Accretion of a giant planet onto a white dwarf star

Abstract

The detection of a dust disc around G29-38 1 and transits from debris orbiting WD 1145+017 2 confirmed that the photospheric trace metals found in many white dwarfs 3 arise from the accretion of tidally disrupted planetesimals 4 . The composition of these planetesimals is similar to that of rocky bodies in the inner solar system 5 . Gravitationally scattering planetesimals towards the white dwarf requires the presence of more massive bodies 6 , yet no planet has so far been detected at a white dwarf. Here we report optical spectroscopy of a ≃ 27 750 K hot white dwarf that is accreting from a circumstellar gaseous disc composed of hydrogen, oxygen, and sulphur at a rate of ≃ 3.3 × 10 9 g s −1 . The composition of this disc is unlike all other known planetary debris around white dwarfs 7 , but resembles predictions for the makeup of deeper atmospheric layers of icy giant planets, with H 2 O and H 2 S being major constituents. A giant planet orbiting a hot white dwarf with a semi-major axis of ≃ 15 solar radii will undergo significant evaporation with expected mass loss rates comparable to the accretion rate onto the white dwarf. The orbit of the planet is most likely the result of gravitational interactions, indicating the presence of additional planets in the system. We infer an occurrence rate of spectroscopically detectable giant planets in close orbits around white dwarfs of ≃ 10 −4 . WD J091405.30+191412.25 (WD J0914+1914) was initially classified as a close interacting white dwarf binary on the basis of a weak Hα emission line detected in its spectrum obtained by the Sloan Digital Sky Survey (SDSS) 8 . Upon closer inspection of this spectrum, we identified additional emission lines of oxygen (O I at wavelengths 7,774Å and 8,446Å), and an emission line near 4,068Å that we tentatively identified as [S II]. The line flux ratios of the hydrogen and oxygen lines are extremely atypical for any white dwarf binary, casting doubt on the published classification.We obtained deep spectroscopy of this star using the X-Shooter spectrograph on the Very Large Telescope of the European Southern Observatory (see Fig. 1) which confirms the presence of [S II] (4,068Å), and contains additional emission lines of [O I] (6,300Å and 6,363Å) as well as a blend of O I and S I lines near 9,200Å.The double-peaked morphology of the Hα and the O I (8,446Å) emission lines (see Fig. 1) indicates an origin in a circumstellar gas disc 9 , reminiscent of several white dwarfs with dusty and gaseous planetary debris discs 10, 11 . However, the spectra of all known gaseous debris discs are dominated by the emission lines of the Ca II triplet (8,600Å), with weaker lines of Fe II, which are absent in the X-Shooter observations of WD J0914+1914. Moreover, none of the other gaseous debris discs around white dwarfs show Hα emission.

The frequency of gaseous debris discs around white dwarfs

“…In a subset of these systems, gaseous debris has also been observed that is co-orbital with the dust ). This gaseous material has so far been detected in seven systems (Gänsicke et al 2006(Gänsicke et al , 2007(Gänsicke et al , 2008Gänsicke 2011;Farihi et al 2012;Melis et al 2012;Wilson et al 2014), via the Dopplerbroadened, double-peaked line emission of the Ca 8600 Å triplet, which results from the Keplerian rotation of a flat disc (Horne & Marsh 1986) that is photoionised by the white dwarf Kinnear 2011;Gänsicke et al 2019).…”

The white dwarf planet WD J0914+1914 b: barricading potential rocky pollutants?

“…No rocky material has been detected in this systemyet -although Gänsicke et al (2019) derived upper limits for abundances of rocky elements in the white dwarf's c 2020 RAS photosphere. The planet's location is uncertain, but was modelled to reside at a distance of about 0.07 au from the star.…”

“…However, in none of these systems with rocky pollution has a major planet been detected so far. Strikingly then, Gänsicke et al (2019) inferred the presence of a likely ice giant planet orbiting the volatile-rich white dwarf WD J091405.30+191412.25 (henceforth WD J0914+1914). This finding was based on the detection of abundant oxygen and sulfur (volatile elements) in combination with hydrogen throughout the system: both in the photosphere and the surrounding gaseous disc.…”

Asteroid belt survival through stellar evolution: dependence on the stellar mass

“…Atomic emission lines suggest the existence of gaseous discs colocated with the compact circumstellar dust (Gänsicke et al 2006;Guo et al 2015). There is one exception to the compactness of gaseous discs, the disc around WD J0914+1914 is thought to be formed from an evaporating giant planet on a close-in orbit around the white dwarf (Gänsicke et al 2019).…”