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Multiwavelength Ground and Space Observations of the Variable White Dwarf BOKS 53856: Nonuniform Metal Absorption in Dark Spots
Stockholm University, Faculty of Science, Department of Astronomy.ORCID iD: 0000-0002-9288-3482
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Number of Authors: 62018 (English)In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 156, no 3, article id 119Article in journal (Refereed) Published
Abstract [en]

We analyze extensive ground- and space-based ultraviolet-infrared observations of the variable white dwarf BOKS 53856. Photometry and spectroscopy were obtained from the Hubble Space Telescope, Spitzer Space Telescope, Kepler and Swift satellites, Palomar Observatory, and Kitt Peak National Observatory. Time series data spanning the entire four-year Kepler mission are used to refine the photometric rotation period of the white dwarf to 0.2557292(9) days. The associated periodic variability, with full (minimum to maximum) amplitudes of approximate to 5% and approximate to 10% in the optical and ultraviolet, respectively, is consistent with the presence of several dark spots on the white dwarf surface. A fit to the observed ultraviolet-infrared spectral energy distribution of BOKS 53856 using a grid of non-LTE synthetic spectra gives best values of effective temperature, T-WD = 31,056(19) K, surface gravity, log g = 7.913(5), mass, M-WD = 0.603(2) M-circle dot, radius, R-WD = 0.0142(4) R-circle dot, reddening, Eb-v = 0.0533(1), and distance, d = 277(10) pc. The Hubble ultraviolet spectrum does not show a detectable signature of accreted metals, and the Spitzer infrared photometry does not reveal an excess attributable to a circumstellar dust disk. Nonetheless, the ground-based optical spectra show a very weak, variable Ca II K absorption line that is phased with the Kepler light curve, being strongest at minimum brightness. We discuss BOKS 53856 and similar objects in the context of a localized enhancement of absorption from metals in dark (possibly magnetic) spots on the white dwarf surface.

Place, publisher, year, edition, pages
2018. Vol. 156, no 3, article id 119
Keywords [en]
circumstellar matter, infrared: stars, stars: individual (BOKS 53856), ultraviolet: stars, white dwarfs
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:su:diva-160215DOI: 10.3847/1538-3881/aad238ISI: 000442639600007OAI: oai:DiVA.org:su-160215DiVA, id: diva2:1250955
Available from: 2018-09-25 Created: 2018-09-25 Last updated: 2019-12-17Bibliographically approved

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