The frequency of gaseous debris discs around white dwarfs

Abstract

1 -3 per cent of white dwarfs are orbited by planetary dusty debris detectable as infrared emission in excess above the white dwarf flux. In a rare subset of these systems, a gaseous disc component is also detected via emission lines of the Ca 8600 Å triplet, broadened by the Keplerian velocity of the disc. We present the first statistical study of the fraction of debris discs containing detectable amounts of gas in emission at white dwarfs within a magnitude and signal-to-noise limited sample. We select 7705 single white dwarfs spectroscopically observed by the Sloan Digital Sky Survey (SDSS) and Gaia with magnitudes g 19. We identify five gaseous disc hosts, all of which have been previously discovered. We calculate the occurrence rate of a white dwarf hosting a debris disc detectable via Ca emission lines as 0.067 ± 0.042 0.025 per cent. This corresponds to an occurrence rate for a dusty debris disc to have an observable gaseous component in emission as 4 ± 4 2 per cent. Given that variability is a common feature of the emission profiles of gaseous debris discs, and the recent detection of a planetesimal orbiting within the disc of SDSS J122859.93+104032.9, we propose that gaseous components are tracers for the presence of planetesimals embedded in the discs and outline a qualitative model. We also present spectroscopy of the Ca triplet 8600 Å region for 20 white dwarfs hosting dusty debris discs in an attempt to identify gaseous emission. We do not detect any gaseous components in these 20 systems, consistent with the occurrence rate that we calculated.

Rapid destruction of planetary debris around white dwarfs through aeolian erosion

“…Metal-polluted WDs, which represent 25 − 50% of the whole WD population (Zuckerman et al 2010;Koester et al 2014), constitute an observational signature of the accretion of heavy elements. A few per cent of the WDs are surrounded by planetary debris disks (Manser et al 2020), and over sixty WD disks have already been discovered (Zuckerman et al 2010;Farihi 2016). Most of the observed disks are dusty, although about twenty disks with gaseous components were discovered as well (Dennihy et al 2020;Melis et al 2020;Gentile Fusillo et al 2020).…”

The lifetimes of planetary debris discs around white dwarfs

“…The exo-planetesimals which do reach the immediate region of the white dwarf, within several solar radii 2 , have now been observed in three successive stages of their evolution, as: (i) Fully or partially intact, sometimes with accompanying fragments, orbiting the white dwarf (Vanderburg et al 2015;Manser et al 2019;Vanderbosch et al 2020), (ii) Broken up into an annulus of dust and sometimes gas around the white dwarf (Zuckerman & Becklin 1987;Farihi 2016;Manser et al 2020), and (iii) Chemically stratified at the atomic level in the photosphere of the white dwarf (van Maanen 1917(van Maanen , 1919Zuckerman et al 2007;Dufour et al 2010;Klein et al 2010;Gänsicke et al 2012;Jura & Young 2014).…”

“…The replenishment timescale -how often exoplanetesimals approach, break-up around and deposit themselves into a white dwarf -is one of the most important unknown parameters in post-main-sequence planetary science. Two reasons are (i) This replenishment timescale is linked to and perhaps constrained by the 25-50 per cent of the white dwarf population which is "metal polluted" (containing photospheric exoplanetary matter), the 1.5 per cent of white dwarfs which are both metal polluted and contain a dusty disc (Wilson et al 2019), the 0.07 per cent of white dwarfs which are metalpolluted and contain both observable dust and gas in a surrounding disc (Manser et al 2020), and the observational biases against their detection (e.g. Bonsor et al 2017).…”

How Jupiters Save or Destroy Inner Neptunes around Evolved Stars

“…About one third of all white dwarfs show atmospheric metal absorption lines that must result from the recent accretion of solid material (Koester et al 2014). Roughly 5 per cent of these white dwarfs show a detectable infrared excess indicative of the presence of a circumstellar debris disk (Barber et al 2012), and about the same fraction of the latter show a detectable gaseous disk component (Manser et al 2020). As first suggested by Jura (2003), metal polluted white dwarfs and the disks around them are the result of the tidal disruption of rocky planetary material (Veras et al 2014;Malamud & Perets 2020).…”