Spitzer IRAC observations of 15 metal-polluted white dwarfs reveal infrared excesses in the spectral energy distributions of HE 0110−5630, GD 61, and HE 1349−2305. All three of these stars have helium-dominated atmospheres, and their infrared emissions are consistent with warm dust produced by the tidal destruction of (minor) planetary bodies. This study brings the number of metal-polluted, helium and hydrogen atmosphere white dwarfs surveyed with IRAC to 53 and 38, respectively. It also nearly doubles the number of metal-polluted helium-rich white dwarfs found to have closely orbiting dust by Spitzer. From the increased statistics for both atmospheric types with circumstellar dust, we derive a typical disk lifetime of log[t disk (yr)] = 5.6 ± 1.1 (ranging from 3 × 10 4 to 5 × 10 6 yr). This assumes a relatively constant rate of accretion over the timescale where dust persists, which is uncertain. We find that the fraction of highly metal-polluted helium-rich white dwarfs that have an infrared excess detected by Spitzer is only 23%, compared to 48% for metal-polluted hydrogen-rich white dwarfs, and we conclude from this difference that the typical lifetime of dusty disks is somewhat shorter than the diffusion timescales of helium-rich white dwarf. We also find evidence for higher time-averaged accretion rates onto helium-rich stars compared to the instantaneous accretion rates onto hydrogen-rich stars; this is an indication that our picture of evolved star-planetary system interactions is incomplete. We discuss some speculative scenarios that can explain the observations.

We have discovered a large number of circular and elliptical shells at 24µm around luminous central sources with the MIPS instrument on-board the Spitzer Space Telescope. Our archival follow-up effort has revealed 90% of these circumstellar shells to be previously unknown. The majority of the shells is only visible at 24µm, but many of the central stars are detected at multiple wavelengths from the midto the near-IR regime. The general lack of optical counterparts, however, indicates that these sources represent a population of highly obscured objects. We obtained optical and near-IR spectroscopic observations of the central stars and find most of these objects to be massive stars. In particular, we identify a large population of sources that we argue represents a narrow evolutionary phase, closely related or identical to the LBV stage of massive stellar evolution.

With the launch of the Wide-field Infrared Survey Explorer (WISE ), a new era of detecting planetary debris and brown dwarfs around white dwarfs (WDs) has begun with the WISE InfraRed Excesses around Degenerates (WIRED) Survey. The WIRED Survey is sensitive to substellar objects and dusty debris around WDs out to distances exceeding 100 pc, well beyond the completeness level of local WDs. In this paper, we present a cross-correlation of the preliminary Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) WD Catalog between the WISE, Two-Micron All Sky Survey (2MASS), UKIRT Infrared Deep Sky Survey (UKIDSS), and SDSS DR7 photometric catalogs. From ∼ 18, 000 input targets, there are WISE detections comprising 344 "naked" WDs (detection of the WD photosphere only), 1020 candidate WD+M dwarf binaries, 42 candidate WD+brown dwarf (BD) systems, 52 candidate WD+dust disk systems, and 69 targets with indeterminate infrared excess. We classified all of the detected targets through spectral energy distribution model fitting of the merged optical, near-IR, and WISE photometry. Some of these detections could be the result of contaminating sources within the large (≈ 6 ′′ ) WISE point spread function; we make a preliminary estimate for the rates of contamination for our WD+BD and WD+disk candidates, and provide notes for each target-of-interest. Each candidate presented here should be confirmed with higher angular resolution infrared imaging or infrared spectroscopy. We also present an overview of the observational characteristics of the detected WDs in the WISE photometric bands, -2including the relative frequencies of candidate WD+M, WD+BD, and WD+disk systems.

We present the ultraviolet spectrum of the SW Sex star and nova-like variable DW UMa in an optical low state, as observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST). The data are well described by a synthetic white dwarf (WD) spectrum with T ef f = 46, 000 ± 1000 K, log g = 7.60 ± 0.15, v sin i = 370 ± 100 km s −1 and Z/Z ⊙ = 0.47 ± 0.15. For this combination of T ef f and log g, WD models predict M W D = 0.48 ± 0.06 M ⊙ and R W D = (1.27 ± 0.18) × 10 9 cm. Combining the radius estimate with the normalization of the spectral fit, we obtain a distance estimate of d = 830 ± 150 pc.During our observations, DW UMa was approximately 3 magnitudes fainter in V than in the high state. A comparison of our low-state HST spectrum to a high-state spectrum obtained with the International Ultraviolet Explorer shows that the former is much bluer and has a higher continuum level shortward of 1450Å. Since DW UMa is an eclipsing system, this suggests that an optically thick accretion disk rim blocks our view of the WD primary in the high state.If self-occulting accretion disks are common among the SW Sex stars, we can account for (i) the preference for high-inclination systems within the class and (ii) their V-shaped continuum eclipses. Moreover, even though the emission lines produced by a self-obscured disk are generally still double-peaked, they are weaker and narrower than those produced by an unobscured disk. This may allow a secondary line emission mechanism to dominate and produce the single-peaked, optical lines that are a distinguishing characteristic of the SW Sex stars.

We present ISAAC spectroscopy and ISAAC, UKIDSS and Spitzer Space Telescope broad-band photometry of SDSS J1228+1040 -a white dwarf for which evidence of a gaseous metal-rich circumstellar disk has previously been found from optical emission lines. The data show a clear excess in the near-and midinfrared, providing compelling evidence for the presence of dust in addition to the previously identified gaseous debris disk around the star. The infrared excess can be modelled in terms of an optically thick but geometrically thin disk. We find that the inner disk temperatures must be relatively high (∼1700 K) in order to fit the SED in the near-infrared. These data provide the first evidence for the co-existence of both gas and dust in a disk around a white dwarf, and show that their presence is possible even around moderately hot (∼22,000 K) stars.