer the Hot Bottom Burning (HBB) phase. The evolution is fully
computed, by assuming a mass loss rate consistent with the observations of
the Magellanic Clouds lithium–rich stars, and modelling convection with the
Full Spectrum of Turbulence model by Canuto and Mazzitelli.
The results are discussed in the framework of their importance for the
evolution of proto–Globular Clusters, whose spectra show that the stars are
very probably formed from matter contaminated by the ejecta of these stars,
or have accreted it after formation.
The main results we find are the following: 1) for metallicities
, masses above
~4
suffer complete CNO cycling in HBB,
so that they show at the surface the result of this process, and the oxygen
abundance is reduced; 2) most models suffer the third dredge up. Although
carbon is processed to nitrogen by HBB, the oxygen burning is so strong in
the lowest metallicities (
) that carbon becomes more
abundant than oxygen: in other words, low-metallicity intermediate mass
stars may show up as carbon stars due to the drastic oxygen burning;
3) if Globular Cluster stars are contaminated by matter processed through
these phases, we must expect a non negligible helium enhancement in
their composition: from a Big Bang abundance
, e.g., we might expect
an abundance
. This may have no practical consequences if pollution
concerns only the external parts of the stars, but is very important
if the stars formed as a whole from a helium rich environment. 4) The
lithium yields, although not important for galactic chemical evolution, are
very interestingly close to the initial Big Bang abundance: processing by HBB
is the only way in which we can obtain substantial amounts of gas which have
gone through full CNO burning, but preserve a reasonable abundance of
lithium.
Key words: stars: AGB and post-AGB / ISM: abundances / globular clusters: general
© ESO, 2002
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