Transiting Disintegrating Planetary Debris Around WD 1145+017

Abstract

More than a decade after astronomers realized that disrupted planetary material likely pollutes the surfaces of many white dwarf stars, the discovery of transiting debris orbiting the white dwarf WD 1145+017 has opened the door to new explorations of this process. We describe the observational evidence for transiting planetary material and the current theoretical understanding (and in some cases lack thereof) of the phenomenon. OverviewA bit more than a decade after astronomers first began to suspect that white dwarf stars occasionally disrupt and accrete asteroids and small planets from their primordial planetary systems (Debes and Sigurdsson 2002;Jura 2003), an impressive body of evidence had emerged in support of this scenario. In early 2015, it was known that (a) a large fraction of white dwarf stars (between 25% and 50%) are "polluted" with trace amounts of elements like silicon, iron, calcium and magnesium in their atmospheres (Zuckerman et al. 2010;Koester et al. 2014), (b) many of these polluted white dwarfs also showed evidence of warm rocky material orbiting the star in a debris disk (Barber et al. 2012), and (c) the abundance ratios of heavy elements in the atmospheres of white dwarfs very closely matched the abundance patterns in rocky bodies in the solar system (Zuckerman et al. 2007;Farihi et al. 2013).The generally accepted explanation for these observations was that these polluted white dwarfs host planetary systems which at least partially survived the white dwarf progenitor's evolution off the main sequence. The host star's evolution was not without ill effects: as the host shed its outer layers and began to cool and contract into a white dwarf, the star's mass loss caused changes to the planetary system's dynamics. Numerical simulations have shown that planetary systems whose host stars have undergone this type of mass loss can occasionally perturb small planets or asteroids into highly eccentric orbits which can occasionally have periastron passages close enough to the host star (by this time a white dwarf) to be tidally disrupted. The planetary, lunar (Payne et al. 2016), or asteroidal remnants would then be pulverized into a fine dust (causing the infrared excesses observed around many polluted white dwarf stars) and slowly accreted onto the white dwarf's surface, where the constituent elements would manifest themselves by the presence of spectral lines.However, the evidence for this scenario was entirely circumstantial, relying on analysis of the after-effects of planetary disruption. There were occasional detections of transient events indicating possible tidal disruptions in progress (Del Santo et al. 2014;Xu and Jura 2014), but no unambiguous detections of disintegrating rocky material until the discovery of transits around a polluted white dwarf called WD 1145+017. For the first time, astronomers had definitively observed the transient process of a white dwarf tidally disrupting a large rocky body (planet, moon, or asteroid),

A dearth of small particles in the transiting material around the white dwarf WD 1145+017

“…Gänsicke et al 2016;Gary et al 2017;Croll et al 2017). The origin, creation mechanism, and lifetimes of these orbiting bodies are currently uncertain at best, with inferred masses in the range of 10 17 − 10 24 g (Vanderburg et al 2015;Rappaport et al 2016). Additional dynamical simulations support the statement that the orbiting objects should be no more massive than Ceres (Gurri et al 2017).…”

“…Followup observations came quickly (e.g. Gänsicke et al 2016;Rappaport et al 2016;Gary et al 2017;Croll et al 2017) and found that the transit durations range from ∼ 3 min to as long as an hour-much longer than expected for a solid body ( Vanderburg et al 2015). The transits are inferred to be caused by the passage of dust clouds rather than solid bodies.…”

“…WD 1145+017 happened to be observed by the K2 mission (in Campaign 1), and it was observed to display transits with multiple periods ranging from 4.5-4.9 hours (Vanderburg et al 2015). Followup observations came quickly (e.g.…”

“…In addition, a uniquely rich set of circumstellar absorption lines from 7 elements was detected. These lines tend to have large velocity dispersions (∼ 300 km s −1 ), and may well be associated with gas orbiting near the white dwarf (i.e., at distances of ∼5-15 white dwarf radii; Redfield et al 2017;Vanderburg & Rappaport 2018). The circumstellar line profiles have changed significantly since their original discovery (Redfield et al 2017).…”

“…The circumstellar line profiles have changed significantly since their original discovery (Redfield et al 2017). In addition, WD 1145+017 displays infrared excess from orbiting dust particles (Vanderburg et al 2015).…”