Silvia Rossi

A stellar relic from the early Milky Way

The chemical composition of the most metal-deficient stars largely reflects the composition of the gas from which they formed. These old stars provide crucial clues to the star formation history and the synthesis of chemical elements in the early Universe. They are the local relics of epochs otherwise observable only at very high redshifts; if totally metal-free (‘population III’) stars could be found, this would allow the direct study of the pristine gas from the Big Bang. Earlier searches for such stars found none with an iron abundance less than 1/10,000 that of the Sun, leading to the suggestion that low-mass stars could form from clouds above a critical iron abundance. Here we report the discovery of a low-mass star with an iron abundance as low as 1/200,000 of the solar value. This discovery suggests that population III stars could still exist—that is, that the first generation of stars also contained long-lived low-mass objects. The previous failure to find them may be an observational selection effect.

The frequency of Carbon-enhanced stars in HERES and SDSS

Recent large surveys of metal-poor stars in the Galaxy have r e aled that a surprising fraction are enhanced in their carbon-to-iron ratios by factors of fr om 10-10,000 relative to the solar ratio. Although most of the stars in the metallicity interval −2.7 ≤ [Fe/H] ≤ −2.0 are likely to have arisen from Asymptotic Giant Branch processing (and subseq uent dumping via mass transfer to a surviving companion), there exist many stars with [Fe/H] < −3.0 (including the two lowest [Fe/H] stars known, with [Fe/H] < −5.0) that cannot be accounted for by this process. Rather, primordial (or nearly primordial) progenitors are implica ted.

CNO Abundances in Metal-Poor Stars

Carbon, nitrogen, and oxygen produced in the early universe come from a variety of possible astrophysical sites. Among these are early supernovae, winds of massive, rapidly-rotating, mega metal-poor stars, and intermediate mass AGB stars. Large-scale surveys such as the HK Survey of Beers and colleagues and the Hamburg/ESO Survey of Christlieb and colleagues have allowed for the identification of numerous metal-poor stars in the Galactic halo. Follow-up observations of these metal-poor stars is necessary to determine CNO abundances. Techniques have been developed such that [C/Fe], [N/Fe], and [O/Fe] can be estimated with considerable accuracy using medium-resolution observations alone. We present estimates of these species for a number of metal-poor stars based on analysis of near-UV, optical, and near-IR spectra. The data come from several different instruments on southern-hemisphere telescopes, including the Goodman HTS and OSIRIS on SOAR, GMOS-S on Gemini, and XSHOOTER on VLT. In this way, we present some of the first metal-poor stars with a full complement of CNO abundances based solely on the analysis of medium-resolution spectra.

A spectroscopic survey of FHB stars near the south galactic pole

We present the results of a medium-resolution spectroscopic survey of 43 field horizontal-branch (FHB) candidates carried out near the south galactic pole, selected from the original list of FHB candidates compiled by Beers et al . (2007). The observation list includes only stars classified as “high-probability” candidates, according to their 2MASS infrared colours. Atmospheric parameters of some stars have been obtained by comparing some spectral features with theoretical models provided by Kurucz (1993). A comparison between the grid of model atmospheres with some parameters of the Hδ line allowed the determination of log g, whilst [Fe/H] was estimated by the equivalent widths of the MgII 4481 and Ca ii lines. About 77% of the sample have been classified as FHB stars, 10% as subdwarfs, whilst the remaining 13% are probably main-sequence A-type stars far from the Galactic plane.