Abstract
We present new Hubble Space Telescope (HST) ultraviolet and ground-based optical observations of the hot, metal-rich white dwarf GD 394. Extreme-ultraviolet (EUV) observations in 1992-1996 revealed a 1.15d periodicity with a 25 percent amplitude, hypothesised to be due to metals in a surface accretion spot. We obtained phase-resolved HST/Space Telescope Imaging Spectrograph (STIS) high-resolution far-ultraviolet (FUV) spectra of GD 394 that sample the entire period, along with a large body of supplementary data. We find no evidence for an accretion spot, with the flux, accretion rate and radial velocity of GD 394 constant over the observed timescales at ultraviolet and optical wavelengths. We speculate that the spot may have no longer been present when our observations were obtained, or that the EUV variability is being caused by an otherwise undetected evaporating planet. The atmospheric parameters obtained from separate fits to optical and ultraviolet spectra are inconsistent, as is found for multiple hot white dwarfs. We also detect non-photospheric, high-excitation absorption lines of multiple volatile elements, which could be evidence for a hot plasma cocoon surrounding the white dwarf.
Highlights
Of the hundreds of known remnant planetary systems at white dwarfs, short time-scale variability has been observed at only a handful
The time-scales for metals to diffuse out of the photospheres of hot white dwarfs are of order days (Paquette et al 1986; Koester & Wilken 2006; Koester 2009), so for the metal abundances to be higher than those supported by radiative levitation GD 394 must be currently, and continuously, accreting material from an external source
As the Goddard High Resolution Spectrograph (GHRS) data are inferior to the Space Telescope Imaging Spectrograph (STIS) FUV data in both wavelength coverage and S/N we do not present this as an alternative result for the atmospheric parameters, but it provides a guide to estimate the systematic uncertainties of the model fit
Summary
Of the hundreds of known remnant planetary systems at white dwarfs, short time-scale variability has been observed at only a handful. The variability was detected in three separate instruments onboard the Extreme Ultraviolet Explorer (EUVE) in observations spanning 1992–1996, leading Dupuis et al (2000) to conclude that it was intrinsic to the star. Their preferred explanation for this variability was that the accreting material is being channelled, presumably by a magnetic field, on to a spot, which is rotating in and out of view over the white dwarf rotation period.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
