Infrared Signatures of Disrupted Minor Planets at White Dwarfs

Abstract

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.

Drifting asteroid fragments around WD 1145+017

“…The composition of the accreted material and total deposited mass can be consistent with the accretion of large asteroids like Ceres or Vesta in our own solar system (Zuckerman et al 2007;Gänsicke et al 2012;Farihi et al 2013). Additionally, dusty debris disks have been found around many of these polluted white dwarfs (Zuckerman et al 1987), linking the presence of elements in the photosphere to circumstellar material near the star's tidal disruption radius for rocky material (Kilic et al 2006;Farihi et al 2009;Barber et al 2012;Rocchetto 2015). The accepted explanation for these observations is that after the white dwarf's progenitor leaves the main sequence and undergoes mass loss, the progenitor's planetary system destabilizes, perturbing planetary orbits close enough to the white dwarf for tidal disruption (Debes & Sigurdsson 2002;Debes et al 2012;Mustill et al 2014;Veras et al 2014;.…”

Circumstellar debris and pollution at white dwarf stars

“…Because the known disks have comparable lifetimes based on both theoretical and observational studies Bochkarev & Rafikov, 2011;Girven et al, 2012), infrared observations are sensitive to any debris orbiting within a few AU but outside the Roche limit. The non-detection of contracting rings of debris (Jura et al, 2007a;Farihi et al, 2009) demonstrates that disks form significantly sooner than from PR drag alone, which specifically cannot pull the largest fragments (and hence masses) into close orbit.…”

Remnant planetary systems around bright white dwarfs

“…An easier way to detect remnant planetary systems around WDs is to look for the tidally disrupted remains of exoplanets, moons, and asteroids in the form of circumstellar debris discs (Debes & Sigurdsson 2002;Jura 2003;Kilic et al 2006;Farihi, Jura & Zuckerman 2009;Veras et al 2013Veras et al , 2014aVeras, Jacobson & Gänsicke 2014b). Consider a planetary system consisting of a WD, an asteroid-belt analogue and a Jupiter analogue.…”