Cool White Dwarfs Research Articles

The magnetic and metallic degenerate G77-50

An accumulation of multi-epoch, high-resolution, optical spectra reveal that the nearby star G77-50 is a very cool DAZ white dwarf externally polluted by Mg, Fe, Al, Ca, and possibly Na, Cr, Mn. The metallic and hydrogen absorption features all exhibit multiple components consistent with Zeeman splitting in a B~120kG magnetic field. Ultraviolet through infrared photometry combined with trigonometric parallaxes yield Teff=5310K, M=0.60Msun, and a cooling age of 5.2Gyr. The space velocity of the white dwarf suggests possible membership in the Galactic thick disk, consistent with an estimated total age of 8.6Gyr. G77-50 is spectrally similar to G165-7 and LHS 2534; these three cool white dwarfs comprise a small group exhibiting both metals and magnetism. The photospheric metals indicate accretion of rocky debris similar to that contained in asteroids, but the cooling age implies a remnant planetary system should be stable. A possibility for G77-50 and similarly old, polluted white dwarfs is a recent stellar encounter that dynamically rejuvenated the system from the outside-in. Metal abundance measurements for these cooler white dwarfs have the potential to distinguish material originating in outer region planetesimals injected via fly-by. If common envelope evolution can generate magnetic fields in white dwarfs, then G77-50 and its classmates may have cannibalized an inner giant planet during prior evolution, with their metals originating in terrestrial bodies formed further out. Although speculative, this scenario can be ruled out if terrestrial planet formation is prohibited in systems where a giant planet has migrated to the inner region nominally engulfed during the post-main sequence.

THE DISCOVERY OF THE MOST METAL-RICH WHITE DWARF: COMPOSITION OF A TIDALLY DISRUPTED EXTRASOLAR DWARF PLANET

Cool white dwarf stars are usually found to have an outer atmosphere that is practically pure in hydrogen or helium. However, a small fraction have traces of heavy elements that must originate from the accretion of extrinsic material, most probably circumstellar matter. Upon examining thousands of Sloan Digital Sky Survey (SDSS) spectra, we discovered that the helium-atmosphere white dwarf SDSS J073842.56+183509.6 shows the most severe metal pollution ever seen in the outermost layers of such stars. We present here a quantitative analysis of this exciting star by combining high signal-to-noise ratio follow-up spectroscopic and photometric observations with model atmospheres and evolutionary models. We determine the global structural properties of our target star, as well as the abundances of the most significant pollutants in its atmosphere, i.e., H, O, Na, Mg, Si, Ca, and Fe. The relative abundances of these elements imply that the source of the accreted material has a composition similar to that of Bulk Earth. We also report the signature of a circumstellar disk revealed through a large infrared excess in JHK photometry. Combined with our inferred estimate of the mass of the accreted material, this strongly suggests that we are witnessing the remains of a tidally disrupted extrasolar body that was as large as Ceres.

ASTROMETRY WITH THEHUBBLE SPACE TELESCOPE: TRIGONOMETRIC PARALLAXES OF PLANETARY NEBULA NUCLEI NGC 6853, NGC 7293, ABELL 31, AND DeHt 5

We present absolute parallaxes and relative proper motions for the central stars of the planetary nebulae NGC 6853 (The Dumbbell), NGC 7293 (The Helix), Abell 31, and DeHt 5. This paper details our reduction and analysis using DeHt 5 as an example. We obtain these planetary nebula nuclei (PNNi) parallaxes with astrometric data from Fine Guidance Sensors FGS 1R and FGS 3, white-light interferometers on the Hubble Space Telescope (HST). Proper motions, spectral classifications and VJHKT_2M and DDO51 photometry of the stars comprising the astrometric reference frames provide spectrophotometric estimates of reference star absolute parallaxes. Introducing these into our model as observations with error, we determine absolute parallaxes for each PNN. Weighted averaging with previous independent parallax measurements yields an average parallax precision, \sigma_{\pi}/\pi = 5 %. Derived distances are: d_{NGC 6853}=405^{+28}_{-25}pc, d_{NGC 7293}=216^{+14}_{-12} pc, d_{Abell 31} = 621^{+91}_{-70} pc, and d_{DeHt 5} = 345^{+19}_{-17} pc. These PNNi distances are all smaller than previously derived from spectroscopic analyses of the central stars. Derived absolute magnitudes and previously measured effective temperatures permit estimates of PNNi radii, through both the Stefan-Boltzmann relation and Eddington fluxes. Comparing absolute magnitudes with post-AGB models provides mass estimates. Masses cluster around 0.57 M(sun), close to the peak of the white dwarf mass distribution. Adding a few more PNNi with well-determined distances and masses, we compare all the PNNi with cooler white dwarfs of similar mass, and confirm, as expected, that PNNi have larger radii than white dwarfs that have reached their final cooling tracks. (Abridged)

The First Mid‐Infrared Spectra of Cool White Dwarfs

We present the first mid-infrared spectra of two cool white dwarfs obtained with the Spitzer Space Telescope. We also present 3.5-8 micron photometry for 19 cool white dwarfs with 5000K < Teff < 9000K. We perform a detailed model atmosphere analysis of these white dwarfs by fitting their UBVRIJHK and Spitzer photometry with state-of-the-art model atmospheres, and demonstrate that the optical and infrared spectral energy distributions of cool white dwarfs are well reproduced by our grid of models. Our mid-IR photometry and 7.5-14.5 micron spectrum of WD0018-267 are consistent with a Teff = 5720K, pure hydrogen white dwarf model atmosphere. On the other hand, LHS 1126 remains peculiar with significant mid-IR flux deficits in all IRAC bands and a featureless spectrum in the 5.2-7.5 micron range. Even though this deficit is attributed to collision induced absorption (CIA) due to molecular hydrogen, the shape of the deficit cannot be explained with current CIA opacity calculations. The infrared portion of the LHS 1126 spectral energy distribution is best-fit with a power law index of -1.99; identical to a Rayleigh-Jeans spectrum. This argues that the deficit may be due to an unrecognized grey-like opacity source in the infrared.

LP 133‐373: A New Chromospherically Active Eclipsing dMe Binary with a Distant, Cool White Dwarf Companion

We report the discovery of the partially eclipsing binary LP 133-373. Nearly identical eclipses along with observed photometric colors and spectroscopy indicate that it is a pair of chromospherically active dM4 stars in a circular 1.6 d orbit. Light and velocity curve modeling to our differential photometry and velocity data show that each star has a mass and radius of 0.340+/-0.014 M_sun and 0.33+/-0.02 R_sun. The binary is itself part of a common proper motion pair with LP 133-374 a cool DC or possible DA white dwarf with a mass of 0.49-0.82 M_sun, which would make the system at least 3 Gyr old.

He Conductivity in Cool White Dwarf Atmospheres

We investigate the conductivity of warm dense helium under conditions found in the atmospheres of cool white dwarfs using ab initio simulations. The calculations performed consist of quantum molecular dynamics simulations where the electronic wavefunction at each time step is obtained using density functional theory, while the ion trajectories are calculated using the resulting quantum mechanical forces. We use both conventional DFT (PW91) and hybrid (PBE0) functionals to calculate the conductivities that provide an estimate of the ionization fraction. While the calculations are in good agreement with the measurements for the equation of state, a significant discrepancy exists with the recently measured conductivity.

The Pseudocontinuum Bound‐Free Opacity of Hydrogen and Its Importance in Cool White Dwarf Atmospheres

We investigate the importance of the pseudocontinuum bound-free opacity from hydrogen atoms in the atmospheres of cool white dwarfs. This source of absorption, when calculated by the occupation probability formalism applied in the modeling of white dwarf atmospheres with Teff < 17,000 K, dominates all other sources of opacity at optical wavelengths. This is unrealistic and not observed. On the other hand, a significant flux suppression in the blue part of the spectra of cool white dwarfs has been reported and mainly interpreted as a result of the pseudocontinuum absorption from atomic hydrogen. We investigate this problem by proposing a new, more realistic approach to calculating this source of opacity. We show that this absorption is orders of magnitude smaller than that predicted by current methods. Therefore, we rule out the pseudocontinuum opacity as a source of the flux deficiency observed in the spectra of cool white dwarfs.

Probing the Faintest Stars in a Globular Star Cluster

NGC 6397 is the second closest globular star cluster to the Sun. Using 5 days of time on the Hubble Space Telescope, we have constructed an ultradeep color-magnitude diagram for this cluster. We see a clear truncation in each of its two major stellar sequences. Faint red main-sequence stars run out well above our observational limit and near to the theoretical prediction for the lowest mass stars capable of stable hydrogen burning in their cores. We also see a truncation in the number counts of faint blue stars, namely white dwarfs. This reflects the limit to which the bulk of the white dwarfs can cool over the lifetime of the cluster. There is also a turn toward bluer colors in the least luminous of these objects. This was predicted for the very coolest white dwarfs with hydrogen-rich atmospheres as the formation of H(2) and the resultant collision-induced absorption cause their atmospheres to become largely opaque to infrared radiation.

Spectroscopic Identification of Cool White Dwarfs in the Solar Neighborhood

The New Luyten Two-Tenths catalog contains a large number of high proper motion white dwarf candidates that remain to be spectroscopically confirmed. We present new spectroscopic observations, as well as SDSS archival spectra of 49 white dwarf candidates selected from the revised NLTT catalog of Salim & Gould. Of these, 34 are cool DA white dwarfs with temperatures ranging from approximately 5000 up to 11,690 K, and 11 are DC white dwarfs with temperatures ranging from 4300 K (NLTT 18555) up to 11,000 K. Three of the DA white dwarfs also display abundances of heavy elements (NLTT 3915, NLTT 44986, and NLTT 43806), and one is a cool magnetic white dwarf (NLTT 44447) with an estimated magnetic field strength of 1.3 MG. We also present a new cool DQ white dwarf (NLTT 31347) with an estimated temperature of 6250 K. We supplement our sample with SDSS ugriz photometry for a fraction of the newly identified white dwarfs. A kinematical study of this sample of white dwarfs, characterized by proper motions ranging from 0136 to 0611 yr-1, suggest that they belong to the thin disk population.

A Far-Ultraviolet Study of the Hot White Dwarf in the Dwarf Nova WW Ceti

We present a synthetic spectral analysis of IUE archival and FUSE (Far Ultraviolet Spectroscopic Explorer) far-UV spectra of the peculiar dwarf nova WW Cet. During the quiescence of WW Cet, a white dwarf with Twd ~ 26,000 ? 1000 K can account for the far-UV flux and yields the proper distance. However, the best agreement with the observations is provided by a two-temperature white dwarf model with a cooler white dwarf at Twd = 25,000 K providing 75% of the far-UV flux and a hotter region (accretion belt or optically thick disk ring) with T = 40,000 K contributing 25% of the flux for the proper distance. We find from the FUSE spectrum that the white dwarf is rotating with V sin i = 600 ? 100 km s-1. Our temperature results provide an additional data point in the distribution of WD Twd versus orbital period above the CV period gap, where few WD Twd values are available.

Discovery of a Cool, Massive, and Metal-rich DAZ White Dwarf

We report the discovery of a new metal-rich DAZ white dwarf, GD 362. High signal-to-noise ratio optical spectroscopy reveals the presence of spectral lines from hydrogen as well as Ca I, Ca II, Mg I, and Fe I. A detailed model atmosphere analysis of this star yields an effective temperature of Teff = 9740 ± 50 K, a surface gravity of log g = 9.12 ± 0.07, and photospheric abundances of log = -5.2 ± 0.1, log = -4.8 ± 0.1, and log = -4.5 ± 0.1. White dwarf cooling models are used to derive a mass of 1.24 M☉ for GD 362, making it the most massive and metal-rich DAZ star uncovered to date. The problems related to the presence of such large metal abundances in a nearby (d ~ 25 pc) white dwarf in terms of an accretion scenario are briefly discussed.

Photometric Identification of Cool White Dwarfs

We investigate the use of a narrowband DDO51 filter for photometric identification of cool white dwarfs. We report photometric observations of 30 known cool white dwarfs with temperatures ranging from 10,000 K down to very cool temperatures (?3500 K). Follow-up spectroscopic observations of a sample of objects selected using this filter and our photometric observations show that DDO51 filter photometry can help select cool white dwarf candidates for follow-up multiobject spectroscopy by rejecting 65% of main-sequence stars with the same broadband colors as the cool white dwarfs. This technique is not selective enough to efficiently feed single-object spectrographs. We present the white dwarf cooling sequence using this filter. Our observations show that very cool white dwarfs form a sequence in the r - DDO versus r - z color-color diagram and demonstrate that significant improvements are needed in white dwarf model atmospheres.

Composite Accretion Disk and White Dwarf Photosphere Analyses of theFUSEandHubble Space TelescopeObservations of EY Cygni

We explore the origin of FUSE and Hubble Space Telescope STIS far-UV spectra of the dwarf nova EY Cygni during its quiescence using combined high-gravity photosphere and accretion disk models, as well as model accretion belts. The best-fitting single-temperature white dwarf model to the FUSE plus STIS spectrum of EY Cyg has Teff = 24,000 K and log g = 9.0 with an Si abundance of 0.1 times solar and C abundance of 0.2 times solar, but the distance is only 301 pc. The best-fitting composite model consists of a white dwarf with Teff = 22,000 K and log g = 9, plus an accretion belt with Tbelt = 36,000 K covering 27% of the white dwarf surface with Vbelt sin i = 2000 km s-1. The accretion belt contributes 63% of the far-UV light and the cooler white dwarf latitudes contribute 37%. This fit yields a distance of 351 pc, which is within 100 pc of our adopted distance of 450 pc. EY Cyg has very weak C IV emission and very strong N V emission, which is atypical of the majority of dwarf novae in quiescence. We also conducted a morphological study of the surroundings of EY Cyg using direct imaging in narrow nebular filters from ground-based telescopes. We report the possible detection of nebular material associated with EY Cyg. Possible origins of the apparently large N V/C IV emission ratio are discussed in the context of nova explosions, contamination of the secondary star, and accretion of nova abundance-enriched matter back to the white dwarf via the accretion disk or as a descendant of a precursor binary that survived thermal-timescale mass transfer. The scenario involving pollution of the secondary by past novae may be supported by the possible presence of a nova remnant–like nebula around EY Cyg.

Discovery of New Ultracool White Dwarfs in the Sloan Digital Sky Survey

We report the discovery of five very cool white dwarfs in the Sloan Digital Sky Survey (SDSS). Four are ultracool, exhibiting strong collision induced absorption (CIA) from molecular hydrogen and are similar in color to the three previously known coolest white dwarfs, SDSS J1337+00, LHS 3250 and LHS 1402. The fifth, an ultracool white dwarf candidate, shows milder CIA flux suppression and has a color and spectral shape similar to WD 0346+246. All five new white dwarfs are faint (g > 18.9) and have significant proper motions. One of the new ultracool white dwarfs, SDSS J0947, appears to be in a binary system with a slightly warmer (T_{eff} ~ 5000K) white dwarf companion.

Radiative Transfer in the Refractive Atmospheres of Very Cool White Dwarfs

We consider the problem of radiative transfer in stellar atmospheres where the index of refraction departs from unity and is a function of density and temperature. We present modified Feautrier and Λ-iteration methods to solve the equation of radiative transfer with refraction in a plane-parallel atmosphere. These methods are general and can be used in any problem with one-dimensional geometry where the index of refraction is a monotonically varying function of vertical optical depth. We present an application to very cool white dwarf atmospheres in which the index of refraction departs significantly from unity. We investigate how ray curvature and total internal reflection affect the limb darkening and the pressure-temperature structure of the atmosphere. Refraction results in a much weakened limb-darkening effect. We find that through the constraint of radiative equilibrium, total internal reflection warms the white dwarf atmosphere near the surface (τ ≲ 1). This effect may have a significant impact on studies of very cool white dwarf stars.

Cool White Dwarfs Revisited: New Spectroscopy and Photometry

In this paper we present new and improved data on 38 cool white dwarfs identified in 2001 by Oppenheimer and coworkers as candidate dark-halo objects. Using the high-resolution spectra obtained with LRIS on Keck I, we measure precise radial velocities for 13 white dwarfs that show an Hα absorption line. We show that accounting for radial velocities on average decreases the U-V plane velocities by only 6%. In two cases, accounting for the radial velocities put original halo candidates below the Oppenheimer and coworkers velocity cut. The radial velocity sample has a velocity dispersion in the direction perpendicular to the Galactic plane of σ_W = 59 km s^(-1), between the values typically associated with the thick-disk and stellar-halo populations. We also see indications of the presence of two populations by analyzing the velocities in the U-V plane. In addition, we present CCD photometry for half of the sample, and with it recalibrate the photographic photometry of the remaining white dwarfs. Using the new photometry in standard bands and applying the appropriate color-magnitude relations for hydrogen and helium atmospheres, we obtain new distance estimates. By recalibrating the distances of the white dwarfs that were not originally selected as halo candidates, we obtain 13 new candidates (and lose two original ones). On average, the new distances produce velocities in the U-V plane that are larger by 10%, with already fast objects gaining more. Using the new data while applying the same U-V plane velocity cut (94 km s^(-1)) and methods of analysis as did Oppenheimer and coworkers, we find a density of cool white dwarfs of 1.7 × 10^(-4) pc^(-3), confirming their value. In addition, we derive the density as a function of the U-V plane velocity cutoff. The density (corrected for losses due to higher U-V plane velocity cuts) starts to flatten out at 150 km s-1 (0.4 × 10^(-4) pc^(-3)) and is minimized (thus minimizing the possible nonhalo contamination) at 190 km s-1 (0.3 × 10^(-4) pc^(-3)). These densities are in rough agreement with the estimates for the stellar-halo white dwarfs, corresponding to values a factor of 1.9 and 1.4 higher.

Search for cool white dwarfs with GSC2

Microlensing experiments have suggested that a significant part of the dark halo of the Milky Way could be composed of matter in the form of massive compact halo objects (MACHOs). Cool ancient white dwarfs (WDs) are the natural candidates. Here we present a new survey of halo WDs and evaluate the local space density using an accurate kinematic analysis. A comparison to a revaluation of the Oppenheimer et al. result is also provided. the local space density estimated for the two independent samples is about ~ 10–5M⊙ pc–3.

A Far‐Ultraviolet Survey of 47 Tucanae. I. Imaging

We present results from the imaging portion of a far-ultraviolet (FUV) survey of the core of 47 Tucanae. We have detected 767 FUV sources, 527 of which have optical counterparts in archival HST/WFPC2 images of the same field. Most of our FUV sources are main-sequence (MS) turn-off stars near the detection limit of our survey. However, the FUV/optical color-magnitude diagram (CMD) also reveals 19 blue stragglers (BSs), 17 white dwarfs (WDs) and 16 cataclysmic variable (CV) candidates. The BSs lie on the extended cluster MS, and four of them are variable in the FUV data. The WDs occupy the top of the cluster cooling sequence, down to an effective temperature of T_{eff} \simeq 20,000 K. Our FUV source catalog probably contains many additional, cooler WDs without optical counterparts. Finally, the CV candidates are objects between the WD cooling track and the extended cluster MS. Four of the CV candidates are previously known or suspected cataclysmics. All of these are bright and variable in the FUV. Another CV candidate is associated with the semi-detached binary system V36 that was recently found by Albrow et al. (2001). V36 has an orbital period of 0.4 or 0.8 days, blue optical colors and is located within 1 arcsec of a Chandra x-ray source. A few of the remaining CV candidates may represent chance superpositions or SMC interlopers, but at least half are expected to be real cluster members with peculiar colors. However, only a few of these CV candidates are possible counterparts to Chandra x-ray sources. Thus it is not yet clear which, if any, of them are true CVs, rather than non-interacting MS/WD binaries or Helium WDs.

A New Wide Pair of Cool White Dwarfs in the Solar Neighborhood

We report the discovery of a wide pair (93'' angular separation) of extremely cool (Teff < 4000 K) white dwarfs with a very large common proper motion (~19 yr-1). The objects were discovered in a high proper motion survey in the poorly investigated southern sky region with δ < -60° using SuperCOSMOS Sky Survey (SSS) data. Both objects, SSSPM J2231-7514 and SSSPM J2231-7515, show featureless optical spectra. Fits of blackbody models to the spectra yield effective temperatures of 3810 and 3600 K, respectively, for the bright (V = 16.60) and faint (V = 16.87) components. Both degenerates are much brighter than other recent discoveries of cool white dwarfs with comparable effective temperatures and/or BJ-R colors. Therefore, they should be relatively nearby objects. The comparison with other cool white dwarfs and a photometric distance determination yield distance estimates between 9 and 14 pc. The latter seems to be more realistic since the good agreement of the proper motion of both components within the errors of about 8 mas yr-1 and the angular separation between the two stars support a distance of about 15 pc, with relatively small masses of the components. With a smaller distance we should be able to measure a differential proper motion due to orbital motion if the orbital plane is not strongly inclined and if the present orbital velocity vector is not close to the line of sight. The space velocity based on that distance and assumptions on radial velocity make the new pair of extremely cool white dwarfs probably some of the oldest members of the Galactic disk population, although the possibility that these objects are part of a Galactic halo dark matter component also cannot yet be ruled out.

Photometric and Spectroscopic Analysis of Cool White Dwarfs with Trigonometric Parallax Measurements

A detailed photometric and spectroscopic analysis of cool (Teff 12,000 K) white dwarf stars is presented. The sample has been drawn from the Yale Parallax Catalog and from a proper motion survey in the southern hemisphere. Optical BVRI and infrared JHK photometry, as well as spectroscopy at Hα, have been secured for a sample of 152 white dwarfs. The discovery of seven new DA white dwarfs, two new DQ white dwarfs, one new magnetic white dwarf, and three weak magnetic white dwarf candidates, is reported. Our sample also identifies 19 known or suspected double degenerates. The photometric energy distributions, the Hα line profiles, and the trigonometric parallax measurements are combined and compared against the predictions of model atmosphere calculations to determine the effective temperature and the radius of each object in the sample and also to constrain the atmospheric composition. New evolutionary sequences with carbon/oxygen cores with thin and thick hydrogen layers are used to derive stellar masses and ages. The results are used to improve our understanding of the chemical evolution of cool white dwarfs. We confirm the existence of a range in effective temperature between ~5000 and 6000 K where almost all white dwarfs have hydrogen-rich atmospheres. Our sample shows little evidence for mixed H/He white dwarfs, with the exception of two helium-rich DA stars, and four (possibly five) C2H white dwarfs which have been interpreted as having mixed H/He/C atmospheres. The observed sequence of DQ stars is found to terminate abruptly near 6500 K, below which they are believed to turn into C2H stars. True DC stars slightly above this temperature are found to exhibit hydrogen-like energy distributions despite the lack of Hα absorption features. The mean mass of our complete sample is 0.65 M☉ with a dispersion of σ ~ 0.20 M☉. Attempts to interpret the chemical evolution of cool white dwarfs show the problem to be complex. Convective mixing is called upon to account for the increase of the non-DA to DA ratio below 12,000 K, as well as the reappearance of helium-rich stars below ~5000 K. The possible presence of helium in cool DA stars, the existence of the non-DA gap, and the nature of the peculiar DC stars are also explained in terms of convective mixing, although our understanding of how this mechanism works needs to be revised in order to account for these observations. Given this chemical evolution uncertainty, it is not clear whether thick or thin hydrogen layer models should be used to determine cooling ages. The oldest object in our sample is ~7.9 Gyr or ~9.7 Gyr old depending on whether thin or thick hydrogen layer models are used, respectively.