Norbert Christlieb

Carbon-enhanced Metal-poor Stars. I. Chemical Compositions of 26 Stars*

The chemical compositions of 26 metal-poor stars that exhibit strong CH and/or C2 molecular bands are determined. Twenty-two stars in our sample satisfy our definition for carbon-enhanced metal-poor (CEMP) stars based on the carbon abundance ratio ([C/Fe]) and the evolutionary status. In addition, we measure Na abundances for nine known carbon-enhanced stars. 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 AGB stars, where the main s-process occurs. (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. The [C/H] values of Ba-normal stars are relatively low, with a wide distribution. (4) The difference in the distributions of evolutionary status between Ba-enhanced and Ba-normal CEMP stars suggested by our previous work is not statistically confirmed by the present, enlarged sample. (5) Excesses of Na are found in stars with extremely large enhancements of C, N, and Ba, suggesting efficient production of this element by AGB nucleosynthesis. The implications of these results on the origins of carbon in CEMP stars are discussed.

HE 0557-4840: Ultra-Metal-Poor and Carbon-Rich

We report the discovery and high-resolution, high-S/N spectroscopic analysis of the ultra-metal-poor red giant HE 0557-4840, which is the third most heavy-element-deficient star currently known. It ...

HE 1327?2326, an Unevolved Star with [Fe/H] < ?5.0. I. A Comprehensive Abundance Analysis

HE 1327-2326, an Unevolved Star with [Fe/H] < -5.0. : I. A Comprehensive Abundance Analysis

Abundances In Very Metal-Poor Dwarf Stars*

We discuss the detailed composition of 28 extremely metal-poor (EMP) dwarfs, 22 of which are from the Hamburg/ESO Survey (HES), based on Keck echelle spectra. Our sample has a median [Fe/H] of -2.7 dex, extends to -3.5 dex, and is somewhat less metal-poor than was expected from [Fe/H](HK, HES) determined from low-resolution spectra. Our analysis supports the existence of a sharp decline in the distribution of halo stars with metallicity below [Fe/H] = -3.0 dex. So far no additional turnoff stars with [Fe/H] < -3.5 have been identified in our follow-up efforts. For the best-observed elements between Mg and Ni, we find that the abundance ratios appear to have reached a plateau, i.e., [X/Fe] is approximately constant as a function of [Fe/H], except for Cr, Mn, and Co, which show trends of abundance ratios varying with [Fe/H]. These abundance ratios at low metallicity correspond approximately to the yield expected from Type II supernovae (SNe) with a narrow range in mass and explosion parameters; high-mass Type II SN progenitors are required. The dispersion of [X/Fe] about this plateau level is surprisingly small and is still dominated by measurement errors rather than intrinsic scatter. These results place strong constraints on the characteristics of the contributing SNe. The dispersion in neutron-capture elements and the abundance trends for Cr, Mn, and Co are consistent with previous studies of evolved EMP stars. We find halo-like enhancements for the α-elements Mg, Ca, and Ti, but solar Si/Fe ratios for these dwarfs. This contrasts with studies of EMP giant stars, which show Si enhancements similar to other α-elements. Sc/Fe is another case where the results from EMP dwarfs and from EMP giants disagree; our Sc/Fe ratios are enhanced compared to the solar value by ~0.2 dex. Although this conflicts with the solar Sc/Fe values seen in EMP giants, we note that α-like Sc/Fe ratios have been claimed for dwarfs at higher metallicity. Two dwarfs in the sample are carbon stars, while two others have significant C enhancements, all with 12C/13C ~7 and with C/N between 10 and 150. Three of these C-rich stars have large enhancements of the heavy neutron capture elements, including lead, which implies a strong s-process contribution, presumably from binary mass transfer; the fourth shows no excess of Sr or Ba.

The Frequency of Carbon-enhanced Metal-poor Stars in the Galaxy from the HERES Sample

We estimate the frequency of carbon-enhanced metal-poor (CEMP) stars among very metal-poor stars, based on an analysis of 349 stars with available high-resolution spectra observed as part of the Hamburg/ESO R-process Enhanced Star (HERES) survey. We obtain that a lower limit of 21% ± 2% of stars with [Fe/H] ≤ -2.0 exhibit [C/Fe] ≥ +1.0. These fractions are higher than those that have been reported by recent examinations of this question, based on substantially smaller samples of stars. We discuss the source of this difference and suggest that in order to take into account effects that result in a decrease of surface carbon abundance with advancing evolution, a definition of CEMP stars based on a [C/Fe] cutoff that varies as a function of luminosity is more appropriate. We discuss the likely occurrence of dilution and mixing for many CEMP stars, which, if properly accounted for, would increase this fraction still further.

Discovery of HE 1523-0901: A Strongly r-Process Enhanced Metal-Poor Star with Detected Uranium

We present age estimates for the newly discovered, very r-process-enhanced metal-poor star HE 1523-0901 ([Fe/H] = -2.95) based on the radioactive decay of Th and U. The bright (V = 11.1) giant was found among a sample of bright metal-poor stars selected from the Hamburg/ESO Survey. From an abundance analysis of a high-resolution (R = 75,000) VLT/UVES spectrum, we find HE 1523-0901 to be strongly overabundant in r-process elements ([r/Fe] = 1.8). The abundances of heavy neutron-capture elements (Z > 56) measured in HE 1523-0901 match the scaled solar r-process pattern extremely well. We detect the strongest optical U line at 3859.57 A. For the first time, we are able to employ several different chronometers, such as the U/Th, U/Ir, Th/Eu, and Th/Os ratios to measure the age of a star. The weighted average age of HE 1523-0901 is 13.2 Gyr. Several sources of uncertainties are assessed in detail.

Bright Metal-poor Stars from the Hamburg/ESO Survey. I. Selection and Follow-up Observations from 329 Fields

We present a sample of 1777 bright (9 1.0) metal-poor ([Fe/H] 20%) and higher values with increasing distance from the Galactic plane. Although the numbers of stars at low metallicity are falling rapidly at the lowest metallicities, there is evidence that the fraction of carbon-enhanced metal-poor stars is increasing rapidly as a function of declining metallicity. For ~60 objects, high-resolution data have already been obtained; one of these, HE 1327-2326, is the new record holder for the most iron-deficient star known.

High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia progenitor survey (SPY). I

We have started a large survey for radial velocity variations in white dwarfs (PI R. Napiwotzki) with the aim of nding close double degenerates, which could be precursor systems for SNe Ia. The UVES spectrograph at the ESO VLT is used to obtain high resolution spectra with good S=N. During this project 1500 white dwarfs will be observed. This unique data set will also allow to derive atmospheric parameters and masses for the largest sample of white dwarfs ever analyzed in a homogenous way. In this paper we present a catalog of objects and report results for the rst sample of about 200 white dwarfs, many of which are spectroscopic conrmations of candidates from the HE, MCT, and EC surveys. Among the peculiar spectra we identify two new magnetic DA, one previously known magnetic DA, several DA with emission cores, in some cases due to a late-type companion, and two new DBA.

HE 1327-2326, An Unevolved Star With Fe/H < -5.0. II. New 3D-1D Corrected Abundances From A Very Large Telescope UVES Spectrum

We present a new abundance analysis of HE 1327?2326, which is currently the most iron-poor star, based on observational data obtained with the VLT Ultraviolet and Visual Echelle Spectrograph (UVES). We correct the one-dimensional (1D) LTE abundances for three-dimensional (3D) effects to provide an abundance pattern that supersedes previous works and should be used to observationally test current models of the chemical yields of the first-generation supernovae (SNe). Apart from confirming the 1D LTE abundances found in previous studies before accounting for 3D effects, we make use of a novel technique to apply the 3D?1D corrections for CNO which are a function of excitation potential and line strength for the molecular lines that comprise the observable CH, NH, and OH features. We find that the fit to the NH band at 3360 ? is greatly improved due to the application of the 3D?1D corrections. This may indicate that 3D effects are actually observable in this star. We also report the first detection of several weak Ni lines. The cosmologically important element Li is still not detected; the new Li upper limit is extremely low, -->A(Li) < 0.62, and in stark contrast with results not only from the Wilkinson Microwave Anisotropy Probe (WMAP) but also from other metal-poor stars. We also discuss how the new corrected abundance pattern of HE 1327?2326 is being reproduced by individual and integrated yields of SNe.

New Extremely Metal-Poor Stars in the Galactic Halo*

We present a detailed abundance analysis based on high-resolution and high signal-to-noise spectra of eight extremely metal-poor (EMP) stars with [ Fe/H ] ≾ − 3.5 dex, four of which are new. Only stars with 4900 K < T_(eff) < 5650 K are included. Two stars of the eight are outliers in each of several abundance ratios. The most metal-poor star in this sample, HE 1424–0241, has [ Fe/H ] ~ − 4 dex and is thus among the most metal-poor stars known in the Galaxy. It has highly anomalous abundance ratios unlike those of any other known EMP giant, with very low Si, Ca, and Ti relative to Fe, and enhanced Mn and Co, again relative to Fe. Only (low) upper limits for C and N can be derived from the nondetection of the CH and NH molecular bands. HE 0132-2429, another sample star, has excesses of N and Sc with respect to Fe. The strong outliers in abundance ratios among the Fe-peak elements in these C-normal stars, not found at somewhat higher metallicities ([ Fe/H ] ~ − 3 dex), are definitely real. They suggest that at such low metallicities we are beginning to see the anticipated and long sought stochastic effects of individual supernova events contributing to the Fe-peak material within a single star. With spectra reaching well into the near-UV we are able to probe the behavior of copper abundances in such extreme EMP stars. A detailed comparison of the results of the analysis procedures adopted by our 0Z project compared to those of the First Stars VLT Large Project finds a systematic difference for [ Fe/H ] of ~0.3 dex, our values always being higher.