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

Abstract

Most observations of polluted white dwarf atmospheres are consistent with accretion of water depleted planetary material. Among tens of known cases, merely two cases involve accretion of objects that contain a considerable mass fraction of water. The purpose of this study is to investigate the relative scarcity of these detections. Based on a new and highly detailed model, we evaluate the retention of water inside icy minor planets during the high luminosity stellar evolution that follows the main sequence. Our model fully considers the thermal, physical, and chemical evolution of icy bodies, following their internal differentiation as well as water depletion, from the moment of their birth and through all stellar evolution phases preceding the formation of the white dwarf. We also account for different initial compositions and formation times. Our results differ from previous studies, that have either underestimated or overestimated water retention. We show that water can survive in a variety of circumstances and in great quantities, and therefore other possibilities are discussed in order to explain the infrequency of water detections. We predict that the sequence of accretion is such that water accretes earlier, and more rapidly than the rest of the silicate disk, considerably reducing the chance of its detection in H-dominated atmospheres. In He-dominated atmospheres, the scarcity of water detections could be observationally biased. It implies that the accreted material is typically intrinsically dry, which may be the result of inside-out depopulation sequence of minor planets.

The Chemical Composition of an Extrasolar Kuiper-Belt-Object*

“…The excess oxygen is attributable to accretion of H 2 O rich objects (Farihi et al 2013). The rarity of water among the objects sampled by polluted white dwarfs to date is likely to be an indicator of low primordial water content because water and volatiles are generally expected to survive the post main sequence evolution of the host star (Jura & Xu 2010;Malamud & Perets 2016). In general, the overall compositions of extrasolar rocky material identified thus far closely resemble those of dry, relatively volatile-poor asteroids in our solar system.…”

Tidal disruption of planetary bodies by white dwarfs I: a hybrid sph-analytical approach

“…Since the irradiation is proportional to the square of the orbital distance, disrupted water-bearing fragments (whose pericentre distances are between ∼ 0.1 − 1R ⊙ ) receive ∼ 10 4 − 10 6 times the intrinsic luminosity of the WD during close approach, when compared to typical Solar system comets at 1 AU. Depending on its cooling age, WD luminosity ranges between 10 3 − 10 −5 L ⊙ , hence we might typically expect 10 −1 − 10 9 the amount of insolation, compared to Solar system comets (see Malamud & Perets (2016) for discussion). However, we recall that in tidal disruptions the characteristic time a fragment spends near perihelion is only ∼ 10 −1 days.…”

“…In particular, it might require an approach which utilizes the hybrid technique in combination with other numerical methods or analytical calculations. E.g., the sublimation evolution of fragments may be studied either through complex numerical simulations like the ones used by Malamud & Perets (2016 or via detailed analytical treatment (Brown et al 2017).…”

Polluted white dwarfs: constraints on the origin and geology of exoplanetary material

“…Evidence for the accretion of water ice in eight systems has been based on an excess abundance of observed oxygen compared with that which could be sequestered in the form of metal oxides using the observed metal abundances Farihi et al 2011;Dufour et al 2012;Gänsicke et al 2012;Raddi et al 2015;Farihi et al 2016;Xu et al 2017). The accretion of water ice is not surprising as ice species are expected to survive the post main sequence evolution of the host star (Jura & Xu 2010;Malamud & Perets 2016). Xu et al (2017) recently detected N for the first time, in the pollutant of WD 1425+540.…”