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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.

We present Keck/HIRES data with model atmosphere analysis of the helium-dominated polluted white dwarf GD 40, in which we measure atmospheric abundances relative to helium of 9 elements: H, O, Mg, Si, Ca, Ti, Cr, Mn, and Fe. Apart from hydrogen whose association with the other contaminants is uncertain, this material most likely accreted from GD 40's circumstellar dust disk whose existence is demonstrated by excess infrared emission. The data are best explained by accretion of rocky planetary material, in which heavy elements are largely contained within oxides, derived from a tidally disrupted minor planet at least the mass of Juno, and probably as massive as Vesta. The relatively low hydrogen abundance sets an upper limit of 10% water by mass in the inferred parent body, and the relatively high abundances of refractory elements, Ca and Ti, may indicate high-temperature processing. While the overall constitution of the parent body is similar to the bulk Earth being over 85% by mass composed of oxygen, magnesium, silicon and iron, we find n(Si)/n(Mg) = 0.30 ± 0.11, significantly smaller than the ratio near unity for the bulk Earth, chondrites, the Sun, and nearby stars. This result suggests that differentiation occurred within the parent body.

We have performed a comprehensive ground-based observational program aimed at characterizing the circumstellar material orbiting three single white dwarf stars previously known to possess gaseous disks. Near-infrared imaging unambiguously detects excess infrared emission towards Ton 345 and allows us to refine models for the circumstellar dust around all three white dwarf stars. We find that each white dwarf hosts gaseous and dusty disks that are roughly spatially coincident, a result that is consistent with a scenario in which dusty and gaseous material has its origin in remnant parent bodies of the white dwarfs' planetary systems. We briefly describe a new model for the gas disk heating mechanism in which the gaseous material behaves like a "Z II" region. In this Z II region, gas primarily composed of metals is photoionized by ultraviolet light and cools through optically thick allowed Ca II-line emission.

We report Keck High Resolution Echelle Spectrometer data and model atmosphere analysis of two helium-dominated white dwarfs, PG1225-079 and HS2253+8023, whose heavy pollutions most likely derive from the accretion of terrestrial-type planet(esimal)s. For each system, the minimum accreted mass is ∼10 22 g, that of a large asteroid. In PG1225-079, Mg, Cr, Mn, Fe and Ni have abundance ratios similar to bulk Earth values, while we measure four refractory elements, Ca, Sc, Ti and V, all at a factor of ∼2-3 higher abundance than in the bulk Earth. For HS2253+8023 the swallowed material was compositionally similar to bulk Earth in being more than 85% by mass in the major element species, O, Mg, Si, and Fe, and with abundances in the distinctive proportions of mineral oxides -compelling evidence for an origin in a rocky parent body. Including previous studies we now know of four heavily polluted white dwarfs where the measured oxygen and hydrogen are consistent with the view that the parents bodies formed with little ice, interior to any snow-line in their nebular environments. The growing handful of polluted white dwarf systems with comprehensive abundance measurements form a baseline for characterizing rocky exoplanet compositions that can be compared with bulk Earth.Subject headings: Planets and satellites: composition -Stars: individual: (HS2253+8023, PG1225-079, G 241-6) -Stars: abundances -white dwarfs required mass influx to explain observed polluted white dwarf (WD) accretion rates. Objects that journey within the Roche radius of the WD will experience tidal shredding, forming a disk of dust and/or gas (Jura 2003(Jura , 2008, and subsequently accrete into the star's, otherwise pure, hydrogen and/or helium atmosphere. This natural mechanism offers the potential to measure the distilled elemental constituents of a planetary parent body or bodies -a unique and powerful tool, indeed.There are a number of benefits of working with helium-dominated WDs. One is that, compared to hydrogendominated WD atmospheres, the lower opacity helium environments more readily display tiny concentrations of numerous contaminant elements, e.g. the trace elements vanadium and scandium are detected in PG1225 at 10 -11 orders of magnitude lower abundance than the helium background they are diluted in. Secondly, detailed model atmosphere analysis of helium-atmosphere WDs indicates the hydrogen in these stars must be accreted (Voss et al. 2007;Dufour et al. 2007); thus when helium is the background constituent, one obtains a measurement of the polluting hydrogen abundance, which is important for understanding the potential accretion of water ice Jura & Xu 2010). A simplifying aspect of the helium-atmosphere WDs is that in our T ef f range of interest (< 20,000 K), all have outer convection zones where the elements are homogeneously mixed. Finally, another benefit of working with helium-rich WDs is that with relatively large convective zones (relatively long settling times), the pollutants are not disappearing so fast from our view. Thus, a ...

We report Keck/HIRES and HST/COS spectroscopic studies of extrasolar rocky planetesimals accreted onto two hydrogen atmosphere white dwarfs, 8 elements are detected, including C, O, Mg, Si, Ca, Ti, Cr and Fe while in GD 133, O, Si, Ca and marginally Mg are seen. These two extrasolar planetesimals show a pattern of refractory enhancement and volatile depletion. For G29-38, the observed composition can be best interpreted as a blend of a chondritic object with some refractory-rich material, a result from post-nebular processing. Water is very depleted in the parent body accreted onto G29-38, based on the derived oxygen abundance. The inferred total mass accretion rate in GD 133 is the lowest of all known dusty white dwarfs, possibly due to non-steady state accretion. We continue to find that a variety of extrasolar planetesimals all resemble to zeroth order the elemental composition of bulk Earth.

The presence of elements heavier than helium in white dwarf atmospheres is often a signpost for the existence of rocky objects that currently or previously orbited these stars. We have measured the abundances of various elements in the hydrogen-atmosphere white dwarfs G149-28 and NLTT 43806. In comparison with other white dwarfs with atmospheres polluted by heavy elements, NLTT 43806 is substantially enriched in aluminum but relatively poor in iron. We compare the relative abundances of Al and eight other heavy elements seen in NLTT 43806 with the elemental composition of bulk Earth, with simulated extrasolar rocky planets, with solar system meteorites, with the atmospheric compositions of other polluted white dwarfs, and with the outer layers of the Moon and Earth. Best agreement is found with a model that involves accretion of a mixture of terrestrial crust and upper mantle material onto NLTT 43806. The implication is that NLTT 43806 is orbited by a differentiated rocky planet, perhaps quite similar to Earth, that has suffered a collision that stripped away some of its outer layers.

Using the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we have obtained high-resolution ultraviolet observations of GD 362 and PG 1225-079, two helium-dominated, externally-polluted white dwarfs. We determined or placed useful upper limits on the abundances of two key volatile elements, carbon and sulfur, in both stars; we also constrained the zinc abundance in PG 1225-079. In combination with previous optical data, we find strong evidence that each of these two white dwarfs has accreted a parent body that has evolved beyond primitive nebular condensation. The planetesimal accreted onto GD 362 had a bulk composition roughly similar to that of a mesosiderite meteorite based on a reduced chi-squared comparison with solar system objects; however, additional material is required to fully reproduce the observed mid-infrared spectrum for GD 362. No single meteorite can reproduce the unique abundance pattern observed in PG 1225-079; the best fit model requires a blend of ureilite and mesosiderite material. From a compiled sample of 9 well-studied polluted white dwarfs, we find evidence for both primitive planetesimals, which are a direct product from nebular condensation, as well as beyond-primitive planetesimals, whose final compositions were mainly determined by post-nebular processing.