A White Dwarf with Transiting Circumstellar Material Far outside the Roche Limit

Abstract

We report the discovery of a white dwarf exhibiting deep, irregularly shaped transits, indicative of circumstellar planetary debris. Using Zwicky Transient Facility DR2 photometry of ZTF J013906.17+524536.89 and follow-up observations from the Las Cumbres Observatory, we identify multiple transit events that recur every ≈107.2 days, much longer than the 4.5-4.9 hr orbital periods observed in WD 1145+017, the only other white dwarf known with transiting planetary debris. The transits vary in both depth and duration, lasting 15-25 days and reaching 20%-45% dips in flux. Optical spectra reveal strong Balmer lines, identifying the white dwarf as a DA with =  T 10, 530 140 K eff and ( ) =  g log 7.86 0.06. A Ca IIK absorption feature is present in all spectra both in and out of transit. Spectra obtained during one night at roughly 15% transit depth show increased Ca IIK absorption with a model atmospheric fit suggesting [Ca/H]=−4.6±0.3, whereas spectra taken on three nights out of transit have [Ca/H] of −5.5, −5.3, and −4.9 with similar uncertainties. While the Ca IIK line strength varies by only 2σ, we consider a predominantly interstellar origin for Ca absorption unlikely. We suggest a larger column density of circumstellar metallic gas along the line of site or increased accretion of material onto the white dwarf's surface are responsible for the Ca absorption, but further spectroscopic studies are required. In addition, high-speed time series photometry out of transit reveals variability with periods of 900 and 1030 s, consistent with ZZ Ceti pulsations.

Horizontal spreading of planetary debris accreted by white dwarfs

“…However, by revisiting the observations Debes & López-Morales (2008) found no evidence for variation on timescales of days to years. Vanderbosch et al (2020) have recently detected marginal calcium line variability at white dwarf ZTF J013906.17+524536.89 with transiting debris. However, they conclude that the spectroscopic variability is likely related to circumstellar debris transiting along the line of sight rather than photospheric variations.…”

“…It is now accepted that 25-50 per cent of all white dwarfs show clear evidence of metal pollution in their surface layers (Zuckerman et al 2010;Koester et al 2014;Farihi et al 2016). For most of these objects the unequivocal origin of this material is the evolved planetary system hosted by the white dwarf (Jura 2003;Gänsicke et al 2012;Vanderburg et al 2015;Gänsicke et al 2019;Manser et al 2019;Vanderbosch et al 2020;Harrison et al 2018;Bonsor et al 2020;Xu et al 2019). The canonical model is that a circumstellar disc borne of one or many tidally disrupted planetesimals is able to constantly feed planetary material onto the white dwarf surface (Jura 2003).…”

Five New Post-main-sequence Debris Disks with Gaseous Emission

“…Still, cyclical changes in emission strength are an intriguing possibility, as the timescale could indicate an ongoing interaction between an existing disk and a larger planetesimal on a wide orbit. Given the dynamic origin of the material that populates white dwarf debris disks, such interactions are expected during the tidal disruption process (Malamud & Perets 2020) and have been observed by the evolution of the disrupting, transiting debris (Vanderburg et al 2015;Vanderbosch et al 2020).…”

A giant planet candidate transiting a white dwarf

“…) is modeled after Eqn. (5) inRappaport et al (1995 122 ) and inferred from Eqn (12). inKalomeni et al (2016 123 ), with some minor modifications.…”

“…During the red giant phase, any close-orbiting planets will be engulfed by the star 2 , but more distant planets can survive this phase and remain in orbit around the white dwarf 3,4 . Some white dwarfs show evidence for rocky material floating in their atmospheres 5 , in warm debris disks [6][7][8][9] , or orbiting very closely [10][11][12] , which has been interpreted as the debris of rocky planets that were scattered inward and tidally disrupted 13 . Recently, the discovery of a gaseous debris disk with a composition similar to ice giant planets 14 demonstrated that massive planets might also find their way into tight orbits around white dwarfs, but it is unclear whether the planets can survive the journey.…”