Surprise Outburst of a New Dwarf Nova

Astronomers had suspected that the star HS 2331+3905 was a dwarf-nova-in-waiting, and sure enough, they were right. The star has just begun its first observed outburst. The flareup was detected by Hiroyuki Mehara of Japan and confirmed by Makoto Uemura at the Higashi-Hiroshima Observatory. The object brightened from m(V) = 14.59 to the most recently reported CCD observation of m(V) = 9.15 (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 a dwarf nova of the WZ Sagittae type. Spectra by Boris Gaensicke initially showed mixed hydrogen-Balmer emission/absorption lines, which have now transitioned to H and He pure absorption lines.

HS 2331+3905 was suspected of being a WZ Sge-class dwarf nova on the basis of it being a close binary star with the very short orbital period of 81.09 minutes (0.0563 day). The primary component is a white dwarf with an unusually low temperature, 10,500 kelvins, and the secondary star is suspected of being a brown dwarf. Both of these suggest a low accretion rate onto the white dwarf, also consistent with the WZ Sge class.

In addition, the white dwarf 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.

Reporting Your Observations

Observers are strongly encouraged to monitor this object intensely during the outburst. You can find it near the chained hand of Andromeda at right ascension 23h 34m 01.55s, declination +39° 21' 42.9" (equinox 2000.0).

Both visual and CCD observations are desired to follow the ourburst's progression. CCD time series (preferably filtered) will be needed to track the expected emergence and evolution of superhumps. Filtered CCD time-series observations will be needed to measure color changes during the course of the outburst.

A chart for HS 2331+3905 may be plotted with VSP using the following link:

Enter "HS 2331+3905" or use the coordinates given above to plot a chart. (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" per year) and so appears offcentered from crosshairs when charts are created from DSS images.)

Please report observations to the AAVSO as "HS2331+390" with the designation "2328+38". Also see "Submitting Observations to the AAVSO" on our website.

Please check the online version of this AstroAlert on for possible minor updates. Any major updates will be announced via subsequent AstroAlert messages. Good luck, and clear skies!

(All information is from Matthew Templeton, AAVSO headquarters.)