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Register a new userIf the MACHOs are Old White Dwarfs
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The microlensing experiments in the direction of the LMC seem to be indicating that about 60% of the dark matter in the Galactic halo is tied up in objects whose masses are about half the mass of the Sun. This mass is a natural one for old white dwarfs although other possibilities do exist. Using a grid of newly constructed models of cooling white dwarfs which incorporate for the first time the effects of molecular opacity in the stellar atmosphere, and assuming that such white dwarfs make up the entire Galactic dark matter, I predict the numbers of old white dwarfs expected in various surveys currently being conducted. In particular, I note the number to be expected in the Hubble Deep Field (HDF), the deepest image of the sky yet obtained.
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Using a new grid of models of cooling white dwarfs, we calculate isochrones and luminosity functions in the Johnson-Kron/Cousins and HST filter sets for systems containing old white dwarfs. These new models incorporate a non-grey atmosphere which is necessary to properly describe the effects of molecular opacity at the cool temperatures of old white dwarfs. The various functions calculated and extensively tabulated and plotted are meant to be as utilitarian as possible for observers so all results are listed in quantities that observers will obtain. The tables and plots developed should eventually prove critical in interpreting the results of HSTs Advanced Camera observations of the oldest white dwarfs in nearby globular clusters, in understanding the results of searches for old white dwarfs in the Galactic halo, and in determining ages for star clusters of all ages using white dwarfs. As a practical application we demonstrate the use of these results by deriving the white dwarf cooling age of the old Galactic cluster M67.
We report the discovery of three nearby old halo white dwarf candidates in the Sloan Digital Sky Survey (SDSS), including two stars in a common proper motion binary system. These candidates are selected from our 2800 square degree proper motion survey on the Bok and U.S. Naval Observatory Flagstaff Station 1.3m telescopes, and they display proper motions of 0.4-0.5 arcsec/yr. Follow-up MMT spectroscopy and near-infrared photometry demonstrate that all three objects are hydrogen-dominated atmosphere white dwarfs with Teff = 3700 - 4100 K. For average mass white dwarfs, these temperature estimates correspond to cooling ages of 9-10 Gyr, distances of 70-80 pc, and tangential velocities of 140-200 km/s. Based on the UVW space velocities, we conclude that they most likely belong to the halo. Furthermore, the combined main-sequence and white dwarf cooling ages are 10-11 Gyr. Along with SDSS J1102+4113, they are the oldest field white dwarfs currently known. These three stars represent only a small fraction of the halo white dwarf candidates in our proper motion survey, and they demonstrate that deep imaging surveys like the Pan-STARRS and Large Synoptic Survey Telescope should find many old thick disk and halo white dwarfs that can be used to constrain the age of the Galactic thick disk and halo.
In a previous study, we analysed the spectra of 230 cool ($T_mathrm{eff}$ < 9000 K) white dwarfs exhibiting strong metal contamination, measuring abundances for Ca, Mg, Fe and in some cases Na, Cr, Ti, or Ni. Here we interpret these abundances in terms of the accretion of debris from extrasolar planetesimals, and infer parent body compositions ranging from crust-like (rich in Ca and Ti) to core-like (rich in Fe and Ni). In particular, two white dwarfs, SDSSJ0823+0546 and SDSSJ0741+3146, which show log[Fe/Ca] > 1.9 dex, and Fe to Ni ratios similar to the bulk Earth, have accreted by far the most core-like exoplanetesimals discovered to date. With cooling ages in the range 1-8 Gyr, these white dwarfs are among the oldest stellar remnants in the Milky Way, making it possible to probe the long-term evolution of their ancient planetary systems. From the decrease in maximum abundances as a function of cooling age, we find evidence that the arrival rate of material on to the white dwarfs decreases by 3 orders of magnitude over a $simeq$6.5 Gyr span in white dwarf cooling ages, indicating that the mass-reservoirs of post-main sequence planetary systems are depleted on a $simeq$1 Gyr e-folding time-scale. Finally, we find that two white dwarfs in our sample are members of wide binaries, and both exhibit atypically high abundances, thus providing strong evidence that distant binary companions can dynamically perturb white dwarf planetary systems.
The old, solar metallicity open cluster Messier 67 has long been considered a lynchpin in the study and understanding of the structure and evolution of solar-type stars. The same is arguably true for stellar remnants - the white dwarf population of M67 provides crucial observational data for understanding and interpreting white dwarf populations and evolution. In this work, we determine the white dwarf masses and derive their progenitor star masses using high signal-to-noise spectroscopy of warm ($gtrsim10,000$ K) DA white dwarfs in the cluster. From this we are able to derive each white dwarfs position on the initial-final mass relation, with an average $M_{mathrm WD} = 0.60pm 0.01 M_{odot}$ and progenitor mass $M_i = 1.52pm 0.04 M_{odot}$. These values are fully consistent with recently published linear and piecewise linear fits to the semi-empirical initial-final mass relation and provide a crucial, precise anchor point for the initial-final mass relation for solar-metallicity, low-mass stars. The mean mass of M67 white dwarfs is also consistent with the sharp narrow peak in the local field white dwarf mass distribution, indicating that a majority of recently-formed field white dwarfs come from stars with progenitor masses of $approx 1.5 M_{odot}$. Our results enable more precise modeling of the Galactic star formation rate encoded in the field WD mass distribution.
Using a novel approach we have reanalized the question of whether the extreme star forming galaxies known as HII galaxies are truly young or rejuvenated old systems. We first present a method of inversion that applies to any monotonic function of time describing the evolution of independent events. We show that, apart from a normalization constant, the ``true time dependence can be recovered from the inversion of its probability density function. We applied the inversion method to the observed equivalent width of Hbeta (EW(Hbeta)) distribution for objects in the Terlevich and collaborators Spectrophotometric Catalogue of HII galaxies and found that their global history of star formation behaves much closer to the expectations of a continuos star formation model than to an instantaneous one. On the other hand, when the inversion method is applied to samples within a restricted metallicity range we find that their history of star formation behaves much closer to what the instantaneous model predicts. Our main conclusion is that, globally, the evolution of HII galaxies seems consistent with a succession of short starbursts separated by quiescent periods and that, while the emission lines trace the properties of the present burst, the underlying stellar continuum traces the whole star formation history of the galaxy. Thus, observables like the EW(Hbeta) that combine an emission line flux, i.e. a parameter pertaining to the present burst, with the continuum flux, i.e. a parameter that traces the whole history of star formation, should not be used alone to characterize the present burst.
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