Likely detection of water-rich asteroid debris in a metal-polluted white dwarf

Abstract

The cool white dwarf SDSS J124231.07+522626.6 exhibits photospheric absorption lines of 8 distinct heavy elements in medium resolution optical spectra, notably including oxygen. The T eff = 13 000 K atmosphere is helium-dominated, but the convection zone contains significant amounts of hydrogen and oxygen. The four most common rock-forming elements (O, Mg, Si, and Fe) account for almost all the accreted mass, totalling at least 1.2×10 24 g, similar to the mass of Ceres. The time-averaged accretion rate is 2 × 10 10 g s −1 , one of the highest rates inferred among all known metal-polluted white dwarfs. We note a large oxygen excess, with respect to the most common metal oxides, suggesting that the white dwarf accreted planetary debris with a water content of ≈ 38 per cent by mass. This star, together with GD 61, GD 16, and GD 362, form a small group of outliers from the known population of evolved planetary systems accreting predominantly dry, rocky debris. This result strengthens the hypothesis that, integrated over the cooling ages of white dwarfs, accretion of water-rich debris from disrupted planetesimals may significantly contribute to the build-up of trace hydrogen observed in a large fraction of helium-dominated white dwarf atmospheres.

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

“…However, it is unclear if the amount of gas in these systems is sufficient to significantly contribute to the circularization process. In contrast to comets where out-gassing of volatiles can free copious quantities of gas, most metalpolluted white dwarfs are best explained by the accretion of volatile-depleted asteroids (e.g., Gänsicke et al 2012;, with possibly a few exceptions, Farihi et al 2013;Raddi et al 2015), where gas production via sublimation is less efficient.…”

Evidence for Eccentric, Precessing Gaseous Debris in the Circumstellar Absorption toward WD 1145 + 017

“…The atmospheric abundances of heavy elements reflects the composition of the orbiting and infalling planetesimal debris, and hence the bulk composition of the disrupted parent body. Polluted white dwarf atmospheres thus offer an exciting window into the chemistry and assembly of the planets and minor bodies formed in the system (Gänsicke et al 2012;Farihi et al 2013;Xu et al 2013;Jura & Young 2014;Raddi et al 2015).…”

Does a Differentiated, Carbonate-Rich, Rocky Object Pollute the White Dwarf SDSS J104341.53+085558.2?

“…White dwarfs that are iron-poor or that otherwise have Earth surface-layer-like compositions have been previously reported (e.g., Zuckerman et al 2011;Farihi et al 2013;Raddi et al 2015). Having securely identified four examples of such systems now with the characterization of SDSS J1043+0855, it seems prudent to evaluate how such systems compare to other well-characterized polluted white dwarfs.…”

“…The large, black filled circle is for the core of Earth which is based on extrapolations from chondrite data and assumes no Mg is in the core (Allègre et al 1995); the Earth core value for Mg/Fe is set to Si/Fe for plotting purposes. White dwarf data come from Dufour et al (2010Dufour et al ( , 2012, Zuckerman et al (2011), Gänsicke et al (2012), Farihi et al (2013), Xu et al (2013Xu et al ( , 2014Xu et al ( , 2016, Wilson et al (2015), Raddi et al (2015), and Farihi et al (2016). Metal abundance ratio errors for white dwarf measurements are adjusted similar to those for SDSS J1043+0855 like in Figure 2.…”