P. Dufour

FROM SHOCK BREAKOUT TO PEAK AND BEYOND: EXTENSIVE PANCHROMATIC OBSERVATIONS OF THE TYPE Ib SUPERNOVA 2008D ASSOCIATED WITH SWIFT X-RAY TRANSIENT 080109

We present extensive early photometric (ultraviolet through near-infrared) and spectroscopic (optical and near-infrared) data on supernova (SN) 2008D as well as X-ray data analysis on the associated Swift X-ray transient (XRT) 080109. Our data span a time range of 5 hr before the detection of the X-ray transient to 150days after its detection, and a detailed analysis allowed us to derive constraints on the nature of the SN and its progenitor; throughout we draw comparisons with results presented in the literature and find several key aspects that differ. We show that the X-ray spectrum of XRT 080109 can be fit equally well by an absorbed power law or a superposition of about equal parts of both power law and blackbody. Our data first established that SN 2008D is a spectroscopically normal SN Ib (i.e., showing conspicuous He lines) and showed that SN 2008D had a relatively long rise time of 18days and a modest optical peak luminosity. The early-time light curves of the SN are dominated by a cooling stellar envelope (for Δt0.1-4days, most pronounced in the blue bands) followed by 56Ni decay. We construct a reliable measurement of the bolometric output for this stripped-envelope SN, and, combined with estimates of E K and M ej from the literature, estimate the stellar radius R ⊙ of its probable Wolf-Rayet progenitor. According to the model of Waxman etal. and Chevalier & Fransson, we derive R W07⊙ = 1.2 0.7R ⊙ and R CF08⊙ = 12 7 R ⊙, respectively; the latter being more in line with typical WN stars. Spectra obtained at three and four months after maximum light show double-peaked oxygen lines that we associate with departures from spherical symmetry, as has been suggested for the inner ejecta of a number of SN Ib cores.

SDSS DR7 WHITE DWARF CATALOG

We present a new catalog of spectroscopically confirmed white dwarf stars from the Sloan Digital Sky Survey (SDSS) Data Release 7 spectroscopic catalog. We find 20,407 white dwarf spectra, representing 19,712 stars, and provide atmospheric model fits to 14,120 DA and 1011 DB white dwarf spectra from 12,843 and 923 stars, respectively. These numbers represent more than a factor of two increase in the total number of white dwarf stars from the previous SDSS white dwarf catalogs based on DR4 data. Our distribution of subtypes varies from previous catalogs due to our more conservative, manual classifications of each star in our catalog, supplementing our automatic fits. In particular, we find a large number of magnetic white dwarf stars whose small Zeeman splittings mimic increased Stark broadening that would otherwise result in an overestimated log g if fit as a non-magnetic white dwarf. We calculate mean DA and DB masses for our clean, non-magnetic sample and find the DB mean mass is statistically larger than that for the DAs.

A disintegrating minor planet transiting a white dwarf

Most stars become white dwarfs after they have exhausted their nuclear fuel (the Sun will be one such). Between one-quarter and one-half of white dwarfs have elements heavier than helium in their atmospheres, even though these elements ought to sink rapidly into the stellar interiors (unless they are occasionally replenished). The abundance ratios of heavy elements in the atmospheres of white dwarfs are similar to the ratios in rocky bodies in the Solar System. This fact, together with the existence of warm, dusty debris disks surrounding about four per cent of white dwarfs, suggests that rocky debris from the planetary systems of white-dwarf progenitors occasionally pollutes the atmospheres of the stars. The total accreted mass of this debris is sometimes comparable to the mass of large asteroids in the Solar System. However, rocky, disintegrating bodies around a white dwarf have not yet been observed. Here we report observations of a white dwarf—WD 1145+017—being transited by at least one, and probably several, disintegrating planetesimals, with periods ranging from 4.5 hours to 4.9 hours. The strongest transit signals occur every 4.5 hours and exhibit varying depths (blocking up to 40 per cent of the star’s brightness) and asymmetric profiles, indicative of a small object with a cometary tail of dusty effluent material. The star has a dusty debris disk, and the star’s spectrum shows prominent lines from heavy elements such as magnesium, aluminium, silicon, calcium, iron, and nickel. This system provides further evidence that the pollution of white dwarfs by heavy elements might originate from disrupted rocky bodies such as asteroids and minor planets.

Rocky planetesimals as the origin of metals in DZ stars

The calcium and hydrogen abundances, Galactic positions and kinematics of 146 DZ white dwarfs from the Sloan Digital Sky Survey are analysed to constrain the possible origin of their externally polluted atmospheres. There are no correlations found between their accreted calcium abundances and spatial–kinematical distributions relative to interstellar material. Furthermore, two thirds of the stars are currently located above the Galactic gas and dust layer, and their kinematics indicate multi-Myr residences in this region where interstellar material is virtually absent. Where detected, the hydrogen abundances for 37 DZA stars show little or no correlation with accreted calcium or spatial–kinematical distributions, though there is a general trend with cooling age. It is found that Eddington-type accretion of interstellar hydrogen can reproduce the observed hydrogen abundances, yet simultaneously fails to account for calcium. The calcium-to-hydrogen ratios for the DZA stars are dominated by supersolar values, as are the lower limits for the remaining 109 DZ stars. All together, these polluted white dwarfs currently contain 10 20±2 g of calcium in their convective envelopes, commensurate with the masses of calcium inferred for large asteroids. A census of current T eff 12 000 K, helium-rich stars from the Sloan Digital Sky Survey suggests the DZ and DC white dwarfs belong to the same stellar population, with similar basic atmospheric compositions, effective temperatures, spatial distributions and Galactic space velocities. Based on this result, pollution by the interstellar medium cannot simultaneously account for both the polluted and non-polluted subpopulations. Rather, it is probable that these white dwarfs are contaminated by circumstellar matter; the rocky remains of terrestrial planetary systems. In this picture, two predictions emerge. First, at least 3.5 per cent of all white dwarfs harbour the remnants of terrestrial planetary systems; this is a concrete lower limit and the true fraction is almost certainly, and perhaps significantly, higher. Therefore, one can infer that at least 3.5 per cent of main-sequence A- and F-type stars build terrestrial planets. Secondly, the DZA stars are externally polluted by both metals and hydrogen, and hence constrain the frequency and mass of water rich, extrasolar planetesimals.

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.

New white dwarf stars in the Sloan Digital Sky Survey Data Release 10

We report the discovery of 9 089 new spectroscopically confirmed white dwarfs and subdwarfs in the Sloan Digital Sky Survey Data Release 10. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), and estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon dominated spectra DQs. We found 1 central star of a planetary nebula, 2 new oxygen spectra on helium atmosphere white dwarfs, 71 DQs, 42 hot DO/PG1159s, 171 white dwarf+main sequence star binaries, 206 magnetic DAHs, 327 continuum dominated DCs, 397 metal polluted white dwarfs, 450 helium dominated white dwarfs, 647 subdwarfs and 6888 new hydrogen dominated white dwarf stars.

KNOW YOUR NEIGHBORHOOD: A DETAILED MODEL ATMOSPHERE ANALYSIS OF NEARBY WHITE DWARFS

We present improved atmospheric parameters of nearby white dwarfs lying within 20 pc of the Sun. The aim of the current study is to obtain the best statistical model of the least-biased sample of the white dwarf population. A homogeneous analysis of the local population is performed combining detailed spectroscopic and photometric analyses based on improved model atmosphere calculations for various spectral types including DA, DB, DC, DQ, and DZ stars. The spectroscopic technique is applied to all stars in our sample for which optical spectra are available. Photometric energy distributions, when available, are also combined to trigonometric parallax measurements to derive effective temperatures, stellar radii, as well as atmospheric compositions. A revised catalog of white dwarfs in the solar neighborhood is presented. We provide, for the first time, a comprehensive analysis of the mass distribution and the chemical distribution of white dwarf stars in a volume-limited sample.

Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs

We present an analysis of spectroscopic and photometric observations of cool DQ white dwarfs based on improved model atmosphere calculations. In particular, we revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz and discuss the astrophysical implications on the temperature scale and mean mass, as well as the chemical evolution of these stars. We also analyze 40 new DQ stars discovered in the First Data Release of the Sloan Digital Sky Survey (SDSS). Our analysis confirms that effective temperatures (Teff) derived from model atmospheres including carbon are significantly lower than the temperatures obtained from pure helium models. Similarly, the mean mass of the trigonometric parallax sample, M = 0.62 M?, is significantly lower than that obtained from pure helium models, M = 0.73 M?, and more consistent with the spectroscopic mean mass of DB stars, M = 0.59 M?, the most likely progenitors of DQ white dwarfs. We find that DQ stars form a remarkably well-defined sequence in a carbon abundance versus effective temperature diagram; below Teff ~ 10,000 K, carbon pollution decreases monotonically with decreasing effective temperature. Improved evolutionary models including diffusion and connecting to the PG 1159 phase are used to infer a typical value for the thickness of the helium layer MHe/M between 10-3 and 10-2, compatible with the predictions of post-AGB models. Several DQ stars in our sample, however, show larger than average carbon abundances. We argue that these DQ stars are all massive white dwarfs and could represent the high-mass tail of the white dwarf mass distribution, with their hotter counterparts corresponding to the hot DQ stars reported recently by Liebert et al. The number distribution of DQ white dwarfs as a function of effective temperature clearly shows a sudden drop at about Teff ~ 7000 K and an abrupt cutoff at Teff ~ 6000 K. The existence of this cutoff is now statistically more significant with the addition of the SDSS stars. The physical mechanism responsible for this cutoff is still unknown, even though it is believed to be somehow related to the existence of the so-called C2H stars at lower temperatures.

White dwarf stars with carbon atmospheres

White dwarfs represent the endpoint of stellar evolution for stars with initial masses between approximately 0.07 and 8–10, where is the mass of the Sun (more massive stars end their life as either black holes or neutron stars). The theory of stellar evolution predicts that the majority of white dwarfs have a core made of carbon and oxygen, which itself is surrounded by a helium layer and, for ∼80 per cent of known white dwarfs, by an additional hydrogen layer. All white dwarfs therefore have been traditionally found to belong to one of two categories: those with a hydrogen-rich atmosphere (the DA spectral type) and those with a helium-rich atmosphere (the non-DAs). Here we report the discovery of several white dwarfs with atmospheres primarily composed of carbon, with little or no trace of hydrogen or helium. Our analysis shows that the atmospheric parameters found for these stars do not fit satisfactorily in any of the currently known theories of post-asymptotic giant branch evolution, although these objects might be the cooler counterpart of the unique and extensively studied PG 1159 star H1504+65 (refs 4–7). These stars, together with H1504+65, might accordingly form a new evolutionary sequence that follows the asymptotic giant branch.

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.