White Dwarf Population Research Articles

Chemical Abundance Constraints on White Dwarfs as Halo Dark Matter

We examine the chemical abundance constraints on a population of white dwarfs in the halo of our Galaxy. We are motivated by microlensing experiments that have reported evidence for massive compact halo objects (MACHOs) in the halo of our Galaxy, with an estimated mass of 0.1-1 M☉; the only conventional dark astrophysical candidates for objects in this mass range are white dwarfs. However, our work constrains white dwarfs in the halo regardless of what the MACHOs are. Further motivation for our work comes from the recent claimed possible detection of a large population of white dwarfs in the Hubble Deep Field. We focus on the composition of the material that would be ejected as the white dwarfs are formed. This material would bear the signatures of nucleosynthesis processing and contain abundance patterns that can be used to constrain white dwarf production scenarios. Using both analytical and numerical chemical evolution models, we confirm previous work that very strong constraints come from Galactic Population II and extragalactic carbon abundances. We also point out that in some cases, depending on the stellar model, significant nitrogen is produced rather than carbon. The combined constraints from carbon and nitrogen give ΩWDh 2 × 10-4 from comparison with the low abundances of these elements measured in the Lyα forest. We note, however, that these results are subject to uncertainties regarding the nucleosynthetic yields of low-metallicity stars. We thus investigate additional constraints from the light elements D and 4He, the nucleosynthesis of which is less uncertain. We find that these elements can be kept within observational limits only for ΩWD 0.003 and for a white dwarf progenitor initial mass function sharply peaked at low mass (2 M☉). Finally, we consider a Galactic wind, which is required to remove the ejecta accompanying white dwarf production from the galaxy. We show that such a wind can be driven by Type Ia supernovae arising from the white dwarfs themselves but find that these supernovae also lead to unacceptably large abundances of iron. The only ways we know of to avoid these constraints are that (1) the ejecta from low-metallicity MACHO progenitors are absent or completely unprocessed or (2) the processed ejecta remain as hot (0.3 keV) gas that is segregated from all observable neutral material to a precision of 99%. Aside from these loopholes, we conclude that abundance constraints exclude white dwarfs as MACHOs.

HUBBLE SPACE TELESCOPE Photometry of the Globular Cluster M4

This paper presents a detailed description of the acquisition and processing of a large body of imaging data for three fields in the globular cluster M4 taken with the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope. Analysis with the ALLFRAME package yielded the deepest photometry yet obtained for this cluster. The resulting data set for 4708 stars (positions and calibrated photometry in V, I, and, in two fields, U) spanning approximately six cluster core radii is presented. The scientific analysis is deferred to three companion papers, which investigate the significant white dwarf population discovered and the main-sequence population.

Monte Carlo simulations of the disc white dwarf population

In order to understand the dynamical and chemical evolution of our Galaxy it is of fundamental importance to study the local neighbourhood. White dwarf stars are ideal candidates to probe the history of the solar neighbourhood, since these `fossil' stars have very long evolutionary time-scales and, at the same time, their evolution is relatively well understood. In fact, the white dwarf luminosity function has been used for this purpose by several authors. However, a long-standing problem arises from the relatively poor statistics of the samples, especially at low luminosities. In this paper we assess the statistical reliability of the white dwarf luminosity function by using a Monte Carlo approach.

Pure Hydrogen Model Atmospheres for Very Cool White Dwarfs

Microlensing events observed in the line of sight toward the LMC indicate that a significant fraction of the mass of the dark halo of the Galaxy is probably composed of white dwarfs. In addition, white dwarf sequences have now be observed in the H-R diagrams of several globular clusters. Because of the unavailability of white dwarf atmospheres for Teff < 4000 K, the cooling timescales for white dwarfs older than ≈10 Gyr are very uncertain. Moreover, the identification of a MACHO white dwarf population by direct observation depends on a knowledge of the colors and bolometric corrections of very cool white dwarfs. In this Letter, we present the first detailed model atmospheres and spectra of very cool hydrogen white dwarfs for Teff < 4000 K. We include the latest description of the opacities of hydrogen, and, significantly, we introduce a nonideal equation of state in the atmosphere calculation. We find that due to strong absorption from H2 in the infrared, very old white dwarfs are brightest in the V, R, and I bands, and we confirm that they become bluer in most color indices as they cool below Teff ≈ 3500 K.

Massive compact halo objects viewed from a cosmological perspective: contribution to the baryonic mass density of the universe

We estimate the contribution of massive compact halo objects (MACHOs) and their stellar progenitors to the mass density of the Universe. If the MACHOs that have been detected reside in the halo of our Galaxy, then a simple extrapolation of the Galactic population (out to 50 kpc) of MACHOs to cosmic scales gives a cosmic density ϱ MACHO=(1–5)×10 9 hM ⊙Mpc −3, which in terms of the critical density corresponds to Ω MACHO = (0.0036-0.017)h −1 . Should the MACHO halo extend out to much further than 50 kpc, then Ω MACHO would only be larger. Such a mass density is comparable to the cosmic baryon density implied by Big Bang nucleosynthesis. If we take the central values of the estimates, then MACHOs dominate the baryonic content of the Universe today, with Ω MACHO Ω B ∼0.7 h . However, the cumulative uncertainties in the density determinations only require that Ω MACHO Ω B ≥ 1 6 hf gal′ , where the fraction of galaxies that contain MACHOs f gal>;0.17 and h is the Hubble constant in units of 100 km s −1 Mpc −1. Our best estimate for Ω MACHO is hard to reconcile with the current best estimates of the baryonic content of the intergalactic medium indicated by measurements of the Lyman-α forest; however, measurements of Ω Lyα are at present uncertain, so that such a comparison may be premature. If the MACHOs are white dwarfs resulting from a single burst of star formation (without recycling), then their main sequence progenitors would have been at least twice more massive: Ω ∗ = (0.007-0.034)h −1 . Thus, far too much gaseous baryonic material would remain in the Galaxy unless there is a Galactic wind to eject it. Indeed a MACHO population of white dwarfs and the gas ejected from their main sequence progenitors accounts for a significant fraction of all baryons. This fact must be taken into account when attempting to dilute the chemical by-products of such a large population of intermediate mass stars. We stress the difficulty of reconciling the MACHO mass budget with the accompanying carbon production in the case of white dwarfs. In the simplest picture, even if the excess carbon is ejected from the Galaxy by a Galactic wind, measurements of carbon abundances in Lyman α forest lines with values 10 −2 solar require that only about 10 −2 of all baryons can have passed through the white dwarf progenitors. Such a fraction can barely be accommodated by our estimates of Ω MACHO and would be in conflict with Ω ∗ .

The LMC Microlensing Events: Evidence for a Warped and Flaring Milky Way Disk?

The simplest interpretation of the microlensing events toward the Large Magellanic Cloud detected by the MACHO and EROS collaborations is that about one-third of the halo of our own Milky Way galaxy exists in the form of objects of around 0.5 M?. There are grave problems with this interpretation. A normal stellar population of 0.5 M? stars should be visible. The other obvious candidate for the lenses is a population of white dwarfs. However, the precursor population must have polluted the interstellar medium with metals, in conflict with current population II abundances. Here we propose a more conventional, but at the moment more speculative, explanation. Some of the lenses are stars in the disk of the Milky Way. They lie along the line of sight to the LMC because of warping and flaring of the Galactic disk. Depending on its scale length and ellipticity, the disk's optical depth may lie anywhere between 0.2?10 -->?7 and 0.9?10 -->?7. Together with contributions from the LMC disk and bar and perhaps even intervening stellar contaminants, the total optical depth may match the data within the uncertainties. Microlensing toward the LMC may be telling us more about the distorted structure and stellar populations of the outer Milky Way disk than the composition of the dark halo.

MACHOs, White Dwarfs, and the Age of the Universe

A favored interpretation of recent microlensing measurements toward the Large Magellanic Cloud implies that a large fraction (i.e., 10%-50%) of the mass of the Galactic dark halo is composed of white dwarfs. However, a ground-based search by Liebert and a recent search of the Hubble Deep Field by Flynn did not detect a substantial dark halo population of white dwarfs; thus the putative halo population is either dim enough or sparse enough to have eluded detection. In this paper we compare model white dwarf luminosity functions to the data from the observational surveys in order to determine a lower bound on the age of any substantial white dwarf halo population (and hence possibly on the age of the universe). In the course of our analysis we pay special attention to the velocity bias in the Liebert and coworkers, survey; we show that (and quantify by how much) the velocity bias renders the survey significantly less sensitive to a cool white dwarf population. We show that the minimum age of a white dwarf halo population depends most strongly on assumptions about three unknown quantities: (1) the white dwarfs' total space density, (2) their atmospheric composition, and (3) the initial mass function (IMF) of their progenitors. We compare various theoretical white dwarf luminosity functions, in which we vary these three parameters, with the abovementioned survey results. From this comparison, we conclude that if white dwarfs do indeed constitute more than 10% (30%) of the local halo mass density and are candidates for explaining the microlensing events, then the universe must be at least 10 Gyr old (12 Gyr old) for our most extreme allowed values of the parameters. When we use cooling curves that account for chemical fractionation and more likely values of the IMF and the bolometric correction, we find tighter limits; a white dwarf MACHO fraction of 10% (30%) requires a minimum age of 14 Gyr (15.5 Gyr). Our analysis also provides evidence that the halo white dwarfs have helium-dominated atmospheres, although this conclusion may change after low-temperature white dwarf atmospheres have been calculated.

The Neutron Star–Helium White Dwarf Population in the Galactic Disk

The population of low-mass binary pulsars in the Galactic disk has grown considerably in the past few years. By low mass, we mean that the companion to the neutron star is, or is very likely to be, a helium white dwarf. The distribution of these objects in the P- plane, where P is the rotation period, suggests both that their initial periods after spin-up are near 3 ms and that the age of the population is near 5 Gyr. This initial spin-up is similar to the inferred rotation rates in low-mass X-ray binaries (LMXBs) with dual-line quasi-periodic oscillations at kilohertz frequencies. Furthermore, the age of 5 Gyr is consistent with models of the LMXB population as evolving from stars with masses just above 1 M☉, although significant uncertainties remain in observationally derived birthrates of the radio and X-ray binaries. The observed distribution of minimum masses in the low-mass binary pulsar population requires a distribution of actual masses and is not the result of a random distribution of inclination angles. This distribution further leads to the conclusion that these millisecond pulsar systems do not have random inclination angles. A preference for orthogonal magnetic and spin axes is a possible configuration.

Further Constraints on White Dwarf Galactic Halos

The suggestion that roughly half the mass of the Galactic halo might be in the form of white dwarfs, together with the limits on its mass fraction in faint red stars and on the initial metallicity of the Galactic disk, would set strong constraints on the initial mass function (IMF) of the halo. Particular IMFs have been proposed to cut off both the numbers of low-mass stars contributing to the infrared background and of high-mass stars that contribute to the growth of metallicity when they promptly explode as gravitational collapse (Type II and Type Ib/c) supernovae. Here we examine the further contribution to metallicity from Type Ia (thermonuclear) supernovae that would later be produced from the halo white dwarf population. We find that, for most of the evolutionary scenarios for the Type Ia supernova progenitor systems proposed so far, constraints on the white dwarf mass fraction in the halo from the predicted production of iron would be extremely severe. When the predicted iron excess is not so large, the exceedingly high Type Ia supernova rate predicted for the present time would also exclude a major contribution of white dwarfs to the halo mass. The white dwarf contribution, in all cases, should be below 5%-10%. Besides, for the IMFs considered, the duration of the halo burst should be shorter than 1 Gyr in order to avoid too large a spread in the iron abundances of Population II halo dwarfs, and the predicted halo [O/Fe] ratio would be at odds with observations.

On the effects of dynamical evolution on the initial mass function of globular clusters

In this paper we show the results of a large set of N-body simulations modelling the evolution of globular clusters driven by relaxation,stellar evolution,disk shocking and including the effects of the tidal field of the Galaxy. We investigate the evolution of multi-mass models with a power-law initial mass function (IMF) starting with different initial masses, concentrations, slopes of the IMF and located at different galactocentric distances. We show to what extent the effects of the various evolutionary processes alter the shape of the IMF and to what extent these changes depend on the position of the cluster in the Galaxy. Both the changes in the global mass function and in the local one (measured at different distances from the cluster center) are investigated showing whether and where the local mass function keeps memory of the IMF and where it provides a good indication of the current global mass function. The evolution of the population of white dwarfs is also followed in detail and we supply an estimate of the fraction of the current value of the total mass expected to be in white dwarfs depending on the main initial conditions for the cluster (mass and position in the Galaxy).Simple analytical expression by which it is possible to calculate the main quantities of interest (total mass, fraction of white dwarfs, slope of the mass function) at any time t for a larger number of different initial conditions than those investigated numerically have been derived.

White Dwarfs in Globular Clusters:Hubble Space TelescopeObservations of M4

Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258 white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial distances from the cluster center were observed and sizeable WD populations were found in all three. The location of these WDs in the color-magnitude diagram, their mean mass of 0.51($ \pm 0.03$)M$_{\odot}$, and their luminosity function confirm basic tenets of stellar evolution theory and support the results from current WD cooling theory. The WDs are used to extend the cluster main-sequence mass function upward to stars that have already completed their nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and found to be at most a few percent of all the main-sequence stars. The most ancient WDs found are about 9 Gyr old, a level which is set solely by the photometric limits of our data. Even though this is less than the age of M4, we discuss how these cooling WDs can eventually be used to check the turnoff ages of globular clusters and hence constrain the age of the Universe.

Hot White Dwarfs in theExtreme Ultraviolet ExplorerSurvey. II. Mass Distribution, Space Density, and Population Age

We present new effective temperature and surface gravity determinations for a sample of 90 hot white dwarfs detected in the Extreme Ultraviolet Explorer (EUVE) all-sky survey. The measurements, based on spectroscopy of the Balmer line series obtained at the Michigan-Dartmouth-MIT Observatory, Mount Stromlo Observatory, Lick Observatory, and Cerro Tololo Inter-American Observatory, are used to constrain the space density as well as the population age and mass distribution of a sample of 110 EUV-selected DA white dwarfs in the solar neighborhood. We find a mass spectrum narrowly peaked over 0.56 M☉, indicative of a C-O core with a thin hydrogen layer, and a significant population of 10 ultramassive (M ≥ 1.1 M☉) white dwarfs; we also find that all objects fall between effective temperatures of ≈ 25,000 and ≈ 75,000 K and that most are younger than 30 Myr. Using Wood's evolutionary models we determine a DA white dwarf birthrate in the solar neighborhood of (0.7-1.0) × 10-12 pc-3 yr-1. Although most objects are on normal C-O cooling tracks, we suggest that a few low-mass white dwarfs and the population of ultramassive white dwarfs may follow different paths with, respectively, He, and, possibly, O-Ne-Mg cores.

Evidence for a new class of extreme ultraviolet sources

Most of the sources detected in the extreme ultraviolet (EUV; 100 Ang to 600 Ang) by the Rosat WFC and EUVE all-sky surveys have been identified with active late-type stars and hot white dwarfs that are near enough to escape absorption by interstellar gas. However, about 15% of EUV sources are as of yet unidentified with any optical counterparts. We examine whether the unidentified EUV sources may consist of the same population of late-type stars and white dwarfs. We present B and R photometry of stars in the fields of seven of the unidentified EUV sources. We detect in the optical the entire main-sequence and white-dwarf population out to the greatest distances where they could still avoid absorption. We use colour-magnitude diagrams to demonstrate that, in most of the fields, none of the observed stars have the colours and magnitudes of late-type dwarfs at distances less than 100 pc. Similarly, none are white dwarfs within 500 pc that are hot enough to be EUV-emitters. The unidentified EUV sources we study are not detected in X-rays, while cataclysmic variables, X-ray binaries, and active galactic nuclei generally are. We conclude that some of the EUV sources may be a new class of nearby objects, that are either very faint at optical bands or which mimic the colours and magnitudes of distant late-type stars or cool white dwarfs. One candidate for optically faint objects is isolated old neutron stars, slowly accreting interstellar matter. Such neutron stars are expected to be abundant in the Galaxy, and have not been unambiguously detected.

Contribution of Brown Dwarfs and White Dwarfs to Recent Microlensing Observations and to the Halo Mass Budget

We examine the recent results of the MACHO collaboration toward the Large Magellanic Cloud in terms of a halo brown dwarf or white dwarf population. The possibility that most of the microlensing events are due to brown dwarfs is totally excluded by large-scale kinematic properties. The white dwarf scenario is examined in detail in the context of the most recent white dwarf cooling theory which explicitly takes into account the extra energy source that derives from the carbon-oxygen differentiation at crystallization and the subsequent Debye cooling. We show that the observational constraints arising from the luminosity function of high-velocity white dwarfs in the solar neighborhood and from the recent Hubble Space Telescope deep field counts are consistent with a white dwarf contribution to the halo missing mass as large as 50%, provided there is (1) an initial mass function strongly peaked around ~1.7 M☉, and (2) a halo age older than ~18 Gyr.

Limits on the Halo White Dwarf Component of Baryonic Dark Matter from the Hubble Deep Field

The MACHO collaboration lensing event statistics suggest that a significant fraction of the dark Galactic halo can be comprised of baryonic matter in the form of white dwarf stars with masses between 0.1 and 1.0 M☉. Such a halo white dwarf population, in order to have escaped detection by those who observe the white dwarf luminosity function of the disk, must have formed from an old population. The observations indicate that the number of halo white dwarfs per cubic parsec per unit bolometric magnitude is less than 10-5 at 10-4.5 L☉; the number must rise significantly at lower luminosities to provide the needed baryonic halo mass. Such white dwarfs may easily escape detection in most current and earlier surveys. Though it is limited in angular extent, the Hubble Deep Field (HDF) probes a sufficient volume of the Galactic halo to provide interesting limits on the number of halo white dwarf stars and on the fraction of the halo mass that they can make up. If the HDF field can be probed for stars down to V = 29.8 then the MACHO result suggests that there could be up to 12 faint halo white dwarfs visible in the HDF. Finding (or not finding) these stars in turn places interesting constraints on star formation immediately following the formation of the galaxy.

Cooling white dwarfs in galactic globulars

view Abstract Citations (12) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cooling White Dwarfs in Galactic Globulars Castellani, V. ; degl'Innocenti, S. ; Romaniello, M. Abstract We study the evolution of hot horizontal-branch models which miss the asymptotic branch (AGB) phase to produce 'low-mass' white dwarfs (WDs) with masses of the order of 0.5 solar mass. One finds that for each given metallicity there exists a maximum value for the mass of the H-rich envelope allowing the models to approach the cooling phase without the onset of CNO H-burning flashes; the larger the metallicity, the lower the mass of such a 'critical envelope.' Moreover, we find that above log L is approximately 1 the cooling rate appears drastically affected by the previous evolutionary history, allowing a meaningful discussion of the cooling laws only below such a luminosity. The dependence of the cooling rates on the mass of the H-rich envelope is investigated for suitable assumptions about the value of this parameter, discussing the contribution of both CNO and p-p H burning to the evolution. The investigation has been further extended to stars with larger He cores, showing that the final cooling structures closely approach available evaluations of post-AGB CO dwarfs, as the result of a convergence toward 'canonical' self-consistent WD structrues. On this basis, we discuss the influence of WD masses on both the cooling rate and the H-R diagram location of the cooling sequence. The application of this theoretical scenario to the WD population in stellar clusters is finally discussed. Publication: The Astrophysical Journal Pub Date: March 1994 DOI: 10.1086/173804 Bibcode: 1994ApJ...423..266C Keywords: Astronomical Models; Globular Clusters; Horizontal Branch Stars; White Dwarf Stars; Hertzsprung-Russell Diagram; Metallicity; Stellar Interiors; Stellar Luminosity; Astrophysics; GALAXY: GLOBULAR CLUSTERS: GENERAL; STARS: EVOLUTION; STARS: INTERIORS; STARS: WHITE DWARFS full text sources ADS |

Dark halos consist of compact stellar remnants

I present the arguments for dark halo matter to consist of stellar remnants, the most innocuous form of which is a population of primordial massive white dwarfs. Several observational tests are described, including the implications of merging into neutron stars of a subset of the white dwarfs that originally underwent tidal capture in photoclusters to form close binaries. Halo neutron stars are born with considerable momentum to form high velocity radio pulsars and an extended isotropic distribution of gamma ray burst sources.

The halo infall timescale

There is an accumulating body of observational evidence which seems to support the view that the duration of the phase of star formation and nucleosynthesis activity in the halo of our Galaxy may have been of the order of 2–3 × 10 9 years. Such evidence arises from abundance determinations for metal-deficient field halo stars and globular cluster stars, as well as from age determinations for the local white dwarf population and for the halo and disk populations of globular clusters. We briefly review these diverse observational constraints on the timescale of halo evolution and discuss some possible implications for dynamical models for the collapse of the halo and the onset of star formation activity in the Galactic disk.

The Evolution of Stars on the AGB: The Mass Loss Intensity and the Formation of Carbon Stars

Simulated populations of white dwarfs and N type carbon stars were generated for a Salpeter initial mass function and constant stellar birth rate history. The effect of very strong mass loss on the mass distribution of white dwarfs and the luminosity distribution of carbon stars is discussed and the results are compared with observations. A significant mass loss by stars on the TP-AGB occurs besides regular stellar wind and planetary nebulae ejection. Thus it is possible to explain the luminosity functions of carbon and M stars in the Magellanic Clouds (with very few stars brighter than Mbol = -6.0), the very narrow mass distribution of white dwarfs, and the very small number of white dwarfs with M &gt; 1.0 MΘ. The luminosity of some AGB stars in the SMC is so high that they may be supernova of type 1 1/2 precursors. There are no such stars in the LMC. Comparison of the theoretical and observed luminosity distributions of high-luminosity AGB stars in the Magellanic Clouds shows that the mass-loss rate of these stars in the LMC is about an order of magnitude larger than in the SMC. In the Galaxy carbon stars may form only from stars with initial mass less than 1.5 MΘ due to the relatively small initial heavy element abundance in these stars; this is perhaps the main reason for the absence of carbon stars in open clusters in the Galaxy.

An Upper Limit to the Space Density of Short-period Noninteracting Binary White Dwarfs

view Abstract Citations (77) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Upper Limit to the Space Density of Short-period Noninteracting Binary White Dwarfs Robinson, Edward L. ; Shafter, Allen W. Abstract We have searched for short-period, detached binary stars in which both stars are white dwarfs by looking for radial velocity variations in spectroscopically identified DA and DB white dwarfs. The search was sensitive to binaries with orbital periods between 30 s and 3 hr and, within that range, we would have detected roughly 90% of all binaries, depending on the distribution of their orbital periods, mass ratios, and spectral types. We observed 44 stars without finding any binaries. The fraction of white dwarfs that are binaries is less than 1/20 with a 90% probability and less than 1/37 with a 70% probability. Other surveys sensitive to longer periods have also failed to find a large population of binary white dwarfs. The space density of binary white dwarfs is too low to account for the rate of Type I supernovae in the Galaxy and too low to agree comfortably with recent estimates of their numbers from theoretical calculations of the evolution of close binaries. Binary white dwarfs are the dominant source of the background gravitational waves at periods between about 30 s and 1 hr. The background is likely to be much lower than previously thought. Publication: The Astrophysical Journal Pub Date: November 1987 DOI: 10.1086/165725 Bibcode: 1987ApJ...322..296R Keywords: STARS: BINARIES; STARS: STELLAR STATISTICS; STARS: SUPERNOVAE; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (38)