<i>HUBBLE SPACE TELESCOPE</i>AND GROUND-BASED OBSERVATIONS OF V455 ANDROMEDAE POST-OUTBURST

Abstract

Hubble Space Telescope spectra obtained in 2010 and 2011, three and four\nyears after the large amplitude dwarf nova outburst of V455 And, were combined\nwith optical photometry and spectra to study the cooling of the white dwarf,\nits spin, and possible pulsation periods after the outburst. The modeling of\nthe ultraviolet (UV) spectra show that the white dwarf temperature remains ~600\nK hotter than its quiescent value at three years post outburst, and still a few\nhundred degrees hotter at four years post outburst. The white dwarf spin at\n67.6 s and its second harmonic at 33.8 s are visible in the optical within a\nmonth of outburst and are obvious in the later UV observations in the shortest\nwavelength continuum and the UV emission lines, indicating an origin in high\ntemperature regions near the accretion curtains. The UV light curves folded on\nthe spin period show a double-humped modulation consistent with two-pole\naccretion. The optical photometry two years after outburst shows a group of\nfrequencies present at shorter periods (250-263 s) than the periods ascribed to\npulsation at quiescence, and these gradually shift toward the quiescent\nfrequencies (300-360 s) as time progresses past outburst. The most surprising\nresult is that the frequencies near this period in the UV data are only\nprominent in the emission lines, not the UV continuum, implying an origin away\nfrom the white dwarf photosphere. Thus, the connection of this group of periods\nwith non-radial pulsations of the white dwarf remains elusive.\n