New lessons from old stars
Extremely metal-poor – EMP – stars ([Fe/H] ≤ -2.5) are very old and have convective envelopes; mapping their detailed abundance pattern requires high-quality spectra and a homogeneous analysis. Chemically peculiar EMP stars are a nuisance for two reasons: (i): Their atmospheric structure and spectra can be very hard to analyse with standard methods, and (ii) their surface layers may not reflect their interior composition. Therefore, they have generally been ignored as products of some form of exotic binary evolution. r-process enhanced EMP stars are rare enough (~3%) to just ignore in a first pass, but carbon-enhanced – CEMP – stars are common, especially at very low metallicity, and their often huge overabundance and prominent molecular bands of such a common element as C demand attention. Accurate long-term radial-velocity monitoring and VLT spectra of such EMP stars have now proved that their chemical peculiarities were in fact intrinsic to their natal clouds, and that non-standard, non-local enrichment processes were active in the early Galaxy.