Hydrogen delivery onto white dwarfs from remnant exo-Oort cloud comets

Abstract

The origin of trace hydrogen in white dwarfs (WDs) with He-dominated atmospheres is a longstanding problem, one that cannot satisfactorily be explained by the historically-favoured hypothesis of accretion from the interstellar medium. Here we explore the possibility that the gradual accretion of exo-Oort cloud comets, which are a rich source of H, contributes to the apparent increase of trace H with WD cooling age. We determine how often remnant exo-Oort clouds, freshly excited from post-main-sequence stellar mass loss, dynamically inject comets inside the WD's Roche radius. We improve upon previous studies by considering a representative range of single WD masses (0.52 − 1.00M ⊙ ) and incorporating different cloud architectures, giant branch stellar mass loss, stellar flybys, Galactic tides and a realistic escape ellipsoid in self-consistent numerical simulations that integrate beyond 8 Gyr ages of WD cooling. We find that ∼ 10 −5 of the material in an exo-Oort cloud is typically amassed onto the WD, and that the H deposits accumulate even as the cloud dissipates. This accumulation may account for the relatively large amount of trace H, 10 22 − 10 25 g, that is determined frequently among WDs with cooling ages 1 Gyr. Our results also reaffirm the notion that exo-Oort cloud comets are not the primary agents of the metal budgets observed in polluted WD atmospheres.

DB white dwarfs in the Sloan Digital Sky Survey data release 10 and 12

“…The size of these average rates is reasonable (Dupuis et al 1993), although the huge spread and the increase toward lower temperatures might be difficult to explain. An alternative proposal, the continuous accretion of comets from an Oort cloud (Veras et al 2014), could also explain such an increase toward older white dwarfs, but this faces the same problems.…”

Post-Main Sequence Evolution of Icy Minor Planets: Implications for Water Retention and White Dwarf Pollution

“…By looking at this particular oxygen-containing subset of WDs, Jura & Young (2014) further show that no more than 5.8% of this oxygen could have been carried in water. A common bias of these studies is that they involve relatively young WDs with cooling ages up to ∼300 Myr (Veras et al 2014c).…”

“…The evidence for this comes from He-dominated WDs with trace hydrogen, that observationally increases with cooling age (Dufour et al 2007). Although biased by both physical effects (the depth of the convection zone) and observational limitations, according to Veras et al (2014c) it could imply the long term accretion of water-bearing planetesimals, and it is not likely to be attributed to primordial hydrogen or interstellar accretion (Bergeron et al 2011). Polluted He-dominated (DBZ) WDs like GD 61 and SDSSJ1242 will eventually resemble typical DBA WDs (with some hydrogen but no heavy elements), as their heavy elements will diffuse out of the convection zone.…”

The fate of exomoons in white dwarf planetary systems

“…In this respect, liberated moons meandering within 1 au may provide a crucial component of the conveyor belt provided by the remaining planets (Bonsor & Wyatt 2012). Such moons may also increase the efficiency rate of cometary impacts (Alcock, Fristrom & Siegelman 1986;Veras & Wyatt 2012;Veras, Shannon & Gänsicke 2014d;Veras et al 2014a;Stone, Metzger & Loeb 2015), although compositionally comets remain disfavoured (Klein et al , 2011Gänsicke et al 2012;Jura et al 2012;Xu et al 2013Xu et al , 2014Wilson et al 2015) even for progenitors which are thought to be water rich (Farihi, Gänsicke & Koester 2013;Raddi et al 2015). …”