Observations Of Metal-poor Stars Research Articles

Metal poor stars

In this contribution I provide an overview of the study of metal poor stars, with an emphasis on the efforts that are on-going to find large number of these rare objects. These efforts will provide, in the next few years, an interesting number of targets for which it will be desirable to have a chemical inventory, as complete as possible. In this respect CUBES, providing access to the near-UV region provides a unique opportunity to access some abundance indicators that have no counterparts in other spectral regions available from ground-based observations. I also take the opportunity to encourage the community to reflect on the possibility of placing a copy of CUBES on a telescope in the northern hemisphere. This contribution is not intended to be an exhaustive review of the field, but aims at stimulating interest in near-UV observations of metal poor stars.

Metal Mixing in Minihalos: The Descendants of Pair-instability Supernovae

The lack of observations of abundance patterns originating in pair-instability supernovae has been a long-standing problem in relation to the first stars. This class of supernovae is expected to have an abundance pattern with a strong odd–even effect, making it substantially different from present-day supernovae. In this study, we use a cosmological radiation hydrodynamics simulation to model such supernovae and the subsequent formation of the second generation of stars. We incorporate streaming velocities for the first time. There are 14 star-forming minihalos in our 1 cMpc h −1 box, leading to 14 supernovae occurring before redshift z = 19.5, where we start reducing the complexity of the simulation. Following the explosions, extremely metal-poor stars form in 10 halos via internal and external enrichment, which makes it the most common outcome. Only one halo does not recollapse during the simulations. This result is at variance with the current (lack of) observations of metal-poor stars with pair-instability supernova abundance patterns, suggesting that these very massive stars might be rare even in the early universe. The results from this simulation also give us insights into what drives different modes of recollapse and what determines the mixing behavior of metals after very energetic supernovae.

A Nuclear Equation of State Inferred from Stellar r-process Abundances

Abstract Binary neutron star mergers (NSMs) have been confirmed as one source of the heaviest observable elements made by the rapid neutron-capture (r-) process. However, modeling NSM outflows—from the total ejecta masses to their elemental yields—depends on the unknown nuclear equation of state (EOS) that governs neutron star structure. In this work, we derive a phenomenological EOS by assuming that NSMs are the dominant sources of the heavy element material in metal-poor stars with r-process abundance patterns. We start with a population synthesis model to obtain a population of merging neutron star binaries and calculate their EOS-dependent elemental yields. Under the assumption that these mergers were responsible for the majority of r-process elements in the metal-poor stars, we find parameters representing the EOS for which the theoretical NSM yields reproduce the derived abundances from observations of metal-poor stars. For our proof-of-concept assumptions, we find an EOS that is slightly softer than, but still in agreement with, current constraints, e.g., by the Neutron Star Interior Composition Explorer, with R 1.4 = 12.25 ± 0.03 km and M TOV = 2.17 ± 0.03 M ⊙ (statistical uncertainties, neglecting modeling systematics).

Neutron star mergers and rare core-collapse supernovae as sources of r-process enrichment in simulated galaxies

ABSTRACT We use cosmological, magnetohydrodynamical simulations of Milky Way-mass galaxies from the Auriga project to study their enrichment with rapid neutron capture (r-process) elements. We implement a variety of enrichment models from both binary neutron star mergers and rare core-collapse supernovae. We focus on the abundances of (extremely) metal-poor stars, most of which were formed during the first ∼Gyr of the Universe in external galaxies and later accreted on to the main galaxy. We find that the majority of metal-poor stars are r-process enriched in all our enrichment models. Neutron star merger models result in a median r-process abundance ratio, which increases with metallicity, whereas the median trend in rare core-collapse supernova models is approximately flat. The scatter in r-process abundance increases for models with longer delay times or lower rates of r-process-producing events. Our results are nearly perfectly converged, in part due to the mixing of gas between mesh cells in the simulations. Additionally, different Milky Way-mass galaxies show only small variation in their respective r-process abundance ratios. Current (sparse and potentially biased) observations of metal-poor stars in the Milky Way seem to prefer rare core-collapse supernovae over neutron star mergers as the dominant source of r-process elements at low metallicity, but we discuss possible caveats to our models. Dwarf galaxies that experience a single r-process event early in their history show highly enhanced r-process abundances at low metallicity, which is seen both in observations and in our simulations. We also find that the elements produced in a single event are mixed with ≈108 M⊙ of gas relatively quickly, distributing the r-process elements over a large region.

The R-process Alliance: First Release from the Southern Search for R-process-enhanced Stars in the Galactic Halo*

Abstract The recent detection of a binary neutron star merger and the clear evidence of the decay of radioactive material observed in this event have, after 60 years of effort, provided an astrophysical site for the rapid neutron-capture (r-) process which is responsible for the production of the heaviest elements in our universe. However, observations of metal-poor stars with highly enhanced r-process elements have revealed abundance patterns suggesting that multiple sites may be involved. To address this issue, and to advance our understanding of the r-process, we have initiated an extensive search for bright (V < 13.5), very metal-poor ([Fe/H] < −2) stars in the Milky Way halo exhibiting strongly enhanced r-process signatures. This paper presents the first sample collected in the southern hemisphere using the echelle spectrograph on du Pont 2.5 m telescope at Las Campanas Observatory. We have observed and analyzed 107 stars with −3.13 < [Fe/H] < −0.79. Of those, 12 stars are strongly enhanced in heavy r-process elements (r-II), 42 stars show moderate enhancements of heavy r-process material (r-I), and 20 stars exhibit low abundances of the heavy r-process elements and higher abundances of the light r-process elements relative to the heavy ones (limited-r). This search is more successful at finding r-process-enhanced stars compared to previous searches, primarily due to a refined target selection procedure that focuses on red giants.

Effects of long-lived 10 MeV-scale sterile neutrinos on primordial elemental abundances and the effective neutrino number

The primordial lithium abundance inferred from observations of metal-poor stars is ~3 times smaller than the theoretical value in standard big bang nucleosynthesis (BBN) model. We assume a simple model including a sterile neutrino nu_H with mass of O(10) MeV which decays long after BBN. We then investigate cosmological effects of a sterile neutrino decay. We formulate the injection spectrum of nonthermal photons induced by electrons and positrons generated at the nu_H decay, as a function of the nu_H mass and the photon temperature. We then consistently solve (1) the cosmic thermal history, (2) nonthermal nucleosynthesis induced by the nonthermal photons, (3) the baryon-to-photon ratio eta, and (4) the effective neutrino number N_eff. Amounts of energy injection at the nu_H decay are constrained from limits on primordial D and 7Li abundances, the N_eff value, and the cosmic microwave background energy spectrum. We find that 7Be is photodisintegrated and the Li problem is partially solved for the lifetime 10^4-10^5 s and the mass >~ 14 MeV. 7Be destruction by more than a factor of 3 is not possible because of an associated D over-destruction. In the parameter region, the eta value is decreased slightly, while the N_eff value is increased by a factor of <~ 1. In this study, errors in photodisintegration cross sections of 7Be(g, a)3He and 7Li(g, a)3H that have propagated through literatures are corrected. It is then found that the new photodisintegration rates are 2.3 to 2.5 times smaller than the old rates, so that efficiencies of 7Be and 7Li photodisintegration are significantly smaller.

REVISED BIG BANG NUCLEOSYNTHESIS WITH LONG-LIVED, NEGATIVELY CHARGED MASSIVE PARTICLES: UPDATED RECOMBINATION RATES, PRIMORDIAL 9 Be NUCLEOSYNTHESIS, AND IMPACT OF NEW 6 Li LIMITS

We extensively reanalyze effects of a long-lived negatively charged massive particle, X-, on big bang nucleosynthesis (BBN). The BBN model with an X- particle was originally motivated by the discrepancy between 6,7Li abundances predicted in standard BBN model and those inferred from observations of metal-poor stars. In this model 7Be is destroyed via the recombination with an X- particle followed by radiative proton capture. We calculate precise rates for the radiative recombinations of 7Be, 7Li, 9Be, and 4He with X-. In nonresonant rates we take into account respective partial waves of scattering states and respective bound states. The finite sizes of nuclear charge distributions cause deviations in wave functions from those of point-charge nuclei. For a heavy X- mass, m_X \gtrsim 100 GeV, the d-wave --> 2P transition is most important for 7Li and 7,9Be, unlike recombination with electrons. Our new nonresonant rate of the 7Be recombination for m_X=1000 GeV is more than 6 times larger than the existing rate. Moreover, we suggest a new important reaction for 9Be production: the recombination of 7Li and X- followed by deuteron capture. We derive binding energies of X-nuclei along with reaction rates and Q-values. We then calculate BBN and find that the amount of 7Be destruction depends significantly on the charge distribution of 7Be. Finally, updated constraints on the initial abundance and the lifetime of the X- are derived in the context of revised upper limits to the primordial 6Li abundance. Parameter regions for the solution to the 7Li problem are revised, and the primordial 9Be abundances is revised.

NUCLEOSYNTHESIS IN CORE-COLLAPSE SUPERNOVA EXPLOSIONS TRIGGERED BY A QUARK-HADRON PHASE TRANSITION

We explore heavy element nucleosynthesis in the explosion of massive stars which are triggered by a quark-hadron phase transition during the early post bounce phase of core-collapse supernovae. The present study is based on general relativistic radiation hydrodynamics simulations with three-flavor Boltzmann neutrino transport in spherical symmetry, which utilize a quark-hadron hybrid equation of state based on the MIT bag model for strange quark matter. The quark-hadron phase transition inside the stellar core forms a shock wave propagating towards the surface of the proto-neutron star. The shock wave results in an explosion and ejects neutron-rich matter which is piled up or accreting on the proto-neutron star. Later, during the cooling phase, the proto-neutron star develops a proton-rich neutrino-driven wind. We present a detailed analysis of the nucleosynthesis outcome in both neutron-rich and proton-rich ejecta and compare our integrated nucleosynthesis with observations of metal poor stars.

“Planetary” noble gas components and the nucleosynthetic history of solar system material

Models capable of explaining the differences between the isotopic compositions of the planetary noble gas components “Q” and “P3” (widespread in primitive meteorites) and average solar system material as sampled by the solar wind are presented, and their implications discussed. Small, variable amounts of known presolar components and 129Xe from 129I decay are present in Q gases alongside a solar composition that has been mass fractionated. These most likely arise either from mixing during parent body processing or co-release from poorly retentive phases during analysis. Thus the heavy noble gas budget of primitive meteorites is dominated by a component derived from material with the average composition of the solar system. In contrast, P3 seems best explained as a presolar component, consistent with isolation from bulk material that subsequently evolved to the solar composition as newly synthesised material was added. Examination of Kr-P3 identifies the addition as having had the signature of the weak s-process, and demonstrates that a second process that contributes the isotopes of krypton not produced in the s-process (residual isotopes) must also have added material to the reservoir as it evolved to the solar composition. Total s-process contributions required of 132Xe and 84Kr are at least ∼1% of the present budget, as is that of the residual krypton “component”. While concentrations of P3 vary with extents of parent body processing, concentrations of 129Xe excess from 129I decay associated with P3 are roughly constant in those least processed meteorites that retain a P3 signature (apart from ALH77307). This has a natural explanation in the different chemical behaviours of iodine and xenon if 129I was alive in P3 in the early solar system. The presence of live 129I in P3 in the early solar system imposes a loose constraint that the P3 component was isolated from a parent reservoir less than ∼100 Myr before the formation of the solar system. The model of evolution from P3 gases to the solar composition requires that residual krypton isotopes are not products of a conventional r-process that also synthesises 129I. Separate sites for synthesis of residual isotopes of krypton and the heavy element r-process are consistent with observations of metal poor stars, but do not correspond to the two r-processes invoked to account for variations between the 182Hf and 129I systems. Since the weak s-process is implicated as the source of the s-process material contributed as the P3 reservoir evolved to a solar composition, the massive stars that host it may also host the process that synthesises residual krypton isotopes. The recent presence of nearby massive stars is consistent with an emerging picture of the environment of solar system formation.

Constraints on Big Bang Nucleosynthesis from observations of metal-poor stars

An overview of observations of 6Li and 7Li in metal-poor stars is given, and possible consequences for Big Bang Nucleosynthesis (BBN) are discussed. Potential solutions for the discrepancy between the 7Li abundance seen in metal-poor stars and the primordial abundance inferred from the value of the baryon density of the Universe, as determined from recent observations of the cosmic microwave background radiation with the Wilkinson microwave anisotropy probe, and standard BBN models, are discussed, and scenarios for the production of 6Li in the early Universe are reviewed.

POSSIBILITY OF A SIGNATURE OF THE RADIATIVE DECAY OF RELIC PARTICLES ON LIGHT ELEMENT ABUNDANCES

Recent spectroscopic observations of metal poor stars have indicated that both 7 Li and 6 Li have abundance plateaus as a function of the metallicity. Abundances of 7 Li are about a factor three lower than the primordial abundance predicted by standard big-bang nucleosynthesis (SBBN), and 6 Li abundances are ~ 1/20 of 7 Li , whereas SBBN predicts negligible amounts of 6 Li compared to the detected level. These discrepancies suggest that 6 Li has another cosmological or Galactic origin. Furthermore, it appear that 7 Li (and also 6 Li ) has been depleted from its primordial abundance by some post-BBN processes. We study the possibility that the radiative decay of long-lived particles has affected the cosmological lithium abundances in reality. We calculate the non-thermal nucleosynthesis associated with the radiative decay, and explore the allowed region of the parameters specifying the properties of long-lived particles. We also impose constraints from observations of the CMB energy spectrum. It is found that non-thermal nucleosynthesis could produces 6 Li at the level detected in metal poor halo stars (MPHSs), when the lifetime of the unstable particles is of the order ~ 108 − 1012 s depending on their initial abundance. We conclude that a combination of two different processes could explain the lithium isotopic abundances in MPHSs. First, a non-thermal cosmological nucleosynthesis associated with the radiative decay of unstable particles; and second, about the same degree of stellar depletion of both primordial lithium isotopic abundances. If MPHSs experience 6 Li depletion of factor much greater than ~ 3, the simple radiative decay process can not be the cause of large 6 Li abundances in MPHSs.

The pulsation modes and masses of carbon-rich long period variables

Following our study of the carbon-rich giants in the HR diagram and of their luminosity function (Paper III), we investigate the pulsation data of the long period variables (LPVs) included in our sample. Pulsation modes (fundamental, over- tone(s)) for carbon LPVs are identified in the period-radius diagram, making use of observed bi-periodicity in a small subsample of those stars, and of comparison to models. Mean pulsation masses are then deduced from theoretical PMR-relations, with due attention paid to a possible bias while averaging. Mean (present) pulsation masses (0:6 4:0 M) are found to increase along the group sequence HC5 to CV6, with still larger masses possibly associated with cool extreme CV7-objects with strong mass loss and thick circumstellar shells. This is con- sistent with the 0.8-4 M range of initial masses found in Paper III for the majority of carbon-rich giants aected by mass loss during their evolution. The pulsation masses found for a few HC-stars (M 0:8 M) are consistent with their low initial masses (Mi < 1:1 M); as inferred from their thick disk membership (age' 11 Gyr?) and locus in the HR diagram. A mean pulsation mass of'0:6 M is found for the three population II Cepheids in the sample. A mass-luminosity diagram is proposed for the Galactic carbon giants. The data from observations is found consistent with theoretical predictions from AGB modeling, specially the third dredge-up (TDU) through thermal pulses (TP) with a carbon star formation line (CSFL) for TP-AGB stars. It appears that the CV-giants are close to the tip and end of their evolutionary tracks in the TP-AGB of the HR diagram. It is confirmed that this end shifts toward lower eective temperatures and higher luminosities, with increasing masses. It is shown that the C/O abundance ratios do correlate with eective temperatures, according to three distinct distributions (halo CH stars, thick disk HC-stars, and thin disk CV-stars). The mean stellar density decreases along the HC5-CV7 sequence, while the surface gravity remains nearly constant at about 0.5 CGS unit logg' 0:3; 5 10 3 SI : The nature of (thin disk) CV-stars as TP-AGB objects being confirmed, the discussion is focused on (thick disk) HC-stars since the origin of these old low-mass giants remains unclear. Unpredicted extra mixing on RGB and/or E-AGB is favored. Evolution from (old, low O/H) dwarf carbon stars is also considered since observations of metal-poor stars and recent calculations point to large supersolar (C=Fe) ratios in Population III objects and contamination through rapid cycling in the interstellar medium.

Early Evolution of the Galactic Halo Revealed from [ITAL]Hipparcos[/ITAL] Observations of Metal-poor Stars

The kinematics of 122 red giant and 124 RR Lyrae stars in the solar neighborhood are studied using accurate measurements of their proper motions obtained by the Hipparcos astrometry satellite, combined with their published photometric distances, metal abundances, and radial velocities. A majority of these sample stars have metal abundances of [Fe/H] ≤ -1 and thus represent the old stellar populations in the Galaxy. The halo component, with [Fe/H] ≤ -1.6, is characterized by a lack of systemic rotation [(U, V, W) = (16 ± 18, -217 ± 21, -10 ± 12) km s-1] and a radially elongated velocity ellipsoid [(σU, σV, σW) = (161 ± 10, 115 ± 7, 108 ± 7) km s-1]. About 16% of such metal-poor stars have low orbital eccentricities (e < 0.4), and we see no evidence of a correlation between [Fe/H] and e. Based on the model for the e-distribution of orbits, we show that this fraction of low-e stars for [Fe/H] ≤ -1.6 is explained by the halo component alone, without introducing the extra disk component claimed by recent workers. This is also supported by the absence of a significant change in the e-distribution with height from the Galactic plane. In the intermediate-metallicity range (-1.6 < [Fe/H] ≤ -1), we find that stars with disklike kinematics have only modest effects on the distributions of rotational velocities and e for the sample at |z| < 1 kpc. This disk component appears to constitute only ~10% for -1.6 < [Fe/H] ≤ -1 and ~20% for -1.4 < [Fe/H] ≤ -1. It is also verified that this metal-weak disk has a mean rotation of ~195 km s-1 and a vertical extent of ~1 kpc, which is consistent with the thick disk's dominating at [Fe/H] = –0.6 to –1. We find no metallicity gradient in the halo, whereas there is an indication of a metallicity gradient in the metal-weak tail of the thick disk. The implications of these results for the early evolution of the Galaxy are also presented.