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We present images of the Vega system obtained with the IRAM Plateau de Bure interferometer at 1.3 millimeters wavelength with sub-mJy sensitivity and $sim2farcs5$ resolution (about 20 AU). These observations clearly detect the stellar photosphere and two dust emission peaks offset from the star by $9farcs5$ and $8farcs0$ to the northeast and southwest, respectively. These offset emission peaks are consistent with the barely resolved structure visible in previous submillimeter images, and they account for a large fraction of the dust emission. The presence of two dust concentrations at the observed locations is plausibly explained by the dynamical influence of an unseen planet of a few Jupiter masses in a highly eccentric orbit that traps dust in principle mean motion resonances.
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The photospheres of some white dwarfs are polluted by accretion of material from their surrounding planetary debris. White dwarfs with dust disks are often heavily polluted and high-resolution spectroscopic observations of these systems can be used to infer the chemical compositions of extrasolar planetary material. Here, we report spectroscopic observation and analysis of 19 white dwarfs with dust disks or candidate disks. The overall abundance pattern very much resembles that of bulk Earth and we are starting to build a large enough sample to probe a wide range of planetary compositions. We found evidence for accretion of Fe-rich material onto two white dwarfs as well as O-rich but H-poor planetary debris onto one white dwarf. In addition, there is a spread in Mg/Ca and Si/Ca ratios and it cannot be explained by differential settling or igneous differentiation. The ratios appear to follow an evaporation sequence. In this scenario, we can constrain the mass and number of evaporating bodies surrounding polluted white dwarfs.
We used chromospheric activity to determine the ages of 2,820 field stars.. We searched these stars for excess emission at 22 um with the Wide-Field Infrared Survey Explorer. Such excess emission is indicative of a dusty debris disk around a star. We investigated how disk incidence trends with various stellar parameters, and how these parameters evolve with time. We found 22 um excesses around 98 stars (a detection rate of 3.5%). Seventy-four of these 98 excess sources are presented here for the first time. We also measured the abundance of lithium in 8 dusty stars in order to test our stellar age estimates.
Clumpy structure in the debris disk around Vega has been previously reported at millimeter wavelengths and attributed to concentrations of dust grains trapped in resonances with an unseen planet. However, recent imaging at similar wavelengths with higher sensitivity has disputed the observed structure. We present three new millimeter-wavelength observations that help to resolve the puzzling and contradictory observations. We have observed the Vega system with the Submillimeter Array (SMA) at a wavelength of 880 um and angular resolution of 5; with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) at a wavelength of 1.3 mm and angular resolution of 5; and with the Green Bank Telescope (GBT) at a wavelength of 3.3 mm and angular resolution of 10. Despite high sensitivity and short baselines, we do not detect the Vega debris disk in either of the interferometric data sets (SMA and CARMA), which should be sensitive at high significance to clumpy structure based on previously reported observations. We obtain a marginal (3-sigma) detection of disk emission in the GBT data; the spatial distribution of the emission is not well constrained. We analyze the observations in the context of several different models, demonstrating that the observations are consistent with a smooth, broad, axisymmetric disk with inner radius 20-100 AU and width >50 AU. The interferometric data require that at least half of the 860 um emission detected by previous single-dish observations with the James Clerk Maxwell Telescope be distributed axisymmetrically, ruling out strong contributions from flux concentrations on spatial scales of <100 AU. These observations support recent results from the Plateau de Bure Interferometer indicating that previous detections of clumpy structure in the Vega debris disk were spurious.
White dwarfs are routinely observed to have polluted atmospheres, and sometimes significant infrared excesses, that indicate ongoing accretion of circumstellar dust and rocky debris. Typically this debris is assumed to be in the form of a (circular) disc, and to originate from asteroids that passed close enough to the white dwarf to be pulled apart by tides. However, theoretical considerations suggest that the circularisation of the debris, which initially occupies highly eccentric orbits, is very slow. We therefore hypothesise that the observations may be readily explained by the debris remaining on highly eccentric orbits, and we explore the properties of such debris. For the generic case of an asteroid originating at several au from the white dwarf, we find that all of the tidal debris is always bound to the white dwarf and that the orbital energy distribution of the debris is narrow enough that it executes similar elliptical orbits with only a narrow spread. Assuming that the tidal field of the white dwarf is sufficient to minimise the effects of self-gravity and collisions within the debris, we estimate the time over which the debris spreads into a single elliptical ring, and we generate toy spectra and lightcurves from the initial disruption to late times when the debris distribution is essentially time steady. Finally we speculate on the connection between these simple considerations and the observed properties of these systems, and on additional physical processes that may change this simple picture.
We present five band imaging of the Vega debris disc obtained using the Herschel Space Observatory. These data span a wavelength range of 70-500 um with full-width half-maximum angular resolutions of 5.6-36.9. The disc is well resolved in all bands, with the ring structure visible at 70 and 160 um. Radial profiles of the disc surface brightness are produced, and a disc radius of 11 (~ 85 AU) is determined. The disc is seen to have a smooth structure thoughout the entire wavelength range, suggesting that the disc is in a steady state, rather than being an ephemeral structure caused by the recent collision of two large planetesimals.
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