We will study the formation history of the Milky Way, and the earliest phases of its chemical enrichment, with a sample of more than 1.5 million stars at high galactic latitude. Elemental abundances of up to 20 elements with a precision of better tha
n 0.2 dex will be derived for these stars. The sample will include members of kinematically coherent substructures, which we will associate with their possible birthplaces by means of their abundance signatures and kinematics, allowing us to test models of galaxy formation. Our target catalogue is also expected to contain 30,000 stars at a metallicity of less than one hundredth that of the Sun. This sample will therefore be almost a factor of 100 larger than currently existing samples of metal-poor stars for which precise elemental abundances are available (determined from high-resolution spectroscopy), enabling us to study the early chemical evolution of the Milky Way in unprecedented detail.
We present an elemental abundance analysis for four newly discovered ultra metal-poor stars from the Hamburg/ESO survey, with $mathrm{[Fe/H]}leq-4$. Based on high-resolution, high signal-to-noise spectra, we derive abundances for 17 elements in the r
ange from Li to Ba. Three of the four stars exhibit moderate to large over-abundances of carbon, but have no enhancements in their neutron-capture elements. The most metal-poor star in the sample, HE~0233$-$0343 ($mathrm{[Fe/H]} = -4.68$), is a subgiant with a carbon enhancement of $mathrm{[C/Fe]}= +3.5$, slightly above the carbon-enhancement plateau suggested by Spite et al. No carbon is detected in the spectrum of the fourth star, but the quality of its spectrum only allows for the determination of an upper limit on the carbon abundance ratio of $mathrm{[C/Fe]} < +1.7$. We detect lithium in the spectra of two of the carbon-enhanced stars, including HE~0233$-$0343. Both stars with Li detections are Li-depleted, with respect to the Li plateau for metal-poor dwarfs found by Spite & Spite. This suggests that whatever site(s) produced C either do not completely destroy lithium, or that Li has been astrated by early-generation stars and mixed with primordial Li in the gas that formed the stars observed at present. The derived abundances for the $alpha$-elements and iron-peak elements of the four stars are similar to those found in previous large samples of extremely and ultra metal-poor stars. Finally, a large spread is found in the abundances of Sr and Ba for these stars, possibly influenced by enrichment from fast rotating stars in the early universe.
We determined the silicon abundances of 253 metal-poor stars in the metallicity range $-4<mathrm{[Fe/H]} <-1.5$, based on non-local thermodynamic equilibrium (NLTE) line formation calculations of neutral silicon and high-resolution spectra obtained w
ith VLT-UT2/UVES. The $T_{mathrm{eff}}$ dependence of [Si/Fe] noticed in previous investigation is diminished in our abundance analysis due to the inclusion of NLTE effects. An increasing slope of [Si/Fe] towards decreasing metallicity is present in our results, in agreement with Galactic chemical evolution models. The small intrinsic scatter of [Si/Fe] in our sample may imply that these stars formed in a region where the yields of type II supernovae were mixed into a large volume, or that the formation of these stars was strongly clustered, even if the ISM was enriched by single SNa II in a small mixing volume. We identified two dwarfs with $mathrm{[Si/Fe]}sim +1.0$: HE 0131$-$3953, and HE 1430$-$1123. These main-sequence turnoff stars are also carbon-enhanced. They might have been pre-enriched by sub-luminous supernovae.
We report on a detailed abundance analysis of two strongly r-process enhanced, very metal-poor stars newly discovered in the HERES project, CS 29491-069 ([Fe/H]=-2.51, [r/Fe]=+1.1) and HE 1219-0312 ([Fe/H]=-2.96, [r/Fe]=+1.5). The analysis is based o
n high-quality VLT/UVES spectra and MARCS model atmospheres. We detect lines of 15 heavy elements in the spectrum of CS 29491-069, and 18 in HE 1219-0312; in both cases including the Th II 4019 {AA} line. The heavy-element abundance patterns of these two stars are mostly well-matched to scaled solar residual abundances not formed by the s-process. We also compare the observed pattern with recent high-entropy wind (HEW) calculations, which assume core-collapse supernovae of massive stars as the astrophysical environment for the r-process, and find good agreement for most lanthanides. The abundance ratios of the lighter elements strontium, yttrium, and zirconium, which are presumably not formed by the main r-process, are reproduced well by the model. Radioactive dating for CS 29491-069 with the observed thorium and rare-earth element abundance pairs results in an average age of 9.5 Gyr, when based on solar r-process residuals, and 17.6 Gyr, when using HEW model predictions. Chronometry seems to fail in the case of HE 1219-0312, resulting in a negative age due to its high thorium abundance. HE 1219-0312 could therefore exhibit an overabundance of the heaviest elements, which is sometimes called an actinide boost.
We determine the metallicity distribution function (MDF) of the Galactic halo by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO objective-prism survey (HES). The sample was corrected for minor biases introduced by the strate
gy for spectroscopic follow-up observations of the metal-poor candidates, namely best and brightest stars first. [...] We determined the selection function of the HES, which must be taken into account for a proper comparison between the HES MDF with MDFs of other stellar populations or those predicted by models of Galactic chemical evolution. The latter show a reasonable agreement with the overall shape of the HES MDF for [Fe/H] > -3.6, but only a model of Salvadori et al. (2007) with a critical metallicity for low-mass star formation of Z_cr = 10^{-3.4} * Z_Sun reproduces the sharp drop at [Fe/H] ~-3.6 present in the HES MDF. [...] A comparison of the MDF of Galactic globular clusters and of dSph satellites to the Galaxy shows qualitative agreement with the halo MDF, derived from the HES, once the selection function of the latter is included. However, statistical tests show that the differences between these are still highly significant. [ABSTRACT ABRIDGED]
We present the quantitative methods used for selecting candidate metal-poor stars in the Hamburg/ESO objective-prism survey (HES). The selection is based on the strength of the Ca II K line, B-V colors (both measured directly from the digital HES spe
ctra), as well as J-K colors from the 2 Micron All Sky Survey. The KP index for Ca II K can be measured from the HES spectra with an accuracy of 1.0 Angstrom, and a calibration of the HES B-V colors, using CCD photometry, yields a 1-sigma uncertainty of 0.07 mag for stars in the color range 0.3 < B-V < 1.4. These accuracies make it possible to reliably reject stars with [Fe/H] > -2.0 without sacrificing completeness at the lowest metallicities. A test of the selection using 1121 stars of the HK survey of Beers, Preston, and Shectman present on HES plates suggests that the completeness at [Fe/H] < -3.5 is close to 100% and that, at the same time, the contamination of the candidate sample with false positives is low: 50% of all stars with [Fe/H] > -2.5 and 97% of all stars with [Fe/H] > -2.0 are rejected. The selection was applied to 379 HES fields, covering a nominal area of 8853 square degrees of the southern high Galactic latitude sky. The candidate sample consists of 20,271 stars in the magnitude range 10 < B < 18. A comparison of the magnitude distribution with that of the HK survey shows that the magnitude limit of the HES sample is about 2 mag fainter. Taking the overlap of the sky areas covered by both surveys into account, it follows that the survey volume for metal-poor stars has been increased by the HES by about a factor of 10 with respect to the HK survey. We have already identified several very rare objects with the HES, including, e.g., the three most heavy-element deficient stars currently known.
We describe the discovery of HE 1327-2326, a dwarf or subgiant with [Fe/H]}=-5.4. The star was found in a sample of bright metal-poor stars selected from the Hamburg/ESO survey. Its abundance pattern is characterized by very high C and N abundances.
The detection of Sr which is overabundant by a factor of 10 as compared to iron and the Sun, suggests that neutron-capture elements had already been produced in the very early Galaxy. A puzzling Li depletion is observed in this unevolved star which contradicts the value of the primordial Li derived from WMAP and other Li studies. Possible scenarios for the origin of the abundance pattern (Pop. II or Pop. III) are presented as well as an outlook on future observations.
We present the elemental abundances of HE1327-2326, the most iron-deficient star known, determined from a comprehensive analysis of spectra obtained with the Subaru Telescope High Dispersion Spectrograph.
We have determined the oxygen abundance of HE0107-5240 from UV-OH lines detected in VLT/UVES spectra. Using a plane-parallel LTE model atmosphere, we derive [O/Fe] = +2.4, and a similar analysis of CD -38 245 yields [O/Fe] = +1.0. We estimate systema
tic errors due to 3D effects to be in the order of 0.3 to 0.4 dex. That is, our derived O abundances are likely overestimates: effects from thermal inhomogeneities due to convection may require that the abundances should be reduced by 0.3-0.4 dex or even more. Radial velocity data for HE0107-5240 based on high-resolution spectra show that over a time span of 373 days the radial velocity was constant at 44.5 km/s, with a 1 sigma scatter of the measurements of 0.5 km/s. However, it can not yet be ruled out that HE0107-5240 is a very long period and/or low amplitude binary. These results provide new constraints on scenarios for the origin of the abundance pattern of HE0107-5240. In particular, it seems unlikely that the large overabundances of CNO have been produced in a medium-mass AGB star which later evolved to a white dwarf. The oxygen abundance of HE0107-5240 is significantly smaller than the prediction of Umeda & Nomoto (2003) from calculated yields of a ~25 solar mass Population III star exploding as a supernova of low explosion energy (E_exp = 3 x 10^50 erg) with mixing and fallback. The scenario of Limongi et al. (2003), involving two Population III supernovae, predicts an oxygen abundance of [O/Fe] = +4.1 for HE0107-5240, in strong contradiction with the observed value. In conclusion, none of the above mentioned scenarios, in their present realizations, can satisfactorly explain the abundance pattern of HE0107-5240.
We report a detailed abundance analysis for HE0107-5240, a halo giant with [Fe/H]_NLTE=-5.3. This star was discovered in the course of follow-up medium-resolution spectroscopy of extremely metal-poor candidates selected from the digitized Hamburg/ESO
objective-prism survey. On the basis of high-resolution VLT/UVES spectra, we derive abundances for 8 elements (C, N, Na, Mg, Ca, Ti, Fe, and Ni), and upper limits for another 12 elements. A plane-parallel LTE model atmosphere has been specifically tailored for the chemical composition of {he}. Scenarios for the origin of the abundance pattern observed in the star are discussed. We argue that HE0107-5240 is most likely not a post-AGB star, and that the extremely low abundances of the iron-peak, and other elements, are not due to selective dust depletion. The abundance pattern of HE0107-5240 can be explained by pre-enrichment from a zero-metallicity type-II supernova of 20-25M_Sun, plus either self-enrichment with C and N, or production of these elements in the AGB phase of a formerly more massive companion, which is now a white dwarf. However, significant radial velocity variations have not been detected within the 52 days covered by our moderate-and high-resolution spectra. Alternatively, the abundance pattern can be explained by enrichment of the gas cloud from which HE0107-5240 formed by a 25M_Sun first-generation star exploding as a subluminous SNII, as proposed by Umeda & Nomoto (2003). We discuss consequences of the existence of HE0107-5240 for low-mass star formation in extremely metal-poor environments, and for currently ongoing and future searches for the most metal-poor stars in the Galaxy.
