The infrared excess around the white dwarf G29-38 can be explained by emission from an opaque flat ring of dust with an inner radius of 0.14 R ⊙ and an outer radius < 1R ⊙ . This ring lies within the Roche region of the white dwarf where an asteroid could have been tidally destroyed, producing a system reminiscent of Saturn's rings. Accretion onto the white dwarf from this circumstellar dust can explain the observed calcium abundance in the atmosphere of G29-38. Either as a bombardment by a series of asteroids or because of one large disruption, the total amount of matter accreted onto the white dwarf may have been ∼ 4 × 10 24 g, comparable to the total mass of asteroids in the Solar System, or, equivalently, about 1% of the mass in the asteroid belt around the main sequence star ζ Lep.

Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T eff 20, 000 K and metal-contaminated photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457−086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal accretion rates dM/dt 3 × 10 8 g s −1 display a warm infrared excess from orbiting dust; the likely result of a tidally-destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 10 6 g s −1 in the solar system zodiacal cloud and 10 10 g s −1 often inferred for debris disks at main sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

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Using the Goddard High Resolution Spectrograph (GHRS) onboard HST, we have obtained high S/N echelle observations of the weak interstellar O I 1356 A absorption toward the stars Gamma Cas, Epsilon Per, Delta Ori, Epsilon Ori, 15 Mon, Tau CMa, and Gamma Ara. In combination with previous GHRS measurements in six other sightlines (Zeta Per, Xi Per, Lambda Ori, Iota Ori, Kappa Ori, and Zeta Oph), these new observations yield a mean interstellar gas-phase oxygen abundance (per 10$^6$ H atoms) of 10$^6$ O/H = 319 +/- 14. The largest deviation from the mean is less than 18%, and there are no statistically significant variations in the measured O abundances from sightline to sightline and no evidence of density-dependent oxygen depletion from the gas phase. Assuming various mixtures of silicates and oxides, the abundance of interstellar oxygen tied up in dust grains is unlikely to surpass 10$^6$ O/H $\approx$ 180. Consequently, the GHRS observations imply that the total abundance of interstellar oxygen (gas plus grains) is homogeneous in the vicinity of the Sun and about 2/3 of the solar value of 10$^6$ O/H = 741 +/- 130. This oxygen deficit is consistent with that observed in nearby B stars and similar to that recently found for interstellar krypton with GHRS. Possible explanations for this deficit include: (1) early solar system enrichment by a local supernova, (2) a recent infall of metal-poor gas in the local Milky Way, or (3) an outward diffusion of the Sun from a smaller galactocentric distance.Comment: 23 pages, LaTeX, 5 Postscript figures; ApJ, in pres

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We also mis-reported the temperature of the silicate and carbon grains in our fit to the HR 7012 IRS spectrum; the grains have a temperature 550 K, not 520 K as reported previously. Lastly, the composition of the enstatite used to fit the HR 7012 spectrum is Mg 0.7 Fe 0.3 SiO 3 , not Mg 0.7 Fe 0.3 SiO 4 . In addition, we noticed an error in the minimum blow-out size for silicate, carbon, and silica grains around HD 113766 and HR 7012; the blow-out sizes are smaller than previously estimated. For HD 113766, we originally estimated minimum silicate and carbon sizes of 1.4 and 1.9 m, respectively; we now estimate 0.35 and 0.46 m, respectively. With the exception of forsterite, all of the grains used to model the HD 113766 spectrum are larger than the minimum grain sizes. The forsterite grains (submicron) possess radii that are similar to the minimum silicate grain size. For HR 7012, we originally estimated minimum silicate, carbon, and silica sizes of 1.1, 1.4, and 1.6 m, respectively; we now estimate 0.9, 1.2, and 1.3 m, respectively. The enstatite and cristobalite grains used to model the infrared HR 7012 spectrum are still smaller than the minimum grain size. We had previously concluded that the minimum grain sizes (>1 m) were inconsistent with presence of submicron-sized grains inferred from the structure of the silicate emission features, suggesting that a recent massive collision must have occurred around HD 113766 and HR 7012. Our new minimum grain size estimates are more consistent with our models for the infrared spectra and do not require a recent massive collision around HD 113766. However, our models do indicate the presence of submicron-sized particles significantly smaller than the blow-out size around HR 7012, suggesting that a recent massive collision may have occurred in this system.

We report the relative abundances of 17 elements in the atmosphere of the white dwarf star GD 362, material that, very probably, was contained previously in a large asteroid or asteroids with composition similar to the Earth/Moon system. The asteroid may have once been part of a larger parent body not unlike one of the terrestrial planets of our solar system.Comment: ApJ, in pres

Infrared studies have revealed debris likely related to planet formation in orbit around ∼30% of youthful, intermediate mass, main sequence stars. We present evidence, based on atmospheric pollution by various elements heavier than helium, that a comparable fraction of the white dwarf descendants of such main sequence stars are orbited by planetary systems. These systems have survived, at least in part, through all stages of stellar evolution that precede the white dwarf. During the time interval (∼200 million years) that a typical polluted white dwarf in our sample has been cooling it has accreted from its planetary system the mass of one of the largest asteroids in our solar system (e.g., Vesta or Ceres). Usually, this accreted mass will be only a fraction of the total mass of rocky material that orbits these white dwarfs; for plausible planetary system configurations we estimate that this total mass is likely to be at least equal to that of the Sun's asteroid belt, and perhaps much larger. We report abundances of a suite of 8 elements detected in the little studied star G241-6 that we find to be among the most heavily polluted of all moderately bright white dwarfs.