[vsnet-alert 9549] Fw: AAVSO Alert Notice 357

[Daisaku NOGAMI]

Begin forwarded message:

Date: Wed, 5 Sep 2007 11:08:35 -0400
From: aavso at aavso.org
Subject: AAVSO Alert Notice 357

AAVSO 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/observing/charts/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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