Accretion of tidally disrupted asteroids onto white dwarfs: direct accretion versus disk processing

Abstract

Atmospheric heavy elements have been observed in more than a quarter of white dwarfs (WDs) at different cooling ages, indicating ongoing accretion of asteroidal material, whilst only a few per cent of the WDs possess a dust disk, and all these WDs are accreting metals.Here, assuming that a rubble-pile asteroid is scattered inside a WD's Roche lobe by a planet, we study its tidal disruption and the long-term evolution of the resulting fragments. We find that after a few pericentric passages, the asteroid is shredded into its constituent particles, forming a flat, thin ring. On a timescale of Myr, tens of per cent of the particles are scattered onto the WD. Fragment mutual collisions are most effective for coplanar fragments, and are thus only important in 10 3 − 10 4 yr before the orbital coplanarity is broken by the planet. We show that for a rubble pile asteroid with a size frequency distribution of the component particles following that of the near earth objects, it has to be roughly at least 10 km in radius such that enough fragments are generated and ≥ 10% of its mass is lost to mutual collisions. At relative velocities of tens of km/s, such collisions grind down the tidal fragments into smaller and smaller dust grains. The WD radiation forces may shrink those grains' orbits, forming a dust disk. Tidal disruption of a monolithic asteroid creates large km-size fragments, and only parent bodies ≥ 100 km are able to generate enough fragments for mutual collisions to be significant.

A road-map to white dwarf pollution: Tidal disruption, eccentric grind-down, and dust accretion

“…where we use that the eccentricities of fragments in these tidal discs are close to unity. We visualise the range of this scattering 1 During the preparation of this manuscript, we became aware of the contemporary manuscript by Li et al (2021). These authors have independently come to similar conclusions as presented here, based on numerical simulations of the continued scattering of eccentric fragments.…”

“…14, plotted at three different times at an emission wavelength of 10 µm. Panel a (t = 0) corresponds to the moment after disruption, assuming that the fragments are spread uniformly over the orbit, which is known to only require a few orbits (Veras et al 2014;Malamud & Perets 2020a;Li et al 2021). Panel b is plotted at 1900 yr, just before the first fragments sublimate and accrete, which happens when their pericentre crosses the sublimation distance (white dashed circle), assumed to be located at 1500 K, corresponding to the temperature where the vapor pressure of silicates becomes significant and they begin to sublimate in vacuum (van Lieshout et al 2014).…”

“…If the inclination of the dust remains small, the amount of stellar light that can reach the dust grains declines and circularisation rates slow down (see also the models by Rafikov 2011b,a). However, if the inclination of the tidal disc does increase from its initial value, for instance due to interactions with neighboring planets (Li et al 2021), the disc can remain entirely optically thin and high dust accretion rates by PR drag are possible. This scenario could explain the few cases where infrared excesses are observed.…”

“…Some of these systems also show evidence of on-going gas production (Gänsicke et al 2006(Gänsicke et al , 2007(Gänsicke et al , 2008Manser et al 2020) and they occasionally contain larger, transiting bodies (Vanderburg et al 2015;Manser et al 2019; Vanderbosch et al 2020Vanderbosch et al , 2021Guidry et al 2021). While no complete description of the accretion process currently exists, the general hypothesis is that small fragments are circularised by Poynting-Robertson (PR) drag (Rafikov 2011b;Veras et al 2015a,b), while larger fragments require a prior phase of collisional grind-down (Jura 2003;Jura et al 2007;Wyatt et al 2011;Li et al 2021). Other mechanisms that induce orbital changes after disruption are the Yarkovski force (Veras et al 2015a,b;Malamud & Perets 2020b), potential interactions with pre-existing material around the star (Grishin & Veras 2019;Malamud et al 2021) and magnetic Alfvénwave drag (Zhang et al 2021).…”

Relentless and Complex Transits from a Planetesimal Debris Disk

“…et al 2011Debes et al 2012;Frewen & Hansen 2014;Smallwood et al 2018;Mustill et al 2018). The process of orbital compaction and circularization is almost certainly driven by collisions (Malamud et al 2021;Li et al 2021), as Poynting-Robertson drag is far too feeble across the bulk of parameter space for white dwarf luminosities, particle sizes, and initial orbits (Veras et al 2015). In contrast, infrared and complementary wavelength disk observations indicate that most, if not all known disks have compact radii that are in the neighborhood of the Roche limit, with no cooling age dependence on the shape of the dust spectral energy distribution (Farihi et al 2009;Rocchetto et al 2015).…”