Andrew Swan

Combining archival photometric observations from multiple large-area surveys spanning the past 17 years, we detect long-term variability in the light curves of ZTF J032833.52−121945.27 (ZTF J0328−1219), ZTF J092311.41+423634.16 (ZTF J0923+4236), and WD 1145+017, all known to exhibit transits from planetary debris. ZTF J0328−1219 showed an overall fading in brightness from 2011 through to 2015, with a maximum dimming of ≃0.3 mag, and still remains ≃0.1 mag fainter compared to 2006. We complement the analysis of the long-term behaviour of these systems with high-speed photometry. In the case of ZTF J0923+4236 and WD 1145+017, the time-series photometry exhibits vast variations in the level of transit activity, both in terms of numbers of transits, as well as their shapes and depths, and these variations correlate with the overall brightness of the systems. Inspecting the current known sample of white dwarfs with transiting debris, we estimate that similar photometric signatures may be detectable in one in a few hundred of all white dwarfs. Accounting for the highly aligned geometry required to detect transits, our estimates imply that a substantial fraction of all white dwarfs exhibiting photospheric metal pollution from accreted debris host close-in planetesimals that are currently undergoing disintegration.

There is a wealth of evidence to suggest that planetary systems can survive beyond the main sequence. Most commonly, white dwarfs are found to be accreting material from tidally disrupted asteroids, whose bulk compositions are reflected by the metals polluting the stellar photospheres. While many examples are known, most lack the deep, high-resolution data required to detect multiple elements, and thus characterize the planetesimals that orbit them. Here, spectra of seven DZ white dwarfs observed with Keck High Resolution Echelle Spectrometer (HIRES) are analysed, where up to nine metals are measured per star. Their compositions are compared against those of Solar system objects, working in a Bayesian framework to infer or marginalize over the accretion history. All of the stars have been accreting primitive material, similar to chondrites, with hints of a Mercury-like composition at one star. The most polluted star is observed several Myr after its last major accretion episode, in which a Moon-sized object met its demise.

A promising progenitor scenario for Type Ia supernovae (SNeIa) is the thermonuclear detonation of a white dwarf in a close binary system with another white dwarf. After the primary star explodes, the surviving donor can be spontaneously released as a hypervelocity runaway. One such runaway donor candidate is LP 398-9, whose orbital trajectory traces back ≈105 yr to a known supernova remnant. Here, we report the discovery of carbon-rich circumstellar material around LP 398-9, revealed by a strong infrared excess and analysed with follow-up spectroscopy. The circumstellar material is most plausibly composed of inflated layers from the star itself, mechanically and radioactively heated by the past companion’s supernova. We also detect a 15.4 h periodic signal in the UV and optical light curves of LP 398-9, which we interpret as surface rotation. The rotation rate is consistent with theoretical predictions from this supernova mechanism, and the brightness variations could originate from surface inhomogeneity deposited by the supernova itself. Our observations strengthen the case for this double-degenerate SNIa progenitor channel, and motivate the search for more runaway SNIa donors.