P. B. Cho | Searcharxiv

arXiv2021

The Heating and Pulsations of V386 Serpentis after its 2019 Dwarf Nova Outburst

Following the pulsation spectrum of a white dwarf through the heating and cooling involved in a dwarf nova outburst cycle provides a unique view of the changes to convective driving that take place on timescales of months versus millenia for non-accreting white dwarfs. In 2019 January the dwarf nova V386 Ser (one of a small number containing an accreting, pulsating white dwarf), underwent a large amplitude outburst. Hubble Space Telescope ultraviolet spectra were obtained 7 and 13 months after outburst along with optical ground-based photometry during this interval and high-speed photometry at 5.5 and 17 months after outburst. The resulting spectral and pulsational analysis shows a cooling of the white dwarf from 21,020 K to 18,750 K (with a gravity log(g) = 8.1) between the two UV observations, along with the presence of strong pulsations evident in both UV and optical at a much shorter period after outburst than at quiescence. The pulsation periods consistently lengthened during the year following outburst, in agreement with pulsation theory. However, it remains to be seen if the behavior at longer times past outburst will mimic the unusual non-monotonic cooling and long periods evident in the similar system GW Lib.

astro-ph.SR↗

arXiv2020

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

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 ${\approx}\,107.2\,$d, much longer than the $4.5{-}4.9\,$h 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 $15{-}25\,$d and reaching $20{-}45\,\%$ dips in flux. Optical spectra reveal strong Balmer lines, identifying the white dwarf as a DA with $T_{\mathrm{eff}}=10{,}530\pm140\,\mathrm{K}$ and $\log(g)=7.86\pm0.06$. A $\mathrm{Ca\,II\,K}$ 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 $\mathrm{Ca\,II\,K}$ absorption with a model atmospheric fit suggesting $[\mathrm{Ca/H}]=-4.6\pm0.3$, whereas spectra taken on three nights out of transit have $[\mathrm{Ca/H}]$ of -5.5, -5.3, and -4.9 with similar uncertainties. While the $\mathrm{Ca\,II\,K}$ line strength varies by only 2-sigma, 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.

astro-ph.SR↗