Formation of planetary debris discs around white dwarfs – II. Shrinking extremely eccentric collisionless rings

Abstract

The formation channel of the tens of compact debris discs which orbit white dwarfs (WDs) at a distance of one Solar radius remains unknown. Asteroids that survive the giant branch stellar phases beyond a few au are assumed to be dynamically thrust towards the WD and tidally disrupted within its Roche radius, generating extremely eccentric (e > 0.98) rings.Here, we establish that WD radiation compresses and circularizes the orbits of super-micron to cm-sized ring constituents to entirely within the WD's Roche radius. We derive a closed algebraic formula which well-approximates the shrinking time as a function of WD cooling age, the physical properties of the star and the physical and orbital properties of the ring particles. The shrinking timescale increases with both particle size and cooling age, yielding age-dependent WD debris disc size distributions.

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

“…The asteroid further would be too large to be circularized by radiation alone, either through the Yarkovsky effect or Poynting-Robertson drag (Veras et al 2015a(Veras et al , 2015c. One possibility is that some fraction of the asteroid has sublimated.…”

“…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). The life times of these disks are thought to be long compared to human timescales, 10 10 4 6 -years (Girven et al 2012), yet, a small number of disks show substantial variability on timescales of years to decades in their infrared flux or in the strength and morphology of optical emission lines from gaseous disk components (Wilson et al 2014(Wilson et al , 2015Manser et al 2016), indicative of ongoing dynamical processes.…”

The fate of exomoons in white dwarf planetary systems

“…Hence, orbiting bodies are not affected by stellar mass-loss (see section 4 of Veras 2016a). Objects under 1000 km in size would be affected by radiation from the parent star on the giant branch phase (Veras, Eggl & Gänsicke 2015a), but not around a white dwarf unless the object was a boulder (approximately 0.1 m) or smaller (Veras et al 2015b) or was outgassing significant volatiles (Veras, Eggl & Gänsicke 2015c). Here, we consider just point-mass gravitational dynamics.…”

“…This notion was quantified by Bonsor, Mustill & Wyatt (2011), Debes, Walsh & Stark (2012) and Frewen & Hansen (2014), who showed how a planet can perturb an asteroid in the vicinity of a white dwarf, as long as some configurations are avoided (Antoniadou & Veras 2016). After tidally breaking up (Debes et al 2012;Veras et al 2014b), the resulting debris is then circularized by stellar radiation (and possibly additional mechanisms, such as gas drag; Veras et al 2015b), forming a disc which eventually accretes on to the white dwarf (Rafikov 2011a,b;Rafikov & Garmilla 2012;Metzger, Rafikov & Bochkarev 2012).…”

The unstable fate of the planet orbiting the A star in the HD 131399 triple stellar system

“…This debris predominately arises from tidal breakup (Debes, Walsh & Stark 2012; Veras et al 2014aVeras et al , 2015aVeras et al , 2016a of progenitor asteroids which have compositions that could be mapped to particular Solar system asteroid families (e.g. fig.…”