The Future is Now: the Formation of Single Low Mass White Dwarfs in the Solar Neighborhood

Low mass helium-core white dwarfs (M < 0.45 Msun) can be produced from interacting binary systems, and traditionally all of them have been attributed to this channel. However, a low mass white dwarf could also result from a single star that experiences severe mass loss on the first ascent giant branch. A large population of low mass He-core white dwarfs has been discovered in the old metal-rich cluster NGC 6791. There is therefore a mechanism in clusters to produce low mass white dwarfs without requiring binary star interactions, and we search for evidence of a similar population in field white dwarfs. We argue that there is a significant field population (of order half of the detected systems) that arises from old metal rich stars which truncate their evolution prior to the helium flash from severe mass loss. There is a consistent absence of evidence for nearby companions in a large fraction of low mass white dwarfs. The number of old metal-rich field dwarfs is also comparable with the apparently single low mass white dwarf population, and our revised estimate for the space density of low mass white dwarfs produced from binary interactions is also compatible with theoretical expectations. This indicates that this channel of stellar evolution, hitherto thought hypothetical only, has been in operation in our own Galaxy for many billions of years. One strong implication of our model is that single low mass white dwarfs should be good targets for planet searches because they are likely to arise from metal-rich progenitors. We also discuss other observational tests and implications, including the potential impact on SN Ia rates and the frequency of planetary nebulae.Comment: ApJ published versio

Resolving the Formation of Protogalaxies. III. Feedback from the First Stars

The first stars form in dark matter halos of masses ~10^6 M_sun as suggested by an increasing number of numerical simulations. Radiation feedback from these stars expels most of the gas from their shallow potential well of their surrounding dark matter halos. We use cosmological adaptive mesh refinement simulations that include self-consistent Population III star formation and feedback to examine the properties of assembling early dwarf galaxies. Accurate radiative transport is modeled with adaptive ray tracing. We include supernova explosions and follow the metal enrichment of the intergalactic medium. The calculations focus on the formation of several dwarf galaxies and their progenitors. In these halos, baryon fractions in 10^8 solar mass halos decrease by a factor of 2 with stellar feedback and by a factor of 3 with supernova explosions. We find that radiation feedback and supernova explosions increase gaseous spin parameters up to a factor of 4 and vary with time. Stellar feedback, supernova explosions, and H_2 cooling create a complex, multi-phase interstellar medium whose densities and temperatures can span up to 6 orders of magnitude at a given radius. The pair-instability supernovae of Population III stars alone enrich the halos with virial temperatures of 10^4 K to approximately 10^{-3} of solar metallicity. We find that 40% of the heavy elements resides in the intergalactic medium (IGM) at the end of our calculations. The highest metallicity gas exists in supernova remnants and very dilute regions of the IGM.Comment: 15 pages, 16 figures, accepted to ApJ. Many changes, including estimates of metal line cooling. High resolution images and movies available at http://www.slac.stanford.edu/~jwise/research/PGalaxies3

Constraining the Metallicity of the Low Density Lyman-alpha Forest Using OVI Absorption

We search for OVI absorption in a Keck HIRES spectrum of the z=3.62 quasar Q1422+231. Comparison of CIV measurements to cosmological simulations shows that \lya forest absorbers with N_HI > 10^{14.5} have [C/H]~=-2.5, for the UV background spectrum of Haardt & Madau (HM). Lower column density absorption arises in lower density gas, where OVI is the most sensitive metal tracer. Since OVI lines lie at wavelengths contaminated by Lyman series absorption, we interpret our Q1422 results by comparing to artificial spectra drawn from an SPH simulation of a Lambda-dominated CDM model. A search for deep, narrow features in Q1422 yields only a few candidate OVI lines, statistically consistent with the number in artificial spectra with no metals; spectra generated with the HM background and [O/H] >= -2.5 predict too many narrow lines. However, applying the optical depth ratio technique of Songaila (1998), we DO find significant OVI associated with CIV systems; matching Q1422 requires [O/C]~=+0.5, implying [O/H]~=-2.0. Taken together these results imply that (a) the metallicity in the low density IGM is at least a factor of three below that in the overdense regions where CIV absorption is detectable, and (b) oxygen is overabundant in these regions, consistent with the enrichment pattern of old halo stars. If the UV background is heavily truncated above 4 Ry, an implausibly high oxygen overabundance ([O/C]>+2) is required by the data; thus a majority of the volume of the universe must have undergone helium reionization by z=3.(Abridged)Comment: Submitted to ApJ, 48 pp including 14 ps figures, uses aaspp4.st

NLTE Strontium and Barium in metal poor red giant stars

We present atmospheric models of red giant stars of various metallicities, including extremely metal poor (XMP, [Fe/H]<-3.5) models, with many chemical species, including, significantly, the first two ionization stages of Strontium (Sr) and Barium (Ba), treated in Non-Local Thermodynamic Equilibrium (NLTE) with various degrees of realism. We conclude that 1) for all lines that are useful Sr and Ba abundance diagnostics the magnitude and sense of the computed NLTE effect on the predicted line strength is metallicity dependent, 2) the indirect NLTE effect of overlap between Ba and Sr transitions and transitions of other species that are also treated in NLTE non-negligibly enhances NLTE abundance corrections for some lines, 3) the indirect NLTE effect of NLTE opacity of other species on the equilibrium structure of the atmospheric model is not significant, 4) the computed NLTE line strengths differ negligibly if collisional b-b and b-f rates are an order of magnitude smaller or larger than those calculated with standard analytic formulae, and 5) the effect of NLTE upon the resonance line of Ba II at 4554.03 AA is independent of whether that line is treated with hyperfine splitting. As a result, the derivation of abundances of Ba and Sr for metal-poor red giant stars with LTE modeling that are in the literature should be treated with caution.Comment: 28 pages, 10 figures. Accepted for publication in April 2006 Astrophysical Journa

Possible Stellar Metallicity Enhancements from the Accretion of Planets

A number of recently discovered extrasolar planet candidates have surprisingly small orbits, which may indicate that considerable orbital migration takes place in protoplanetary systems. A natural consequence of orbital migration is for a series of planets to be accreted, destroyed, and then thoroughly mixed into the convective envelope of the central star. We study the ramifications of planet accretion for the final main sequence metallicity of the star. If maximum disk lifetimes are on the order of 10 Myr, stars with masses near 1 solar mass are predicted to have virtually no metallicity enhancement. On the other hand, early F and late A type stars with masses of 1.5--2.0 solar masses can experience significant metallicity enhancements due to their considerably smaller convection zones during the first 10 Myr of pre-main-sequence evolution. We show that the metallicities of an aggregate of unevolved F stars are consistent with an average star accreting about 2 Jupiter-mass planets from a protoplanetary disk having a 10 Myr dispersal time.Comment: 14 pages, AAS LaTeX, 3 figures, accepted to ApJ Letter

Extended Stromgren Photoelectric Photometry in NGC 752

Photoelectric photometry on the extended Stromgren system (uvbyCa) is presented for 7 giants and 21 main sequence stars in the old open cluster, NGC 752. Analysis of the hk data for the turnoff stars yields a new determination of the cluster mean metallicity. From 10 single-star members, [Fe/H] = -0.06 +/- 0.03, where the error quoted is the standard error of the mean and the Hyades abundance is set at [Fe/H] = +0.12. This result is unchanged if all 20 stars within the limits of the hk metallicity calibration are included. The derived [Fe/H] is in excellent agreement with past estimates using properly-zeroed m1 data, transformed moderate-dispersion spectroscopy, and recent high dispersion spectroscopy.Comment: 14 tex'd pages including 2 tables; 2 separate files with eps figures Accepted for PASP March 200

The IntraCluster Medium: An Invariant Stellar IMF

Evidence supporting the hypothesis of an invariant stellar Initial Mass Function is strong and varied. The intra-cluster medium in rich clusters of galaxies is one of the few contrary locations where recent interpretations of the chemical abundances have favoured an IMF that is biased towards massive stars, compared to the `normal' IMF. This interpretation hinges upon the neglect of Type Ia supernovae to the ICM enrichment, and a particular choice of the nucleosynthesis yields of Type II supernovae. We demonstrate here that when one adopts yields determined empirically from observations of Galactic stars, rather than the uncertain model yields, a self-consistent picture may be obtained with an invariant stellar IMF, and about half of the iron in the ICM being produced by Type Ia supernovae.Comment: 9 pages, LateX (aaspp4 macro), including one postscript figure. Accepted, ApJ Letter

LiBeB: High and low energy cosmic ray production and comparison with neutrino induced nucleosynthesis in SNII

We present new calculations of LiBeB production by accelerated particles with various compositions and energy spectra ranging from low energies to relativistic energies, and various ambient medium metallicities ($Z/Z_\odot$). The observed, essentially constant, Be/Fe ratio as a function of $Z/Z_\odot$ can be best understood if the metallicity of the accelerated particles (high energy or low energy) does not vary much with $Z/Z_\odot$. This could be achieved if the particles are accelerated directly from the ejecta of Type II supernovae (SNII) and not from the interstellar medium. Using the observed Be/Fe and the fact that most of the Fe at low $Z/Z_\odot$ is from SNII, we derive the energy content in accelerated particles per SNII (a few times 10$^{50}$ erg). We show that additional $^{11}$B production by neutrinos is consistent with the available data, allowing a neutrino yield from SNII less than or equal to the nominal published yields. We further show that the observed $^6$Li at low $Z/Z_\odot$ suggests that the accelerated particles responsible for the LiBeB at such $Z/Z_\odot$ are confined to low energies and have a relatively high He/O abundance.Comment: 9 pages with 4 emdedded figures, uuencoded, compressed, tared, style files included. In press Nuclei In the Cosmos, Nuclear Physics, Conference Proceeding

Chemical abundances for 11 bulge stars from high-resolution, near-IR spectra

It is debated whether the Milky Way bulge has the characteristics of a classical bulge sooner than those of a pseudobulge. Detailed abundance studies of bulge stars is a key to investigate the origin, history, and classification of the bulge. The aim is to add to the discussion on the origin of the bulge and to study detailed abundances determined from near-IR spectra for bulge giants already investigated with optical spectra, the latter also providing the stellar parameters which are very significant for the results of the present study. Especially, the important CNO elements are better determined in the near-IR. High-resolution, near-infrared spectra in the H band are recorded using the CRIRES spectrometer on the Very Large Telescope. The CNO abundances can all be determined from the numerous molecular lines in the wavelength range observed. Abundances of the alpha elements are also determined from the near-IR spectra. [O/Fe], [Si/Fe] and [S/Fe] are enhanced up to metallicities of at least [Fe/H]=-0.3, after which they decline. This suggests that the Milky Way bulge experienced a rapid and early star-formation history like that of a classical bulge. However, a similarity between the bulge trend and the trend of the local thick disk seems present. Such a similarity could suggest that the bulge has a pseudobulge origin. Our [C/Fe] trend does not show any increase with [Fe/H] which could have been expected if W-R stars have contributed substantially to the C abundances. No "cosmic scatter" can be traced around our observed abundance trends; the scatter found is expected, given the observational uncertainties.Comment: Accepted for publication in A&