Strengthening the Case for Asteroidal Accretion: Evidence for Subtle and Diverse Disks at White Dwarfs

Abstract

Spitzer Space Telescope IRAC 3 − 8 µm and AKARI IRC 2 − 4 µm photometry are reported for ten white dwarfs with photospheric heavy elements; nine relatively cool stars with photospheric calcium, and one hotter star with a peculiar high carbon abundance. A substantial infrared excess is detected at HE 2221−1630, while modest excess emissions are identified at HE 0106−3253 and HE 0307+0746, implying these latter two stars have relatively narrow (∆r < 0.1 R ⊙ ) rings of circumstellar dust. A likely 7.9 µm excess is found at PG 1225−079 and may represent, together with G166-58, a sub-class of dust ring with a large inner hole. The existence of attenuated disks at white dwarfs substantiates the connection between their photospheric heavy elements and the accretion of disrupted minor planets, indicating many polluted white dwarfs may harbor orbiting dust, even those lacking an obvious infrared excess.

Circumstellar debris and pollution at white dwarf stars

“…Subsequent disk searches with Spitzer began to include all known types of polluted white dwarfs, and attempted to analyze all the stars on the same basis of inferred accretion rate (Farihi et al, 2009). Within a few years, the number of disks orbiting helium-rich white dwarfs more than doubled (Farihi et al, 2010b), and currently there are at least ten similar systems.…”

“…While it is possible some of these brighter disk candidates are contaminated by additional, unresolved sources, it can also be the case that more subtle, authentic disk excesses (e.g. PG 1457−086, HE 0106−3253; Farihi et al 2010b) are often missed. However, the McCook & Sion (1999) sample is likely to contain a significant fraction of white dwarfs with T eff < 8000 K, and based on the combined Spitzer studies, will likely result in a smaller measured fraction of detectable disks.…”

The Drop During Less Than 300 Days of a Dusty White Dwarf's Infrared Luminosity

“…For the pre-March 2010 data, we find the fractional luminosity L IR /L wd = 0.03, the highest of all dusty white dwarfs (Farihi et al 2010). A face-on disk with a small inner disk radius, correspondingly a high inner disk temperature, is needed to fit the SED.…”

“…To increase the inner radius from 10.5 R wd to 14 R wd , the disk has decreased 5 × 10 10 km 2 in surface area, much larger than the typical size of an asteroid in the solar system. For comparison, there exist dusty white dwarfs with much larger inner holes, e.g., PG 1225-079 and G166-58, possibly also due to planetesimal impacts (Farihi et al 2010). Recently, the tidal disruption of an entire planet by a white dwarf has been invoked to explain the light curve of a source in the center of a globular cluster (Del Santo et al 2014).…”

ALMA and Herschel observations of the prototype dusty and polluted white dwarf G29-38

“…Over 30 metal-lined white dwarfs are now known to exhibit T ∼ 1000 K thermal emission from disks (e.g. Brinkworth et al 2012;Girven et al 2012;Farihi et al 2012Farihi et al , 2010bFarihi et al , 2009Jura et al 2007;von Hippel et al 2007;Kilic et al 2006); their properties are precisely as expected for material contained within the Roche limit of the star and feeding the stellar surface (Metzger et al 2012;Rafikov 2011;Bochkarev & Rafikov 2011). A fraction of these exhibit metallic, gaseous emission Melis et al 2012Melis et al , 2011Gänsicke et al 2008Gänsicke et al , 2007Gänsicke et al , 2006 that is spatially coincident with the particulate disks Melis et al 2010).…”