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Alert Notice 357: Outburst of HS 2331+3905

September 5, 2007

Object: 2328+38 HS2331+390

Coordinates: RA: 23:34:01.55 Dec: +39:21:42.9

The WZ Sge-candidate HS 2331+3905 is currently undergoing its first observed outburst. The outburst was detected by Hiroyuki Mehara of Japan and reported via VSNET, and confirmed by Makoto Uemura at the Higashi-Hiroshima Observatory. This object went into outburst some time between JD 2454347.3931 (a fainter-than observation at m(vis) < 14.6 by Eddy Muyllaert, Oostende, Belgium) and 2454348.2752 (m(V) = 14.59 by Hiroyuki Mehara of VSNET, Japan). The object has risen from m(V) = 14.59 to the most recently reported CCD observation of m(V) = 9.149 (Martin Nicholson, Northamptonshire, UK) in approximately 24 hours. The rise of more than seven magnitudes from the quiescent level of m(V) ~ 16.4 strongly suggests that this is indeed a WZ Sge-type dwarf nova. Spectra by Boris Gaensicke reported to VSNET initially showed mixed H-Balmer emission/absorption lines, which have now transitioned to H and He pure absorption lines.

HS 2331+3905 was already suspected of being a WZ Sge-class dwarf nova on the basis of its short orbital period of 81.09 minutes (0.0563 d). The white dwarf temperature is very low at T=10500 K (Araujo-Betancor et al., 2005, A&A 430, 629) and the secondary star is suspected of being a brown dwarf. Both of these together suggest a low accretion rate, also consistent with the WZ Sge class. In addition, the white dwarf primary is known to be a pulsator of the ZZ Ceti class, making this object particularly interesting from a physical standpoint; if the outburst results in temperature or other changes in the white dwarf, these may change its pulsation behavior and provide new clues for white dwarf asteroseismology.

Observers are strongly encouraged to observe this object intensely during this ouburst. Both visual and CCD observations are needed to follow the outburst itself. CCD time series (preferably filtered, if available) will be needed to track the expected emergence and evolution of superhumps, and filtered CCD time series observations of this object will be needed to measure color changes in this object during the course of the outburst.

A chart for HS 2331+3905 may be plotted with VSP using the following link: http://www.aavso.org/vsp

Enter "HS 2331+3905" or use the coordinates given above to plot a chart. Please note that for default magnitude limits, crosshairs will not appear on charts of "D" scale or larger. Note also that HS 2331+3905 is a high-proper motion star (0.14"/yr) and so appears offcentered from crosshairs when creating charts from DSS images.

CCD observers may use the existing comparison stars throughout the course of the outburst. The following bright extension of the visual sequence has been provided by W. Renz, and will be available on VSP shortly:

	RA               Dec                 V               B       visual
----------------------------------------------------------------------
23:37:10.49     +39:27:09.7      8.366 (0.014)   9.430 (0.025)   84
23:31:57.51     +39:19:43.4      9.248 (0.024)   9.969 (0.031)   93
23:33:05.63     +39:23:11.8      9.572 (0.029)  10.693 (0.059)   96
23:33:53.74     +38:57:22.6     10.007 (0.042)  11.115 (0.087)  100
23:34:46.69     +39:16:44.0     10.310 (0.043)  10.961 (0.054)  103
23:33:23.41     +39:17:58.8     10.481 (0.055)  11.167 (0.070)  105
23:34:23.38     +39:15:34.9     10.900 (0.078)  11.393 (0.076)  109

(Magnitudes from Kharchenko 2001)

Please report all observations to the AAVSO as "HS2331+390" with designation "2328+38".

Clear skies and good observing,

Matthew Templeton
AAVSO Headquarters

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