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F. Herwig (LANL), J. Johnson (DAO), T.C. Beers (Michigan State), N. Christlieb (Hamburger Sternwarte, Germany)
Roughly 25% of all very metal-poor stars ([Fe/H] < -2) are strongly C-, and to a lesser extent, N-enhanced. In particular, the over-abundance of C and N, in addition to their s-process signatures, has led to the assumption that the AGB star progenitor of the current WD companion to the Carbon-Enhanced Metal-Poor (CEMP) star is responsible for the observed abundance pattern. Observations of the CH and CN bands in CEMP stars indicate that -0.5 < [C/N] < 1.5 at C-abundances around [C/Fe] ~2. Hence, these CEMP stars are simultaneously enriched by a primary nuclear production source of C and N, but C is produced somewhat more efficiently than N. This specific abundance pattern seems to defy the abundance predictions of AGB stellar evolution calculations.
In order to quantify the comparison with observations we have calculated a grid of intermediate-mass AGB stars of sufficiently low metallicity ([Fe/H] = -2.3; see Herwig 2004). These models show that, similar to the situation at higher metallicity, there exits a dichotomy of the C/N ratio in the ejecta as a function of initial mass. Large initial masses result in efficient hot-bottom burning (HBB), which turns most dredged-up primary carbon into nitrogen, resulting in [C/N] < -1 and [C/Fe] < 1.5, whereas the lower-mass cases cannot produce nitrogen via HBB. For [C/Fe] = 2 these lower-mass stars should have [C/N] = 1.5. These models predict that most of the [C/N] ratios of CEMP stars are too small to correspond to non-HBB AGB stars, and too large for HBB AGB stars.
We have carried out observations of extremely metal-poor (EMP) stars with 0.5 < [C/Fe] < 1, using the CH and the NH bands for abundance determination. Specifically, we wanted to find out whether the paucity of EMP stars with [C/N] < -1 is a selection effect or a systematic observational bias. We DID NOT find the EMP stars polluted by the N-producing HBB AGB stars. This leaves us with two important open questions: (1) Where are the EMP stars polluted by massive AGB stars ? and (2) Where does the N in the CEMP stars come from ? We offer some speculative answers to these questions.
The author(s) of this abstract have provided an email address for comments about the abstract: fherwig@lanl.gov
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.