DZ White Dwarfs Research Articles

AN ULTRAMASSIVE 1.28 M⊙ WHITE DWARF IN NGC 2099*

ABSTRACT With the Keck I Low-Resolution Imaging Spectrometer we have observed nine white dwarf candidates in the very rich open cluster NGC 2099 (M37). The spectroscopy shows seven to be DA white dwarfs, one to be a DB white dwarf, and one to be a DZ white dwarf. Three of these DA white dwarfs are consistent with singly evolved cluster membership: an ultramassive (1.28 − 0.08 + 0.05 ?> M ⊙ ?> ) and two intermediate-mass (0.70 and 0.75 M ⊙ ?> ) white dwarfs. Analysis of their cooling ages allows us to calculate their progenitor masses and establish new constraints on the initial–final mass relation. The intermediate-mass white dwarfs are in strong agreement with previous work over this mass regime. The ultramassive white dwarf has V = 24.5, ∼2 mag fainter than the other two remnants. The spectrum of this star has lower quality, so the derived stellar properties (e.g., T eff ?> , log g) have uncertainties that are several times higher than the brighter counterparts. We measure these uncertainties and establish the star’s final mass as the highest-mass white dwarf discovered thus far in a cluster, but we are unable to calculate its progenitor mass because at this high mass and cooler T eff ?> its inferred cooling age is highly sensitive to its mass. At the highest temperatures, however, this sensitivity of cooling age to an ultramassive white dwarf’s mass is only moderate. This demonstrates that future investigations of the upper-mass end of the initial–final mass relation must identify massive, newly formed white dwarfs (i.e., in young clusters with ages 50–150 Myr).

The incidence of magnetic fields in cool DZ white dwarfs

Little is known about the incidence of magnetic fields among the coolest white dwarfs. Their spectra usually do not exhibit any absorption lines as the bound-bound opacities of hydrogen and helium are vanishingly small. Probing these stars for the presence of magnetic fields is therefore extremely challenging. However, external pollution of a cool white dwarf by, e.g., planetary debris, leads to the appearance of metal lines in its spectral energy distribution. These lines provide a unique tool to identify and measure magnetism in the coolest and oldest white dwarfs in the Galaxy. We report the identification of 7 strongly metal polluted, cool (T_eff < 8000 K) white dwarfs with magnetic field strengths ranging from 1.9 to 9.6 MG. An analysis of our larger magnitude-limited sample of cool DZ yields a lower limit on the magnetic incidence of 13+/-4 percent, noticeably much higher than among hot DA white dwarfs.

Temperature dependence of sodium and ionized calcium resonance lines perturbed by helium

Traces of heavy metals in cool DZ white dwarf stars may be attributed to the accretion of circumstellar dust thought to originate from tidal disruption of rocky parent bodies. Spectra of such stars therefore provide a unique opportunity to study the composition of extrasolar planetary systems. The determination of metal abundances from stellar spectra depends on stellar atmospheric parameters and an accurate prior knowledge of the collision broadening of the line profiles by the most common constituents of the stellar atmosphere. For this purpose, we present theoretical absorption spectra of Na and Ca+ broadened by He for the conditions prevailing in cool white dwarfs.

Signs of a faint disc population at polluted white dwarfs

Observations of atmospheric metals and dust discs around white dwarfs provide important clues to the fate of terrestrial planetary systems around intermediate mass stars. We present Spitzer IRAC observations of 15 metal polluted white dwarfs to investigate the occurrence and physical properties of circumstellar dust created by the disruption of planetary bodies. We find subtle infrared excess emission consistent with warm dust around KUV 15519+1730 and HS 2132+0941, and weaker excess around the DZ white dwarf G245-58, which, if real, makes it the coolest white dwarf known to exhibit a 3.6 micron excess and the first DZ star with a bright disc. All together our data corroborate a picture where 1) discs at metal-enriched white dwarfs are commonplace and most escape detection in the infrared (possibly as narrow rings), 2) the discs are long lived, having lifetimes on the order of 10^6 yr or longer, and 3) the frequency of bright, infrared detectable discs decreases with age, on a timescale of roughly 500 Myr, suggesting large planetesimal disruptions decline on this same timescale.

Search for carbon stars and DZ white dwarfs in SDSS spectra survey through machine learning

Carbon stars and DZ white dwarfs are two types of rare objects in the Galaxy. In this paper, we have applied the label propagation algorithm to search for these two types of stars from Data Release Eight (DR8) of the Sloan Digital Sky Survey (SDSS), which is verified to be efficient by calculating precision and recall. From nearly two million spectra including stars, galaxies and QSOs, we have found 260 new carbon stars in which 96 stars have been identified as dwarfs and 7 identified as giants, and 11 composition spectrum systems (each of them consists of a white dwarf and a carbon star). Similarly, using the label propagation method, we have obtained 29 new DZ white dwarfs from SDSS DR8. Compared with PCA reconstructed spectra, the 29 findings are typical DZ white dwarfs. We have also investigated their proper motions by comparing them with proper motion distribution of 9,374 white dwarfs, and found that they satisfy the current observed white dwarfs by SDSS generally have large proper motions. In addition, we have estimated their effective temperatures by fitting the polynomial relationship between effective temperature and g-r color of known DZ white dwarfs, and found 12 of the 29 new DZ white dwarfs are cool, in which nine are between 6000K and 6600K, and three are below 6000K.

Collisional profiles of ionized calcium perturbed by helium

The spectra of many cool DZ white dwarfs show extremely broad resonance lines of ionized calcium and magnesium in the optical and ultraviolet region. When the line center is strongly saturated, these wings may become a significant source of additional opacity in cool white dwarf atmospheres. The omission of this opacity results in systematic errors in the predicted flux distribution and hence in the effective temperatures and element abundances. In a continuation of Homeier et al. (2007) and Allard et al. (2012), where we presented absorption spectra of Na–He in cool white dwarfs, we extend this work to the Ca+–He system.

Density Effects in Sodium and ionized Calcium perturbed by helium in Cool White Dwarf Stars

Traces of heavy metal in cool DZ white dwarfs may be attributed to the accretion of circumstellar dust. This dust is thought to originate from the tidal disruption of some rocky material, they provide a unique opportunity to study the composition of extra-solar planetary systems. The determinations of precise atmospheric parameters and abundances require accurate description of the line profiles of the identified features. We present absorption spectra of Na-He and Ca + -He for the conditions prevailing in cool white dwarfs.

A search for the hidden population of AM CVn binaries in the Sloan Digital Sky Survey

We present the latest results from a spectroscopic survey designed to uncover the hidden population of AM Canum Venaticorum (AM CVn) binaries in the photometric database of the Sloan Digital Sky Survey (SDSS). We selected ~2000 candidates based on their photometric colours, a relatively small sample which is expected to contain the majority of all AM CVn binaries in the SDSS (expected to be ~50). We present two new candidate AM CVn binaries discovered using this strategy: SDSS J104325.08+563258.1 and SDSS J173047.59+554518.5. We also present spectra of 29 new cataclysmic variables, 23 DQ white dwarfs and 21 DZ white dwarfs discovered in this survey. The survey is now approximately 70 per cent complete, and the discovery of seven new AM CVn binaries indicates a lower space density than previously predicted. From the essentially complete g <= 19 sample, we derive an observed space density of (5 +/- 3) x10^-7 pc^-3; this is lower than previous estimates by a factor of 3. The sample has been cross-matched with the GALEX All-Sky Imaging Survey database, and with Data Release 9 of the UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey (UKIDSS). The addition of UV photometry allows new colour cuts to be applied, reducing the size of our sample to ~1100 objects. Optimising our followup should allow us to uncover the remaining AM CVn binaries present in the SDSS, providing the larger homogeneous sample required to more reliably estimate their space density.

Cool DZ white dwarfs in the SDSS

We report the identification of 26 cool DZ white dwarfs that lie across and below the main sequence in the Sloan Digital Sky Survey (SDSS) u-g vs. g-r two-color diagram; 21 of these stars are new discoveries. The sample was identified by visual inspection of all spectra of objects that fall below the main sequence in the two-color diagram, as well as by an automated search for characteristic spectral features over a large area in color space that included the main sequence. The spectra and photometry provided by the SDSS project are interpreted with model atmospheres, including all relevant metals. Effective temperatures and element abundances are determined, while the surface gravity has to be assumed and was fixed at the canonical value of log g = 8. These stars represent the extension of the well-known DZ sequence towards cooler temperatures and fill the gap around Teff = 6500 K present in a previous study. The metal abundances are similar to those in the hotter DZ, but the lowest abundances are missing, probably because of our selection procedures. The interpretation is complicated in terms of the accretion/diffusion scenario, because we do not know if accretion is still occurring or has ended long ago. Independent of that uncertainty, the masses of the metals currently present in the convection zones - and thus an absolute lower limit of the total accreted masses - of these stars are similar to the largest asteroids in our solar system.

Origin of debris disks and the supply of metals in DZ white dwarfs

AbstractWe discuss the dynamical evolution of minor planetary bodies in the outer regions of planetary systems around the progenitors of DZ white dwarfs. We show that during the planetary-nebula phase of these stars, mass loss can lead to the expansion of all planetary bodies. The orbital eccentricity of the minor bodies, as relics of planetesimals, may be largely excited by the perturbation due to both gas drag effects and nearby gas giant planets. Some of these bodies migrate toward the host star, while others are scattered out of the planetary system. The former have modest probability of being captured by the sweeping secular resonances of giant planets, and induced to migrate toward the host star. When they venture close to their host stars, their orbits are tidally circularized so that they form compact disks where they may undergo further collisionally driven evolution. During the subsequent post main sequence evolution of their host stars, this process may provide an avenue which continually channels heavy elements onto the surface of the white dwarfs. We suggest that this scenario provides an explanation for the recently discovered Calcium line variation in G29-38.

On the Spectral Evolution of Cool, Helium‐Atmosphere White Dwarfs: Detailed Spectroscopic and Photometric Analysis of DZ Stars

We present a detailed analysis of a large spectroscopic and photometric sample of DZ white dwarfs based on our latest model atmosphere calculations. We revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz (12 stars) and analyze 147 new DZ white dwarfs discovered in the SDSS. The inclusion of metals and hydrogen in our model atmosphere calculations leads to different atmospheric parameters than those derived from pure helium models. Calcium abundances are found in the range from log(Ca/He) = -12 to -8. We also find that fits of the coolest objects show peculiarities, suggesting that our physical models may not correctly describe the conditions of high atmospheric pressure encountered in the coolest DZ stars. We find that the mean mass of the 11 DZ stars with trigonometric parallaxes, M = 0.63 M☉, is significantly lower than that obtained from pure helium models, M = 0.78 M☉, and in much better agreement with the mean mass of other types of white dwarfs. We determine hydrogen abundances for 27% of the DZ stars in our sample, while only upper limits are obtained for objects with low-S/N spectroscopic data. We confirm with a high level of confidence that the accretion rate of hydrogen is at least 2 orders of magnitude smaller than that of metals (and up to 5 in some cases) to be compatible with the observations. We find a correlation between the hydrogen abundance and the effective temperature, suggesting for the first time empirical evidence of a lower temperature boundary for the hydrogen screening mechanism. Finally, we speculate on the possibility that the DZA white dwarfs could be the result of the convective mixing of thin hydrogen-rich atmospheres with the underlying helium convection zone.

Model Atmosphere Analysis of the Weakly Magnetic DZ White Dwarf G165‐7

A reanalysis of the strongly metal-blanketed DZ white dwarf G165-7 is presented. An improved grid of model atmospheres and synthetic spectra is used to analyze BVRI, JHK, and ugriz photometric observations as well as a high quality Sloan Digital Sky Survey spectrum covering the energy distribution from 3600 to 9000 A. The detection of splitting in several lines of Ca, Na, and Fe, suggesting a magnetic field of Bs~650 kG, is confirmed by spectropolarimetric observations that reveal as much as +/- 7.5% circular polarization in many of the absorption lines, most notably Na, Mg, and Fe. Our combined photometric and spectroscopic fit yields Teff = 6440 K, log g=7.99, log H/He=-3.0 and log Ca/He=-8.1. The other heavy elements have solar ratios with respect to calcium, with the exception of Na and Cr that had to be reduced by a factor of two and three, respectively. A crude polarization model based upon the observed local spectral flux gradient yields a longitudinal field of 165 kG, consistent with the mean surface field inferred from the Zeeman splitting. The inclusion of this weak magnetic field in our synthetic spectrum calculations, even in an approximate fashion, is shown to improve our fit significantly.

Observations of White Dwarfs in the Solar Neighborhood

High proper motion surveys are still adding much to our understanding of the local white dwarf population and, possibly, to the Galactic halo membership of some objects. As part of our study of white dwarfs in the solar neighborhood we have observed seven white dwarfs from the revised New Luyten Two Tenths (NLTT) catalog of Salim & Gould. We found four DA white dwarfs (NLTT 529, 49985, 53468, 55932), one DZ white dwarf (NLTT 40607) that shows a close resemblance to the unusual DZ white dwarf G165-7, and two DC white dwarfs (NLTT 19138, NLTT 52404). The white dwarf candidates were chosen using the V-J reduced proper-motion diagram of Salim & Gould and an optical-infrared diagram (V-J vs. J-H). We also observed five stars from the list of local white dwarfs by Holberg, Oswalt, & Sion, that required confirmation of their spectral classification. We confirm that GD 1212 and WD 1717-345 are white dwarfs but that BPM 17113 (WD 0311-543), KUV 05097+1649, and BPM 19929 (WD 1013-559) are main-sequence F stars rather than white dwarfs. An analysis of a color-selected sample of white dwarf candidates extracted from the rNLTT catalog could contribute more than 200 new white dwarfs, most belonging to the thin disk, with a few (N < 15) kinematically selected members of an older population (thick disk or halo).

The Century Survey Galactic Halo Project. I. Stellar Spectral Analysis

The Century Survey Galactic Halo Project is a photometric and spectroscopic survey from which we select relatively blue stars (V-R < 0.30 mag) as probes of the Milky Way halo. The survey strip spans the range of Galactic latitude 35° < b < 88°, allowing us to study the nature of populations of stars and their systematic motions as a function of Galactic latitude. One of our primary goals is to use blue horizontal-branch stars to trace potential star streams in the halo, and to test the hierarchical model for the formation of the Galaxy. In this paper we discuss spectroscopy and multipassband photometry for a sample of 764 blue stars in the Century Survey region. Our sample consists predominantly of A- and F-type stars. We describe our techniques for determination of radial velocities, effective temperatures, metallicities, and surface gravities. Based on these measurements, we derive distance estimates by comparison with a set of calibrated isochrones. We devote special attention to the classification of blue horizontal-branch stars, and compare the results obtained from the application of the techniques of Kinman et al., Wilhelm et al., and Clewley et al. We identify 55 blue horizontal-branch stars. Our large sample of stars also uncovers a number of unusual objects, including three carbon-enhanced stars, a late B-type star located 0.8 kpc above the Galactic plane, and a DZ white dwarf.

Element abundances in cool white dwarfs

We present a small data base of homogeneously derived photospheric element abundances of DZ white dwarfs and related objects. Our previous investigations are supplemented with the analysis of ultraviolet spectra for nine white dwarfs. Of particular interest is the detection of Lα absorption in van Maanen 2 and a determination of the effective temperature of this star. The new value is about 1000 K lower than previous results due to the strong ultraviolet absorption by metals which has to be considered consistently. The metal abundances of our sample stars are compatible with the predictions from the two-phase accretion model of Dupuis et al. ([CITE], [CITE], [CITE]). Small deviations can be observed for the abundance ratios in some objects. This could indicate non-solar metal-to-metal ratios in the accreted material. Hydrogen can be detected in virtually all of our objects. However, its average accretion rate must be at least two orders of magnitude lower than the metal accretion rate.

Discovery of a Magnetic DZ White Dwarf with Zeeman-Split Lines of Heavy Elements

A spectroscopic survey of unstudied Luyten half-second proper-motion stars has resulted in the discovery of an unusual new magnetic white dwarf. LHS 2534 proves to be the first magnetic DZ, showing Zeeman-split Na I and Mg I components, as well as Ca I and Ca II lines for which Zeeman components are blended. The Na I splittings result in a mean surface field strength estimate of 1.92 MG. Apart from the magnetic field, LHS 2534 is one of the most heavily blanketed and coolest DZ white dwarfs at Teff ~ 6000 K.

Spectroscopy and spectral types for 387 stellar objects from the large, bright QSO survey

Spectroscopy and spectral types for 387 stellar objects brighter than B J = 18.75 found in the Large, Bright QSO Survey (LBQS) are reported. The spectra were observed with the MMT at moderate resolution (6 A FWHM) and low S/N, and cover the interval λλ3200-7400. They represent all of the northern hemisphere Galactic LBQS objects observed through 1989 September. We report the identification and present spectra of one DQA, 10 DB, two DC, three DQ, and three DZ white dwarfs, as well as seven composite objects and five cataclysmic variables and unredshifted emission objects

Abundances in the hot DZ star CBS 127 - How efficient is hydrogen screening?

A revised and updated grid of cool helium-rich model atmospheres which includes the effect of varying hydrogen abundance is used to analyze the temperature and abundance of the DZ white dwarf CBS 127. Comparisons of the star to several other DZ white dwarfs with similar T(e)s show that CBS 137 is the hottest DZ star and lies near the boundary temperature above which it has been proposed that hydrogen accretion may be prevented. Based on the results, a specifically defined single lower boundary temperature for screening is rejected, and it is demonstrated that the propeller must operate over a very broad range of T(eff), down to T(eff) as low as 5800 K, with highly variable efficiency.

An atlas of optical spectra of DZ white dwarfs and related objects

view Abstract Citations (33) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Atlas of Optical Spectra of DZ White Dwarfs and Related Objects Sion, Edward M. ; Kenyon, Scott J. ; Aannestad, Per A. Abstract An atlas of optical spectra and equivalent width measurements for DZ stars and several related objects is described. These data should improve abundance measurements for Ca/He, Mg/He, and Fe/He in these stars and provide tests for calculations of accretion, diffusion, and radiative transfer in white-dwarf atmospheres. Also reported is the possible detection of He I (3888-A) in three DZ white dwarfs, 0246 + 735, 1705 + 030, and 2215 + 388. Publication: The Astrophysical Journal Supplement Series Pub Date: March 1990 DOI: 10.1086/191429 Bibcode: 1990ApJS...72..707S Keywords: Stellar Spectra; Visible Spectrum; White Dwarf Stars; Radiative Transfer; Signal To Noise Ratios; Spectral Line Width; Stellar Atmospheres; Stellar Spectrophotometry; Astronomy; LINE IDENTIFICATIONS; SPECTROPHOTOMETRY; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (23)

Model atmosphere analysis of the DZ white dwarf K789 - 37

This paper presents new observations and an analysis using model atmosphere techniques of the recently discovered DZA white dwarf K789 - 37. The effective temperature derived is 9500 + or - 500 K. Abundances are determined for the elements hydrogen, magnesium, calcium, and iron, as well as upper limits for carbon and silicon. The abundances are very low compared to solar values and within the range typical for this spectral class. The implications of the results for the currently accepted explanation of trace elements in cool white dwarfs in terms of the diffusion/accretion scenario are discussed. It is concluded that the accreted matter cannot have had solar composition.