[vsnet-campaign-news 52] AstroAlert: Two dwarf novae undergoing rare outbursts

[Taichi Kato <tkato>]

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               Two Dwarf Novae Undergoing Rare Outbursts
                  DV Ursae Majoris and V359 Centauri
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DV Urase Majoris
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   On 2001 Apr. 24.9049 UT, Christopher Jones (UK) discovered a rare outburst
of DV UMa, a variable star belonging to a rare class of eclipsing SU UMa
type dwarf novae.  Gary Poyner (UK) also independently detected the
outburst.  Dwarf novae are short-period (usually 1.3 to 12 hours) binary
systems composed of white dwarfs and orbiting red dwarfs.  In very lucky
cases, when the orbital plane is seen edge-on, the red dwarf eclipses
the central white dwarf and the surrounding accretion disk in every
orbit.  Since dwarf novae sometimes erupt (outbursts), detailed observation
of eclipses during outburst provides crucial information why dwarf novae
erupt.  There are only a handful of this class of such systems.
Especially important are eclipsing SU UMa-type systems, which show
both normal outbursts (short outbursts) and superoutbursts (very bright,
long outbursts).  The existence of these two types of outbursts has
puzzled many astronomers for these thirty years, and has been one of
central topics in researchers of cataclysmic variables.
Previously reported outbursts of DV UMa are 1995 Feb. (normal outburst),
1997 Apr. (superoutburst) and 1999 Dec. (superoutburst), all of which have
result rich results leading to our better understanding to outbursting
dwarf nova disks, some of which are already published as scientific
papers.  The 1999 Dec. superoutburst was very well observed by the VSNET
collaboration team, and the data are being intensively analyzed using
the recently developed computational techinque, and will be compared
with theories.  The summary of the 1999 observation cal be seen at the
URL below.

    http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/DNe/dvuma9912.html

   Please join us to track the evolution of this rare outburst, and observe
very exciting eclipses.  DV UMa is curently at about 14.5 mag, and has
an orbital period of 2h 3m 38s and deep eclipses recur with this period.
The prediction of immediate eclipses is seen at

    http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/Mail/alert5000/msg00840.html

   DV UMa is located at 09h 46m 37s, +44o 46' 45" (J2000.0).

V359 Centauri
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   V359 Cen was discovered by A. Opolski on Franklin-Adams plates.
The object was visible on 19 plates between 1939 April 20 and 27, and
had been considered as a possible nova, until the second-ever discovery
of an outburst in 1999 July by Rod Stubbings (Australia).  The next
outburst (the third historical outburst = the last outburst) was
detected by Rod Stubbings on 2000 May 15.  A private communication on
the 1999 outburst reported the confirmation of the outburst, and the
possible existence of superhumps, which are characteristic to SU UMa-type
dwarf nova (see the DV UMa section).  Since the present outburst is most
favorably situated among the recent three outburst, time-resolved photometry
in order to confirm possible superhumps is very strongly encouraged.
Spectroscopic confirmation (as a dwarf nova) is also encouraged.

   V359 Cen is located at 11h 58m 15.3s -41o 46' 08" (J2000.0)

Regards,
Taichi Kato
On behalf of the VSNET Collaboration Team

   We have been undertaking on photometric campaigns on several super-
outbursting dwarf novae and peculiar variable stars like WR104.  News and
call for observations on these objects will be issued on our regular weekly
"News from VSNET" bulletins.  Those who immediately need information
should make an access to the VSNET website, and click on VSNET campaign list.

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  VSNET (Variable Star Network) is an international variable star observing
network, covering various areas of novae, supernovae, cataclysmic variables
(CVs), X-ray transients, and other classical eruptive, pulsating, and
eclipsing variables.  VSNET is one of invited contributing organizations
to the SkyPub AstroAlert system.

       VSNET Home Page: http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/

CCD observations of such targets are a relatively easy task for a 20-40cm
telescope; simply take as many CCD frames (with exposure times 10-30 sec)
as possible, spanning several hours per night.  The only requirements are
the weather and your patience!  If you need more help on the observing
technique, please feel free to ask on the vsnet-campaign list.

   We would sincerely appreciate volunteers who would join the VSNET
Collaboration team to study the wonders of these exotic variable stars.
To join the VSNET campaign collaborative list, send an e-mail to

         vsnet-adm@kusastro.kyoto-u.ac.jp (VSNET administrator)

with a line "SUBSCRIBE vsnet-campaign." or your comment to join the
collaboration team.

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