First stars IV. CS 29497-030: Evidence for operation of the s-process at very low metallicity

We present an abundance analysis of the very metal-poor, carbon-enhanced star CS 29497 - 030. Our results indicate that this unusually hot turnoff star (T-eff = 6650 K, log g = 3.5) has a metallicity [Fe/H] = - 2.8, and exhibits large overabundances of carbon ([C/Fe] = + 2.38), nitrogen ([N/Fe] = + 1.88), and oxygen ([O/Fe] = + 1.67). This star also exhibits a large enhancement in its neutron-capture elements; the pattern follows that expected to arise from the s-process. In particular, the Pb abundance is found to be very high with respect to iron ([Pb/Fe] = + 3.5), and also with respect to the second peak s-process elements (e.g., Ba, La, Ce, Nd), which fits into the newly introduced classification of lead (Pb) stars. The known spectroscopic binary status of this star, along with the observed s-process abundance pattern, suggest that it has accreted matter from a companion, which formerly was an Asymptotic Giant-Branch (AGB) star. In a preliminary analysis, we have also identified broad absorption lines of metallic species that suggest a large axial rotational velocity for this star, which may be the result of spin-up associated with the accretion of material from its previous AGB companion. In addition, this star is clearly depleted in the light element Li. When considered along with its rather high inferred temperature, these observations are consistent with the expected properties of a very low metallicity halo blue straggler.