The frequency of planetary debris around young white dwarfs

Abstract

Context. Heavy metals in the atmospheres of white dwarfs are thought in many cases to be accreted from a circumstellar debris disk, which was formed by the tidal disruption of a rocky planetary body within the Roche radius of the star. The abundance analysis of photospheric elements and conclusions about the chemical composition of the accreted matter are a new and promising method of studying the composition of extrasolar planetary systems. However, ground-based searches for metal-polluted white dwarfs that rely primarily on the detection of the Ca ii K line become insensitive at T eff > 15 000 K because this ionization state depopulates.Aims. We present the results of the first unbiased survey for metal pollution among hydrogen-atmosphere (DA type) white dwarfs with cooling ages in the range 20-200 Myr and 17 000 K < T eff < 27 000 K. Methods. The sample was observed with the Cosmic Origins Spectrograph on board the Hubble Space Telescope in the far ultraviolet range between 1130 and 1435 Å. The atmospheric parameters were obtained using these spectra and optical observations from the literature. Element abundances were determined using theoretical models, which include the effects of element stratification due to gravitational settling and radiative levitation. Results. We find 48 of the 85 DA white dwarfs studied, or 56% show traces of heavy elements. In 25 stars (showing only Si and occasionally C), the elements can be explained by radiative levitation alone, although we argue that accretion has very likely occurred recently. The remaining 23 white dwarfs (27%), however, must be currently accreting. Together with previous studies from the ground and adopting bulk Earth abundances for the debris, accretion rates range from a few 10 5 g s −1 to a few 10 8 g s −1 , with no evident trend in cooling age from ≈40 Myr to ≈2 Gyr. Only a single, modest case of metal pollution (Ṁ < 10 6 g s −1 ) is found among ten white dwarfs with T eff > 23 000 K, in excellent agreement with the absence of infrared excess from dust around these warmer stars. The median, main sequence progenitor of our sample corresponds to an A-type star of ≈2 M , and we find 13 of 23 white dwarfs descending from main sequence 2-3 M , late B-and A-type stars to be currently accreting. Only one of 14 targets with M wd > 0.8 M is found to be currently accreting, which suggests a large fraction of these stars result from double-degenerate mergers, and the merger disks do not commonly reform large planetesimals or otherwise pollute the remnant. We reconfirm our previous finding that two 625 Myr Hyades white dwarfs are currently accreting rocky planetary debris. Conclusions. At least 27% of all white dwarfs with cooling ages 20-200 Myr are accreting planetary debris, but that fraction could be as high as ≈50%. At T eff > 23 000 K, the luminosity of white dwarfs is probably sufficient to vaporize circumstellar dust grains, so no stars with strong metal-pollution are found. Planetesimal disruption events should occur in this cooling age and temper...

Circumstellar debris and pollution at white dwarf stars

“…More specifically and importantly, the predictions of PR drag on white dwarf disks can account for the full range of instantaneous accretion rates (Rafikov, 2011a;, including all the DAZ stars with diffusion timescales less than 1000 yr. That is, the PR drag model successfully describes the subset of white dwarfs where ongoing, steady-state accretion is likely. Moreover, the disk lifetimes predicted by these models are a good match to those inferred from observations based on 1) the fraction of polluted white dwarfs over a given range of cooling ages Koester et al, 2014), 2) the mass of metals in the outer layers of convective stars divided by typical accretion rates , or 3) the steady-state accretion phase necessary for self-consistent oxygen chemistry (Klein et al, 2011). These combined theoretical and empirical data support a picture where the most prominent disks have m 10 20 g and hence parent bodies with d 40 km .…”

Infrared observations of white dwarfs and the implications for the accretion of dusty planetary material

“…A literature search reveals that at least 39 stars in the sample have known pollution (7%). HST COS observations of a sub-set (85 stars) find a pollution rate of at least 27% (Koester et al 2014), suggesting a similar rate of pollution for the full sample. This compares well to typical pollution rates observed for other samples of white dwarfs.…”

High-Speed Photometry of the Disintegrating Planetesimals at Wd1145+017: Evidence for Rapid Dynamical Evolution

“…This expectation is corroborated by the detection of debris accreted into the photospheres of white dwarfs, resulting from the tidal disruption (Debes & Sigurdsson 2002;Jura 2003) of planetary bodies among ;25%-50% of all white dwarfs (Zuckerman et al 2003;Koester et al 2014). Little is known so far regarding the detailed nature of the disrupted objects, the exact origin within their planetary systems, and the processes resulting in their disintegration and subsequent circularization (Debes et al 2012;Veras et al 2014bVeras et al , 2015c) and leading to dusty debris disks with typical radii of ;1 R  , which have been detected as infrared excess to ;40 white dwarfs (Rocchetto et al 2015).…”