We discuss the importance of very metal-poor stars to develop an understanding of the nature of the first stars that formed in the Universe and the nucleosynthesis events associated with them, as well as to refine models of galaxy formation, in particular for large spiral galaxies such as the Milky Way. After briefly reviewing the history of the search for very metal-deficient stars in the Galaxy, we summarize ongoing efforts, concentrating on the two large objective-prism surveys that have led to the discovery of the majority of stars with [Fe/H] < −2.0 known at present: the HK survey of Beers and collaborators and the Hamburg/ESO survey of Christlieb and collaborators. We then consider the wealth of information that can be gleaned from high-resolution spectroscopic study of very metal-poor stars. We close with a list of open questions and a discussion of new survey techniques that will expand the sample of recognized very metal-deficient stars in the Galaxy by several orders of magnitude. 0066-4146/05/0922-0531$20.00 DISCOVERY AND ANALYSIS OF METAL-POOR STARS 533In this review we discuss these and other applications of analyses based on observations of very metal-poor stars. We do not discuss metal-poor postasymptotic giant branch (post-AGB) stars, because it is believed that their low atmospheric metallicities do not reflect the composition of the gas clouds from which these stars formed (see, e.

The chemical compositions of 26 metal-poor stars that exhibit strong CH and/or C 2 molecular bands are determined based on high-resolution spectroscopy. We define carbon-enhanced stars taking account of the carbon abundance ratio ([C/Fe]) and the evolutionary status, which is a slight modification over previous definitions. Twenty two stars in our sample satisfy our modified definition for Carbon-Enhanced Metal-Poor (CEMP) stars. In addition, we measure Na abundances for nine other carbon-enhanced stars for which abundances of other elements have been previously reported. Combining our new sample with the results of previous work, we investigate the abundance and evolutionary status of a total of 64 CEMP stars. The following results are obtained:(1) All but one of the 37 stars with [Fe/H] ≥ −2.6 exhibit large excesses of barium ([Ba/Fe] > +0.5), while the other 27 stars with lower metallicity exhibit a large scatter in their barium abundance ratios (−1.2 < [Ba/Fe]<+3.3). (2) A correlation between the carbon and barium abundance ratios ([C/Fe] and [Ba/Fe]) is found in Ba-enhanced objects (comprising 54 stars), suggesting that the origin of the observed carbon excess in Ba-enhanced stars is nucleosynthesis in asymptotic giant branch (AGB) stars, where the main s-process occurs. The correlation between the barium abundance ratio and that of carbon plus nitrogen ([(C+N)/Fe]) is relatively weak, because of the large excesses of nitrogen in some extremely metal-poor stars. (3) The majority of the Ba-enhanced stars have −1.0 < [C/H] < 0.0, and a clear cutoff exists at [C/H] ∼ 0, which we take as the limit of carbon-enrichment by metal-poor AGB stars. Within the above range,

Abstract. We present the results of analysis of "snapshot" spectra of 253 metal-poor halo stars −3.8 ≤ [Fe/H] ≤ −1.5 obtained in the HERES survey. The snapshot spectra have been obtained with VLT/UVES and have typically S /N ∼ 54 per pixel (ranging from 17 to 308), R ∼ 20 000, λ = 3760-4980 Å. This sample represents the major part of the complete HERES sample of 373 stars; however, the CH strong content of the sample is not dealt with here. The spectra are analysed using an automated line profile analysis method based on the Spectroscopy Made Easy (SME) codes of Valenti & Piskunov. Elemental abundances of moderate precision (absolute rms errors of order 0.25 dex, relative rms errors of order 0.15 dex) have been obtained for 22 elements, C, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, and Eu, where detectable. Of these elements, 14 are usually detectable at the 3σ confidence level for our typical spectra. The remainder can be detected in the least metal-poor stars of the sample, spectra with higher than average S /N, or when the abundance is enhanced. Among the sample of 253 stars, disregarding four previously known comparison stars, we find 8 r-II stars and 35 r-I stars. The r-II stars, including the two previously known examples CS 22892-052 and CS 31082-001, are centred on a metallicity of [Fe/H] = −2.81, with a very small scatter, on the order of 0.16 dex. The r-I stars are found across practically the entire metallicity range of our sample. We also find three stars with strong enhancements of Eu which are s-process rich. A significant number of new very metal-poor stars are confirmed: 49 stars with [Fe/H] < −3 and 181 stars with −3 < [Fe/H] < −2. We find one star with [Fe/H] < −3.5. We find the scatter in the abundance ratios of Mg, Ca, Sc, Ti, Cr, Fe, Co, and Ni, with respect to Fe and Mg, to be similar to the estimated relative errors and thus the cosmic scatter to be small, perhaps even non-existent. The elements C, Sr, Y, Ba and Eu, and perhaps Zr, show scatter at [Fe/H] < ∼ −2.5 significantly larger than can be explained from the errors in the analysis, implying scatter which is cosmic in origin. Significant scatter is observed in abundance ratios between light and heavy neutron-capture elements at low metallicity and low levels of r-process enrichment.

Original article can be found at: http://www.nature.com/nature/index.html--Copyright Nature Publishing Group --DOI : 10.1038/nature0345

The chemical composition of the most metal-deficient stars 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 1,2 ; 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 3,4 , leading to the suggestion 5,6 that low-mass stars could only 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 star HE 0107−5240, at coordinates right ascension R.A.(2000.0) = 01 h 09 m 29.1 s and declination δ = −52 • 24 34 , is a giant star of the Galactic halo population with apparent magnitude B = 15.86. It was found during medium-resolution spectroscopic follow-up observations of candidate metal-poor stars selected from the Hamburg/ESO objective prism survey (HES) 7,8 . This survey, which covers the entire southern high-galactic-latitude sky to an apparent magnitude limit of B ≈ 17.5, extends the total survey volume for metal-poor stars in the Galaxy by almost one order of magnitude compared to the total volume explored by previous spectroscopic surveys.A medium-resolution (δλ ≈ 0.2 nm) spectrum of HE 0107−5240 was obtained by M. Spring Observatory 2.3-m telescope on 12 November 2001. The Ca II K (λ = 393.4 nm) line was barely visible in that spectrum, indicating that the star was likely to be extremely metal deficient. Shortly thereafter, a high-resolution, high signal-to-noise ratio spectrum was obtained with the 8-m Unit Telescope 2 (UT2) of the Very Large Telescope at the European Southern Observatory (ESO), Paranal, Chile. Figure 1: A portion of the spectrum of HE 0107−5240, shown compared to the spectrum of CD −38 • 245, the previously most iron-poor giant star known. Both spectra were obtained with VLT-UT2, and the UltravioletVisual Echelle Spectrograph (UVES). We note the strong molecular CH and C 2 lines and extremely weak lines of Fe I in the spectrum of HE 0107−5240. The spectra used in our analysis have a resolution of R = λ/∆λ = 40, 000, and a signal-to-noise ratio of more than 100 per pixel at λ > 400.0 nm. The covered wavelength ranges are 329.0-452.0 nm, 478.0-576.0 nm, and 583.0-681.0 nm.We derive an effective temperature T eff = 5100 ± 150 K for HE 0107−5240 by means of broad-band visual and infrared photometry. The absence of Fe II lines, through the Fe I/Fe II ionisation equilibrium, constrains the star to have a logarithmic surface gravity of log(g) > 2.0 dex, while main-seq...