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...

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 À2.7 dex, extends to À3.5 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 ½Fe=H ¼ À3:0 dex. So far no additional turnoff stars with ½Fe=H < À3:5 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 -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 -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 -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 12 C/ 13 C $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.

We report a detailed abundance analysis for HE 0107−5240, a halo giant with [Fe/H] NLTE = −5.3. This star was discovered in the course of follow-up mediumresolution 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 0107−5240. Scenarios for the origin of the abundance pattern observed in the star are discussed. We argue that HE 0107−5240 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 HE 0107−5240 can be explained by pre-enrichment from a zero-metallicity type-II supernova of 20-25 M ⊙ , 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 1 Based on observations collected at the European Southern Observatory, Paranal, Chile (Proposal Number 268.D-5745).

Previous genome-wide association studies (GWAS) have identified hundreds of genetic loci to be associated with body mass index (BMI) and risk of obesity. Genetic effects can differ between individuals depending on lifestyle or environmental factors due to gene-environment interactions. In this study, we examine gene-environment interactions in 362,496 unrelated participants with Caucasian ancestry from the UK Biobank resource. A total of 94 BMI-associated SNPs, selected from a previous GWAS on BMI, were used to construct weighted genetic scores for BMI (GSBMI). Linear regression modeling was used to estimate the effect of gene-environment interactions on BMI for 131 lifestyle factors related to: dietary habits, smoking and alcohol consumption, physical activity, socioeconomic status, mental health, sleeping patterns, as well as female-specific factors such as menopause and childbirth. In total, 15 lifestyle factors were observed to interact with GSBMI, of which alcohol intake frequency, usual walking pace, and Townsend deprivation index, a measure of socioeconomic status, were all highly significant (p = 1.45*10−29, p = 3.83*10−26, p = 4.66*10−11, respectively). Interestingly, the frequency of alcohol consumption, rather than the total weekly amount resulted in a significant interaction. The FTO locus was the strongest single locus interacting with any of the lifestyle factors. However, 13 significant interactions were also observed after omitting the FTO locus from the genetic score. Our analyses indicate that many lifestyle factors modify the genetic effects on BMI with some groups of individuals having more than double the effect of the genetic score. However, the underlying causal mechanisms of gene-environmental interactions are difficult to deduce from cross-sectional data alone and controlled experiments are required to fully characterise the causal factors.

The chemical abundance patterns observed in metal-poor Galactic halo stars contain the signature of the first supernovae, and thus allow us to probe the first stars that formed in the universe. We construct a theoretical model for the early chemical enrichment history of the Milky Way, aiming in particular at the contribution from pair-instability supernovae (PISNe). These are a natural consequence of current theoretical models for primordial star formation at the highest masses. However, no metal-poor star displaying the distinct PISN signature has yet been observed. We here argue that this apparent absence of any PISN signature is due to an observational selection effect. Whereas most surveys traditionally focus on the most metal-poor stars, we predict that early PISN enrichment tends to ''overshoot,'' reaching enrichment levels of ½Ca/H ' À2:5 that would be missed by current searches. We utilize existing observational data to place constraints on the primordial initial mass function (IMF). The number fraction of PISNe in the primordial stellar population is estimated to be <0.07, or P40% by mass, assuming that metal-free stars have masses in excess of 10 M . We further predict, based on theoretical estimates for the relative number of PISNe, that the expected fraction of second-generation stars below ½Ca/H ¼ À2 with a dominant (i.e., >90%) contribution from PISNe is merely $10 À4 to 5 ; 10 À4 . The corresponding fraction of stars formed from gas exclusively enriched by PISNe is a factor of $4 smaller. With the advent of next-generation telescopes and new, deeper surveys, we should be able to test these predictions.

The emergence of the first sources of light at redshifts of z ∼ 10 − 30 signaled the transition from the simple initial state of the Universe to one of increasing complexity. We review recent progress in our understanding of the formation of the first stars and galaxies, starting with cosmological initial conditions, primordial gas cooling, and subsequent collapse and fragmentation. We emphasize the important open question of how the pristine gas was enriched with heavy chemical elements in the wake of the first supernovae. We conclude by discussing how the chemical abundance patterns conceivably allow us to probe the properties of the first stars, and allow us to test models of early metal enrichment.1 In this review, we make a clear distinction between a chemical fingerprint and a chemical signature (see Sec. IV). The term chemical fingerprint is used when the elemental abundances provide direct evidence of a specific reaction mechanism: the r-process or triple-α process, for example. This review is concerned with identifying the chemical signatures of the first stars in the surface abundances of the oldest stellar populations. A chemical signature is inherently more complex because the elemental yields expelled from a dying star are likely to depend on more than one physical process. The signature then reflects multiple parameters, such as stellar mass, rotation, explosion energy, and the amount of fallback onto the remnant. 2

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In many species, the offspring of related parents suffer reduced reproductive success, a phenomenon known as inbreeding depression. In humans, the importance of this effect has remained unclear, partly because reproduction between close relatives is both rare and frequently associated with confounding social factors. Here, using genomic inbreeding coefficients (FROH) for >1.4 million individuals, we show that FROH is significantly associated (p < 0.0005) with apparently deleterious changes in 32 out of 100 traits analysed. These changes are associated with runs of homozygosity (ROH), but not with common variant homozygosity, suggesting that genetic variants associated with inbreeding depression are predominantly rare. The effect on fertility is striking: FROH equivalent to the offspring of first cousins is associated with a 55% decrease [95% CI 44–66%] in the odds of having children. Finally, the effects of FROH are confirmed within full-sibling pairs, where the variation in FROH is independent of all environmental confounding.