Browsing by Subject "1st stars"
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Item The Chemical Abundances Of Stars In The Halo (CASH) Project. II. A Sample Of 14 Extremely Metal-Poor Stars(2011-11) Hollek, Julie K.; Frebel, Anna; Roederer, Ian U.; Sneden, Christopher; Shetrone, Matthew; Beers, Timothy C.; Kang, Sung-Ju; Thom, Christopher; Hollek, Julie K.; Sneden, Christopher; Shetrone, MatthewWe present a comprehensive abundance analysis of 20 elements for 16 new low-metallicity stars from the Chemical Abundances of Stars in the Halo (CASH) project. The abundances have been derived from both Hobby-Eberly Telescope High Resolution Spectrograph snapshot spectra (R similar to 15,000) and corresponding high-resolution (R similar to 35,000) Magellan Inamori Kyocera Echelle spectra. The stars span a metallicity range from [Fe/H] from -2.9 to -3.9, including four new stars with [Fe/H] < -3.7. We find four stars to be carbon-enhanced metal-poor (CEMP) stars, confirming the trend of increasing [C/Fe] abundance ratios with decreasing metallicity. Two of these objects can be classified as CEMP-no stars, adding to the growing number of these objects at [Fe/H]< -3. We also find four neutron-capture-enhanced stars in the sample, one of which has [Eu/Fe] of 0.8 with clear r-process signatures. These pilot sample stars are the most metal-poor ([Fe/H] less than or similar to -3.0) of the brightest stars included in CASH and are used to calibrate a newly developed, automated stellar parameter and abundance determination pipeline. This code will be used for the entire similar to 500 star CASH snapshot sample. We find that the pipeline results are statistically identical for snapshot spectra when compared to a traditional, manual analysis from a high-resolution spectrum.Item The Chemical Imprint Of Silicate Dust On The Most Metal-Poor Stars(2014-02) Alexander, P.; Frebel, Anna; Bromm, Volker; Bromm, VolkerWe investigate the impact of dust-induced gas fragmentation on the formation of the first low-mass, metal-poor stars (<1 M-circle dot) in the early universe. Previous work has shown the existence of a critical dust-to-gas ratio, below which dust thermal cooling cannot cause gas fragmentation. Assuming that the first dust is silicon-based, we compute critical dust-to-gas ratios and associated critical silicon abundances ([Si/H](crit)). At the density and temperature associated with protostellar disks, we find that a standard Milky Way grain size distribution gives [Si/H](crit) = -4.5 +/- 0.1, while smaller grain sizes created in a supernova reverse shock give [Si/H](crit) = -5.3 +/- 0.1. Other environments are not dense enough to be influenced by dust cooling. We test the silicate dust cooling theory by comparing to silicon abundances observed in the most iron-poor stars ([Fe/H] < -4.0). Several stars have silicon abundances low enough to rule out dust-induced gas fragmentation with a standard grain size distribution. Moreover, two of these stars have such low silicon abundances that even dust with a shocked grain size distribution cannot explain their formation. Adding small amounts of carbon dust does not significantly change these conclusions. Additionally, we find that these stars exhibit either high carbon with low silicon abundances or the reverse. A silicate dust scenario thus suggests that the earliest low-mass star formation in the most metal-poor regime may have proceeded through two distinct cooling pathways: fine-structure line cooling and dust cooling. This naturally explains both the carbon-rich and carbon-normal stars at extremely low [Fe/H].Item The Cosmic Near Infrared Background. III. Fluctuations, Reionization, And The Effects Of Minimum Mass And Self-Regulation(2012-05) Fernandez, Elizabeth R.; Iliev, Illian T.; Komatsu, Eiichiro; Shapiro, Paul R.; Komatsu, Eiichiro; Shapiro, Paul R.Current observations suggest that the universe was reionized sometime before z similar to 6. One way to observe this epoch of the universe is through the Near Infrared Background (NIRB), which contains information about galaxies which may be too faint to be observed individually. We calculate the angular power spectrum (C-l) of the NIRB fluctuations caused by the distribution of these galaxies. Assuming a complete subtraction of any post-reionization component, C-l will be dominated by galaxies responsible for completing reionization (e.g., z similar to 6). The shape of C-l at high l is sensitive to the amount of nonlinear bias of dark matter halos hosting galaxies. As the nonlinear bias depends on the mass of these halos, we can use the shape of C-l to infer typical masses of dark matter halos responsible for completing reionization. We extend our previous study by using a higher-resolution N-body simulation, which can resolve halos down to 10(8) M-circle dot. We also include improved radiative transfer, which allows for the suppression of star formation in small-mass halos due to photoionization heating. As the nonlinear bias enhances the dark matter halo power spectrum on small scales, we find that C-l is steeper for the case with a complete suppression of small sources or partial suppression of star formation in small halos (the minimum galaxy mass is M-min = 10(9)M(circle dot) in ionized regions and M-min = 10(8)M(circle dot) in neutral regions) than for the case in which these small halos were unsuppressed. In all cases, we do not see a turnover toward high l in the shape of l(2)C(l).Item The Cosmic Near-Infrared Background. II. Fluctuations(2010-02) Fernandez, Elizabeth R.; Komatsu, Eiichiro; Iliev, Illian T.; Shapiro, Paul R.; Komatsu, Eiichiro; Shapiro, Paul R.The near-infrared background (NIRB) is one of a few methods that can be used to observe the redshifted light from early stars at a redshift of 6 and above, and thus it is imperative to understand the significance of any detection or nondetection of the NIRB. Fluctuations of the NIRB can provide information on the first structures, such as halos and their surrounding ionized regions in the intergalactic medium (IGM). We combine, for the first time, N-body simulations, radiative transfer code, and analytic calculations of luminosity of early structures to predict the angular power spectrum (C(l)) of fluctuations in the NIRB. We study in detail the effects of various assumptions about the stellar mass, the initial mass spectrum of stars, the metallicity, the star formation efficiency (f(*)), the escape fraction of ionizing photons (f(esc)), and the star formation timescale (t(SF)), on the amplitude as well as the shape of C(l). The power spectrum of NIRB fluctuations is maximized when f(*) is the largest (as C(l) proportional to f(*)(2))and f(esc) is the smallest (as more nebular emission is produced within halos). A significant uncertainty in the predicted amplitude of C(l) exists due to our lack of knowledge of t(SF) of these early populations of galaxies, which is equivalent to our lack of knowledge of the mass-to-light ratio of these sources. We do not see a turnover in the NIRB angular power spectrum of the halo contribution, which was claimed to exist in the literature, and explain this as the effect of high levels of nonlinear bias that was ignored in the previous calculations. This is partly due to our choice of the minimum mass of halos contributing to NIRB (similar to 2 x 10(9) M(circle dot)), and a smaller minimum mass, which has a smaller nonlinear bias, may still exhibit a turnover. Therefore, our results suggest that both the amplitude and shape of the NIRB power spectrum provide important information regarding the nature of sources contributing to the cosmic reionization. The angular power spectrum of the IGM, in most cases, is much smaller than the halo angular power spectrum, except when f(esc) is close to unity, t(SF) is longer, or the minimum redshift at which the star formation is occurring is high. In addition, low levels of the observed mean background intensity tend to rule out high values of f(*) greater than or similar to 0.2.Item Cosmological Impact Of Population III Binaries(2015-03) Chen, Ke-Jung; Bromm, Volker; Heger, Alexander; Jeon, Myoungwon; Woosley, Stan; Chen, Ke-Jung; Woosley, StanWe present the results of the stellar feedback from Population III (Pop III) binaries by employing improved, more realistic Pop III evolutionary stellar models. To facilitate a meaningful comparison, we consider a fixed mass of 60 M-circle dot incorporated in Pop III stars, either contained in a single star, or split up in binary stars of 30 M-circle dot each or an asymmetric case of one 45 and one 15 M-circle dot star. Whereas the sizes of the resulting H II regions are comparable across all cases, the He III regions around binary stars are significantly smaller than that of the single star. Consequently, the He+ 1640 angstrom recombination line is expected to become much weaker. Supernova (SN) feedback exhibits great variety due to the uncertainty in possible explosion pathways. If at least one of the component stars dies as a hypernova about 10 times more energetic than conventional core-collapse SNe, the gas inside the host minihalo is effectively blown out, chemically enriching the intergalactic medium (IGM) to an average metallicity of 10(-4)-10(-3) Z(circle dot), out to similar to 2 kpc. The single star, however, is more likely to collapse into a black hole, accompanied by at most very weak explosions. The effectiveness of early chemical enrichment would thus be significantly reduced, in contrast to. the lower mass binary stars, where at least one component is likely to contribute to heavy element production and dispersal. Important new feedback physics is also introduced if close binaries can form high-mass X-ray binaries, leading to the pre-heating and -ionization of the IGM beyond the extent of the stellar H II regions.Item Effects Of Rotation On The Minimum Mass Of Primordial Progenitors Of Pair-Instability Supernovae(2012-03) Chatzopoulos, Emmanouil; Wheeler, J. Craig; Chatzopoulos, Emmanouil; Wheeler, J. CraigThe issue of which stars may reach the conditions of electron/positron pair-formation instability is of importance to understand the final evolution both of the first stars and of contemporary stars. The criterion to enter the pair-instability regime in density and temperature is basically controlled by the mass of the oxygen core. The main-sequence masses that produce a given oxygen core mass are, in turn, dependent on metallicity, mass loss, and convective and rotationally induced mixing. We examine the evolution of massive stars to determine the minimum main-sequence mass that can encounter pair-instability effects, either a pulsational pair-instability supernova (PPISN) or a full-fledged pair-instability supernova (PISN). We concentrate on zero-metallicity stars with no mass-loss subject to the Schwarzschild criterion for convective instability, but also explore solar metallicity and mass loss and the Ledoux criterion. As expected, for sufficiently strong rotationally induced mixing, the minimum main-sequence mass is encountered for conditions that induce effectively homogeneous evolution such that the original mass is converted almost entirely to helium and then to oxygen. For this case, we find that the minimum main-sequence mass is about 40 M-circle dot to encounter PPISN and about 65 M-circle dot to encounter a PISN. The implications of these results for the first stars and for contemporary supernovae are discussed.Item Effects Of Varying The Three-Body Molecular Hydrogen Formation Rate In Primordial Star Formation(2011-01) Turk, Matthew J.; Clark, Paul; Glover, Simon C. O.; Greif, Thomas H.; Abel, Tom; Klessen, Ralf; Bromm, Volker; Bromm, VolkerThe transformation of atomic hydrogen to molecular hydrogen through three-body reactions is a crucial stage in the collapse of primordial, metal-free halos, where the first generation of stars (Population III stars) in the universe is formed. However, in the published literature, the rate coefficient for this reaction is uncertain by nearly an order of magnitude. We report on the results of both adaptive mesh refinement and smoothed particle hydrodynamics simulations of the collapse of metal-free halos as a function of the value of this rate coefficient. For each simulation method, we have simulated a single halo three times, using three different values of the rate coefficient. We find that while variation between halo realizations may be greater than that caused by the three-body rate coefficient being used, both the accretion physics onto Population III protostars as well as the long-term stability of the disk and any potential fragmentation may depend strongly on this rate coefficient.Item Emission From Pair-Instability Supernovae With Rotation(2015-01) Chatzopoulos, Emmanouil; van Rossum, Daniel R.; Wheeler, Craig J.; Whalen, Daniel J.; Smidt, Joseph; Wiggins, Brandon; Wheeler, Craig J.Pair-instability supernovae (PISNe) have been suggested as candidates for some superluminous supernovae, such as SN 2007bi, and as one of the dominant types of explosion occurring in the early universe from massive, zero-metallicity Population III stars. The progenitors of such events can be rapidly rotating, therefore exhibiting different evolutionary properties due to the effects of rotationally induced mixing and mass-loss. Proper identification of such events requires rigorous radiation hydrodynamics and radiative transfer calculations that capture not only the behavior of the light curve but also the spectral evolution of these events. We present radiation hydrodynamics and radiation transport calculations for 90-300M(circle dot) rotating PISNe covering both the shock breakout and late light curve phases. We also investigate cases of different initial metallicity and rotation rate to determine the impact of these parameters on the detailed spectral characteristics of these events. In agreement with recent results on non-rotating PISNe, we find that for a range of progenitor masses and rotation rates these events have intrinsically red colors in contradiction with observations of superluminous supernovae. The spectroscopic properties of rotating PISNe are similar to those of non-rotating events with stripped hydrogen and helium envelopes. We find that the progenitor metallicity and rotation rate properties are erased after the explosion and cannot be identified in the resulting model spectra. It is the combined effects of pre-supernova mass-loss and the basic properties of the supernova ejecta such as mass, temperature, and velocity that have the most direct impact in the model spectra of PISNe.Item Gravitational Fragmentation In Turbulent Primordial Gas And The Initial Mass Function Of Population III Stars(2011-02) Clark, Paul C.; Glover, Simon C. O.; Klessen, Ralf S.; Bromm, Volker; Bromm, VolkerWe report results from numerical simulations of star formation in the early universe that focus on the dynamical behavior of metal-free gas under different initial and environmental conditions. In particular we investigate the role of turbulence, which is thought to ubiquitously accompany the collapse of high-redshift halos. We distinguish between two main cases: the birth of Population III. 1 stars-those which form in the pristine halos unaffected by prior star formation-and the formation of Population III. 2 stars-those forming in halos where the gas has an increased ionization fraction. We find that turbulent primordial gas is highly susceptible to fragmentation in both cases, even for turbulence in the subsonic regime, i.e., for rms velocity dispersions as low as 20% of the sound speed. Fragmentation is more vigorous and more widespread in pristine halos compared to pre-ionized ones. If such levels of turbulent motions were indeed present in star-forming minihalos, Population III. 1 stars would be on average of somewhat lower mass, and form in larger groups, than Population III. 2 stars. We find that fragment masses cover over two orders of magnitude, suggesting that the Population III initial mass function may have been much broader than previously thought. This prompts the need for a large, high-resolution study of the formation of dark matter minihalos that is capable of resolving the turbulent flows in the gas at the moment when the baryons become self-gravitating. This would help to determine the applicability of our results to primordial star formation.Item GRB Cosmology: Probing The Early Universe(2007-10) Bromm, V.; Loeb, A.; Bromm, V.Current observations are about to open up a direct observational window into the final frontier of cosmology: the crucial first billion years in cosmic history when the first stars and galaxies formed. Even before the launch of the James Webb Space Telescope, it would be possible to utilize Gamma-Ray Bursts (GRBs) as unique probes of cosmic star formation and the state of the intergalactic medium up to redshifts of the first stars. The ongoing Swift mission might be the first observatory to detect individual Population III stars, provided that massive metal-free stars were able to trigger GRBs. Swift will empirically constrain the redshift at which Population III star formation was terminated, thus providing crucial input to models of cosmic reionization and metal enrichment.Item HE 1327-2326, An Unevolved Star With Fe/H <-5.0. III. Does Its Atmosphere Reflect Its Natal Composition?(2009-06) Korn, A. J.; Richard, O.; Mashonkina, L.; Bessell, Michael S.; Frebel, Anna; Aoki, Wako; Frebel, AnnaBased on spectroscopic constraints derived from nonlocal thermodynamic equilibrium line formation, we explore the likely range of stellar parameters (T-eff and log g) for the hyper-metal-poor (HMP) star HE 1327-2326. Combining the constraints from Balmer line profiles and the Ca I/II ionization equilibrium, a subgiant stage of evolution is indicated. This result is further supported by spectrophotometric observations of the Balmer jump. If a higher T-eff value was used (as favored by some photometric calibrations), the spectroscopic analysis would indicate a turnoff-point stage of evolution. Using a stellar-structure code that treats the effects of atomic diffusion throughout the star in detail, we evolve a low-mass model star to reach the Hertzsprung-Russell-diagram position of HE 1327-2326 after roughly 13 Gyr. While the surface abundances are modified significantly (by more than 1 dex for the case of uninhibited diffusion), such corrections cannot resolve the discrepancy between the abundance inferred from the nondetection of the Li I resonance line at 6707 angstrom and the Wilkinson Microwave Anisotropy Probe based primordial lithium abundance. As there are numerous processes that can destroy lithium, any cosmological interpretation of a lower-than-expected lithium abundance at the lowest metallicities will have to await sample sizes of unevolved HMP stars that are 1 order of magnitude larger. The situation remains equally inconclusive concerning atomic-diffusion corrections. Here, attempts have to be made to better constrain internal mixing processes, both observationally and by means of sophisticated modeling. With constraints on additional mixing processes taken from a recent globular-cluster study, the likeliest scenario is that HE 1327-2326's surface abundances have undergone mild depletion (of order 0.2 dex).Item High-Resolution Spectroscopy Of Extremely Metal-Poor Stars In The Least Evolved Galaxies: Ursa Major II And Coma Berenices(2010-01) Frebel, Anna; Simon, Joshua D.; Geha, Marla; Willman, Beth; Frebel, AnnaWe present spectra of six metal-poor stars in two of the ultra-faint dwarf galaxies orbiting the Milky Way (MW), Ursa Major II, and Coma Berenices obtained with the Keck/High Resolution Echelle Spectrometer (HIRES). These observations include the first high-resolution spectroscopic observations of extremely metal-poor ([Fe/H] < -3.0) stars not belonging to the MW halo field star population. We obtain abundance measurements and upper limits for 26 elements between carbon and europium. The entire sample of stars spans a range of -3.2 < [Fe/H] < -2.3, and we confirm that each galaxy contains a large intrinsic spread of Fe abundances. A comparison with MW halo stars of similar metallicities reveals substantial agreement between the abundance patterns of the ultra-faint dwarf galaxies and the MW halo for the light, alpha, and iron-peak elements (C to Zn). This agreement contrasts with the results of earlier studies of more metal-rich stars (-2.5 less than or similar to [Fe/H] less than or similar to -1.0) in more luminous dwarf spheroidal galaxies, which found significant abundance discrepancies with respect to the MW halo data. The abundances of neutron-capture elements (Sr to Eu) in the ultra-faint dwarf galaxies are extremely low, consistent with the most metal-poor halo stars, but not with the typical halo abundance pattern at [Fe/H] greater than or similar to -3.0. Not only are our results broadly consistent with a galaxy formation model that predicts that massive dwarf galaxies are the source of the metal-rich component ([Fe/H] > -2.5) of the MW halo, but they also suggest that the faintest known dwarfs may be the primary contributors to the metal-poor end of the MW halo metallicity distribution.Item The Inhomogeneous Background Of H-2-Dissociating Radiation During Cosmic Reionization(2009-04) Ahn, Kyungjin; Shapiro, Paul R.; Iliev, Illian T.; Mellema, Garrelt; Pen, Ue-Li; Shapiro, Paul R.The first, self-consistent calculations are presented of the cosmological, H-2-dissociating UV background produced during the epoch of reionization by the sources of reionization. Large-scale radiative transfer simulations of reionization trace the impact of all the ionizing starlight on the intergalactic medium (IGM) from all the sources in our simulation volume down to dwarf galaxies of mass similar to 10(8) M-circle dot, identified by very high resolution N-body simulations, including the self-regulating effect of IGM photoheating on dwarf galaxy formation. The UV continuum emitted below 13.6 eV by each source is then transferred through the same IGM, attenuated by atomic H Lyman series resonance lines, to predict the evolution of the inhomogeneous radiation background in the Lyman-Werner (LW) bands of H-2 between 11 and 13.6 eV. On average, the intensity of this LW background is found to rise to the threshold level at which dissociation suppresses H-2 cooling and star formation inside minihalos, long before reionization is complete. Spatial variations in the LW background are found which result from the clustering of sources associated with large-scale structure formation, such that intensity fluctuations correlate with matter density fluctuations. As a result, the LW background rises to the threshold level for H-2 suppression earlier in the vicinity of the reionization sources and their H II regions.Item Line Lists for the A(2)Pi-X-2 Sigma(+) (Red) and B-2 Sigma(+)-X-2 Sigma(+) (Violet) Systems of Cn, (Cn)-C-13-N-14, and (Cn)-C-12-N-15, and Application To Astronomical Spectra(2014-10) Sneden, Christopher; Lucatello, Sara; Ram, Ram S.; Brooke, James S. A.; Bernath, Peter; Sneden, ChristopherNew red and violet system line lists for the CN isotopologues (CN)-C-13-N-14 and (CN)-C-12-N-15 have been generated. These new transition data are combined with those previously derived for (CN)-C-12-N-14, and applied to the determination of CNO abundances in the solar photosphere and in four red giant stars: Arcturus, the bright, very low-metallicity star HD 122563, and the carbon-enhanced metal-poor stars HD 196944 and HD 201626. When both red and violet system lines are detectable in a star, their derived N abundances are in good agreement. The mean N abundances determined in this work are also generally in accord with published values.Item Multi-Dimensional Simulations Of Rotating Pair-Instability Supernovae(2013-10) Chatzopoulos, Emmanouil; Wheeler, J. Craig; Couch, Sean M.; Chatzopoulos, Emmanouil; Wheeler, J. CraigWe study the effects of rotation on the dynamics, energetics, and Ni-56 production of pair instability supernova (PISN) explosions by performing rotating two-dimensional ("2.5D") hydrodynamics simulations. We calculate the evolution of eight low-metallicity (Z = 10(-3), 10(-4) Z(circle dot)) massive (135-245 M-circle dot) PISN progenitors with initial surface rotational velocities of 50% of the critical Keplerian value using the stellar evolution code MESA. We allow for both the inclusion and the omission of the effects of magnetic fields in the angular momentum transport and in chemical mixing, resulting in slowly rotating and rapidly rotating final carbon-oxygen cores, respectively. Increased rotation for carbon-oxygen cores of the same mass and chemical stratification leads to less energetic PISN explosions that produce smaller amounts of Ni-56 due to the effect of the angular momentum barrier that develops and slows the dynamical collapse. We find a non-monotonic dependence of Ni-56 production on rotational velocity in situations when smoother composition gradients form at the outer edge of the rotating cores. In these cases, the PISN energetics are determined by the competition of two factors: the extent of chemical mixing in the outer layers of the core due to the effects of rotation in the progenitor evolution and the development of angular momentum support against collapse. Our 2.5D PISN simulations with rotation are the first presented in the literature. They reveal hydrodynamic instabilities in several regions of the exploding star and increased explosion asymmetries with higher core rotational velocity.Item Probing Pre-Galactic Metal Enrichment With High-Redshift Gamma-Ray Bursts(2012-11) Wang, F. Y.; Bromm, Volker; Greif, Thomas H.; Stacy, Athena; Dai, Z. G.; Loeb, Abraham; Cheng, K. S.; Wang, F. Y.; Bromm, VolkerWe explore high-redshift gamma-ray bursts (GRBs) as promising tools to probe pre-galactic metal enrichment. We utilize the bright afterglow of a Population III (Pop III) GRB exploding in a primordial dwarf galaxy as a luminous background source, and calculate the strength of metal absorption lines that are imprinted by the first heavy elements in the intergalactic medium (IGM). To derive the GRB absorption line diagnostics, we use an existing highly resolved simulation of the formation of a first galaxy which is characterized by the onset of atomic hydrogen cooling in a halo with virial temperature greater than or similar to 10(4) K. We explore the unusual circumburst environment inside the systems that hosted Pop III stars, modeling the density evolution with the self-similar solution for a champagne flow. For minihalos close to the cooling threshold, the circumburst density is roughly proportional to (1 + z) with values of about a few cm(-3). In more massive halos, corresponding to the first galaxies, the density may be larger, n greater than or similar to 100 cm(-3). The resulting afterglow fluxes are weakly dependent on redshift at a fixed observed time, and may be detectable with the James Webb Space Telescope and Very Large Array in the near-IR and radio wavebands, respectively, out to redshift z greater than or similar to 20. We predict that the maximum of the afterglow emission shifts from near-IR to millimeter bands with peak fluxes from mJy to Jy at different observed times. The metal absorption line signature is expected to be detectable in the near future. GRBs are ideal tools for probing the metal enrichment in the early IGM, due to their high luminosities and featureless power-law spectra. The metals in the first galaxies produced by the first supernova (SN) explosions are likely to reside in low-ionization stages (C II, OI, Si II and Fe II). We show that, if the afterglow can be observed sufficiently early, analysis of the metal lines may distinguish whether the first heavy elements were produced in a pair-instability supernova or a core-collapse (Type II) SN, thus constraining the initial mass function of the first stars.Item Simulations On A Moving Mesh: The Clustered Formation Of Population III Protostars(2011-08) Greif, Thomas H.; Springel, Volker; White, Simon D. M.; Glover, Simon C. O.; Clark, Paul C.; Smith, Rowan J.; Klessen, Ralf S.; Bromm, Volker; Bromm, VolkerThe cosmic dark ages ended a few hundred million years after the big bang, when the first stars began to fill the universe with new light. It has generally been argued that these stars formed in isolation and were extremely massive-perhaps 100 times as massive as the Sun. In a recent study, Clark and collaborators showed that this picture requires revision. They demonstrated that the accretion disks that build up around Population III stars are strongly susceptible to fragmentation and that the first stars should therefore form in clusters rather than in isolation. We here use a series of high-resolution hydrodynamical simulations performed with the moving mesh code AREPO to follow up on this proposal and to study the influence of environmental parameters on the level of fragmentation. We model the collapse of five independent minihalos from cosmological initial conditions, through the runaway condensation of their central gas clouds, to the formation of the first protostar, and beyond for a further 1000 years. During this latter accretion phase, we represent the optically thick regions of protostars by sink particles. Gas accumulates rapidly in the circumstellar disk around the first protostar, fragmenting vigorously to produce a small group of protostars. After an initial burst, gravitational instability recurs periodically, forming additional protostars with masses ranging from similar to 0.1 to 10 M-circle dot. Although the shape, multiplicity, and normalization of the protostellar mass function depend on the details of the sink-particle algorithm, fragmentation into protostars with diverse masses occurs in all cases, confirming earlier reports of Population III stars forming in clusters. Depending on the efficiency of later accretion and merging, Population III stars may enter the main sequence in clusters and with much more diverse masses than are commonly assumed.Item The Stellar Content Of The Hamburg/ESO Survey V. The Metallicity Distribution Function Of The Galactic Halo(2009-11) Schorck, T.; Christlieb, N.; Cohen, J. G.; Beers, T. C.; Shectman, S.; Thompson, I.; McWilliam, A.; Bessell, M. S.; Norris, J. E.; Melendez, J.; Ramirez, S.; Haynes, D.; Cass, P.; Hartley, M.; Russell, K.; Watson, F.; Zickgraf, F. J.; Behnke, B.; Fechner, C.; Fuhrmeister, B.; Barklem, P. S.; Edvardsson, B.; Frebel, A.; Wisotzki, L.; Reimers, D.; Frebel, A.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 strategy for spectroscopic follow-up observations of the metal-poor candidates, namely >best and brightest stars first>. Comparison of the metallicities [Fe/H] of the stars determined from moderate-resolution (i.e., R similar to 2000) follow-up spectra with results derived from abundance analyses based on high-resolution spectra (i.e., R > 20 000) shows that the [Fe/H] estimates used for the determination of the halo MDF are accurate to within 0.3 dex, once highly C-rich stars are eliminated. 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(circle dot) reproduces the sharp drop at [Fe/H] similar to -3.6 present in the HES MDF. Although currently about ten stars at [Fe/H] < -3.6 are known, the evidence for the existence of a tail of the halo MDF extending to [Fe/H] similar to -5.5 is weak from the sample considered in this paper, because it only includes two stars [Fe/H] < -3.6. Therefore, a comparison with theoretical models has to await larger statistically complete and unbiased samples. 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.Item An Upper Limit On The Sulphur Abundance In HE 1327-2326(2012-08) Bonifacio, P.; Caffau, E.; Venn, K. A.; Lambert, D. L.; Lambert, D. L.Context. Star HE 1327-2326 is a unique object, with the lowest measured iron abundance ([Fe/H] similar to -6) and a peculiar chemical composition that includes large overabundances of C, N, and O with respect to iron. One important question is whether the chemical abundances in this star reflect the chemical composition of the gas cloud from which it was formed or if they have been severely affected by other processes, such as dust-gas winnowing. Aims. We measure or provide an upper limit to the abundance of the volatile element sulphur, which can help to discriminate between the two scenarios. Methods. We observed HE 1327-2326 with the high resolution infra-red spectrograph CRIRES at the VLT to observe the S i lines of Multiplet 3 at 1045 nm. Results. We do not detect the S i line. A 3 sigma upper limit on the equivalent width (EW) of any line in our spectrum is EW < 0.66 pm. Using either one-dimensional static or three-dimensional hydrodynamical model-atmospheres, this translates into a robust upper limit of [S/H] < -2.6. Conclusions. This upper limit does not provide conclusive evidence for or against dust-gas winnowing, and the evidence coming from other elements (e. g., Na and Ti) is also inconclusive or contradictory. The formation of dust in the atmosphere versus an origin of the metals in a metal-poor supernova with extensive >fall-back> are not mutually exclusive. It is possible that dust formation distorts the peculiar abundance pattern created by a supernova with fall-back, thus the abundance ratios in HE 1327-2326 may be used to constrain the properties of the supernova(e) that produced its metals, but with some caution.