[vsnet-chat 330] V1830 Sgr

[bas@bom.gov.au (Bruce Sumner)]

			V1830 SAGITARII                              

                          Bruce Sumner

Abstract

V1830 Sgr is a bright, large range, UG type variable star that has never been observed
in quiescence. This is due mainly to the very crowded nature of the field, causing
difficulty in identifying the variable, even when at maximum. Accurate coordinates of
the variable are here presented. A discussion of the outburst behaviour leads to the
suggestion that V1830 Sgr is an SU UMa sub-type variable.

Coordinates

A visual outburst observed by the author in 1978 was confirmed by a photograph
obtained at Perth Observatory. A subsequent outburst was photographed at ESO in
Chile, a copy of which I was able to obtain (Vogt, 1979). The finder chart in the Vogt
and Bateson Dwarf Nova Atlas (Vogt & Bateson, 1982) was prepared from the ESO
outburst photograph, and correctly identifies the variable. However the Downes and
Shara Atlas (Downes & Shara, 1993) shows the vicinity of this variable from a POSS
photograph when the variable was presumably at minimum. There is no indication of
the variable on this finder chart. 

Comparison of the Perth and ESO outburst photographs with a non-outburst POSS
photograph has enabled the location of this variable to be confirmed.

Careful measurement of a copy of the ESO outburst print, in conjunction with
comparison star positions obtained from the GSC, COSMOS/UKST Southern Sky
Catalogue and DSS, yield the following offsets of the variable from GSC 6851-1802
(V=12.01, Vogt 1981):

     9.4 10.4  S 
     0.8 10.2  E = 0.06s 10.02s  E

The absolute position of V1830 Sgr therefore depends on knowing an accurate position
for GSC 6851-1802. Fortuitously this is available in an Astrometric Catalogue of Dwarf
Nova (Bruch, et.al, 1992), where the authors there incorrectly identified V1830 Sgr with
GSC 6851-1802. This mistake apparently arose because of the equality in brightness of
the two stars when V1830 Sgr is in outburst (~ magnitude 12) and because of their
closeness (9 arcsec separation).

Confirmation of the above assumption is obtained from independent positions of GSC
6851-1802 obtained from other sources:

     J2000 position of GSC 6851-1802

     18h 13m 50.58s   -27 42 13.3     GSC
     18 13 50.54      -27 42 13.0     Bruch, 1992 (precessed position)
     18 13 50.54      -27 42 11.1     COSMOS/UKST
     18 13 50.60      -27 42 13.7     DSS 
     18 13 50.56      -27 42 13.0     USNO A1.0

COSMOS/UKST declinations in this field have a +2.0  bias (independent investigation)
resulting in a corrected end-figure of 13.1 .


     J2000 position of V1830 Sgr 

Using the Bruch position for GSC 6851-1802, together with the previously determined
offsets of the variable from this star, results in the following accurate position for 
V1830 Sgr:

     18h 13m 50.60s    -27  42  22.4    J2000


Discussion of Outburst Behaviour

The following discussion is based on Monthly Notices of VSS, RASNZ, and must be
treated as preliminary.

During the 15 years that this star has been under observation by members of the VSS,
RASNZ, some 18 outbursts have been observed. Three outbursts have been observed at
11.5v, and four other outbursts have been observed in the range 11.6-11.9v. Eleven
additional maxima at about 12.0v have also been observed.

The best observed bright outburst remained bright for 16 days. Several other bright
outbursts remained at maximum for many days. However the outbursts at about 12.0v
were generally only bright for a day or so. There thus appear to be two types of
outburst - long and bright at about 11.5v and slightly fainter and narrow at about
12.0v. 

It is proposed that the brighter outbursts are supermaxima of an SU UMa sub-type
variable. An investigation of the outburst recurrence time of the brighter maxima does
indeed suggest a recurrence cycle of between 330-390 days, with approximately 2 or 3
fainter outbursts between the brighter outburst. The recurrence cycle of the fainter
maxima can be satisfied by a period of approximately 100 days, but this must be treated
with extreme caution as this star is not observed as frequently as other bright UG
variables.

The minimum magnitude for this variable is unknown. The GCVS lists the minimum
as 17.5p. The absence of an image on the POSS photograph suggests the minimum
magnitude is <18B. Thus V1830 Sgr varies from approximately 11.5 to <18B, a range
of at least 6.5 magnitudes. This together with infrequent outbursts and occasional
brighter and longer duration (super)outbursts, suggest this to be a candidate for SU
UMa sub-type.


References 

Bruch, A. et.al.         An Astrometric Catalogue of Southern and Equatorial
                         Dwarf Nova, A & AS, 1992, 93, 463.

Downes, R.  & Shara. M.  Catalog and Atlas of Cataclysmic Variables, PASP, 1993,
                         105, 684.

Vogt. N.            	 Publication No. 5, VSS, RASNZ, 1977, p.45.

Vogt. N.                 Personal correspondence, 1979. 

Vogt, N. & Bateson. F.M. An Atlas of Southern and Equatorial Dwarf Nova,
                         A & AS, 1982, 48, 383.

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