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Blobrana


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V455 Andromedae
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Accretion on a Very Rapidly Spinning White Dwarf Observed Using an L3CCD

Recent William Herschel Telescope observations have revealed a new view of the accretion process in V455 And, an intermediate polar with a very rapidly spinning white dwarf. This white dwarf, which is a star that is only as large as the Earth but about half as massive as the Sun, spins around its axis in just over one minute, and is the third fastest-spinning white dwarf known.
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Title: Remarkable spectral variability on the spin period of the accreting white dwarf in V455 And
Authors: S. Bloemen, D. Steeghs, K. De Smedt, J. Vos, B. T. Gänsicke, T. R. Marsh, P. Rodriguez-Gil

We present spin-resolved spectroscopy of the accreting white dwarf binary V455 And. With a suggested spin period of only 67s, it has one of the fastest spinning white dwarfs known. To study the spectral variability on the spin period of the white dwarf, we observed V455 And with 2s integration times, which is significantly shorter than the spin rate of the white dwarf. To achieve this cadence, we used the blue arm of the ISIS spectrograph at the 4.2-m William Herschel Telescope, equipped with an electron multiplying CCD (EMCCD). Strong coherent signals were detected in our time series, which lead to a robust determination of the spin period of the white dwarf (Pspin=67.619 ±0.002 s). Folding the spectra on the white dwarf spin period uncovered very complex emission line variations in Hgamma, He I 4472 and He II 4686. We attribute the observed spin phase dependence of the emission line shape to the presence of magnetically controlled accretion onto the white dwarf via accretion curtains, consistent with an intermediate polar type system. We are, however, not aware of any specific model that can quantitatively explain the complex velocity variations we detect in our observations. The orbital variations in the spectral lines indicate that the accretion disc of V455 And is rather structureless, contrary to the disc of the prototype of the intermediate polars, DQ Her. This work demonstrates the potential of electron multiplying CCDs to observe faint targets at high cadence, as readout noise would make such a study impossible with conventional CCDs.

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Blobrana


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HS 2331+3905
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Title: HS 2331+3905 : the cataclysmic variable that has it all
Authors: Araujo-Betancor, S. and Gänsicke, B. T. (Boris T.) and Hagen, H.-J. and Marsh, T. R. and Harlaftis, E. T. and Thorstensen, John Robert and Fried, R. E. and Schmeer, P. and Engels, Dieter

We report detailed follow-up observations of the cataclysmic variable HS 2331+3905, identified as an emissionline object in the Hamburg Quasar Survey. An orbital period of 81.08 min is unambiguously determined from the detection of eclipses in the light curves of HS 2331+3905. A second photometric period is consistently detected at P 83.38 min, ~2.8% longer than P orb , which we tentatively relate to the presence of permanent superhumps. High time resolution photometry exhibits short-timescale variability on time scales of 5-6 min which we interpret as non-radial white dwarf pulsations, as well as a coherent signal at 1.12 min, which is likely to be the white dwarf spin period. A large-amplitude quasi-sinusoidal radial velocity modulation of the Balmer and Helium lines with a period ~3.5 h is persistently detected throughout three seasons of time-resolved spectroscopy. However, this spectroscopic period, which is in no way related to the orbital period, is not strictly coherent but drifts in period and/or phase on time scales of a few days. Modelling the far-ultraviolet to infrared spectral energy distribution of HS 2331+3905, we determine a white dwarf temperature of T_eff 10 500 K (assuming M wd = 0.6 M ), close to the ZZ Ceti instability strip of single white dwarfs. The spectral model implies a distance of d = 90 ± 15 pc, and a low value for the distance is supported by the large proper motion of the system, µ = 0.14 yr^-1 . The non-detection of molecular bands and the low J, H, and K fluxes of HS 2331+3905 make this object a very likely candidate for a brown-dwarf donor.

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Position (J2000):     R.A. 3 34 01.45  |  Dec. +39° 21' 40.9''



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Blobrana


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Posts: 131433
Date:
V455 ANDROMEDAE
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HS 2331+3905 = V455 ANDROMEDAE
HS 2331+3905 was known to be a close binary with a orbital period of only 81.09 minutes (0.0563 day). The primary component is a white dwarf with an a temperature of 10,500 Kelvin, and the secondary star may be a brown dwarf. Material is probably accreting from the brown dwarf onto the white dwarf, which also shows pulsations signs of a ZZ Ceti class, which makes the star system quite unusual.
Because of this close orbit the star HS 2331+3905 had been suspected of being a WZ Sagittae type.

Ands the star has just had its first observed outburst.

A flare-up was detected by observers Hiroyuki Mehara and Makoto Uemura of Japan.
The object brightening from m(V) = 14.59 on Sept 3, 2007, 21:26 UT, to m(V) = 9.15 within 24 hours.

V455 ANDROMEDAE.kmz
Google Sky file



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