From Danie Overbeek dao@pixie.co.za
To AAVSO observations@aavso.org and VSNet
Subject Dwarf Novae in Outburst
V1830 SGR: 1997 05 12.0 <12.2 OB
05 13.022 12.6 OB
05 13.137 12.6 OB
05 13.987 12.4 OB
The dwarf nova V1830 Sgr is undergoing one of its rare outbursts as indicated by the following observations. The object is apparently rising. No positive observations have been reported to VSNET before this. Follow-up observations are encouraged.
YYMMDD(UT) mag code 970505.580 <126 MKA 970509.8 <122 MLF 970511.9 <122 MLF 970512.0 <122 OBK 970513.022 126 OBK 970513.137 126 OBK 970513.987 124 OBK 970515.0 125: MLF 970515.747 119 StuRegards,
I have long suspected V1830 Sgr as being an UGSU type dwarf nova, but superhumps have never been observed. The current bright outburst is an ideal opportunity to search for superhumps. Any takers?
Regards,
Bruce Sumner
Melbourne
I perfectly agree with you. The quoted magnitude range in Downes and Shara is 11.5-17.5p -- already well deserves a "TOAD". My own CCD estimate suggested even fainter quiescence. Is there any evidence of being UGSU from its outburst behavior?
Regards,
Taichi Kato
Dear colleagues,
Regarding the comments on V1830 Sgr, that appeared on this vsnet-chat, I'd like to add following information.
V1830 Sgr has been reported several times in outburst since 1991. The list below is an extract of the Outburst Activity Database on Selected Cataclysmic Variables.
V1830 SGR UG 11.5p 17.5p Outburst date JD Max mag Observer Reference 1991.0708 48446 12.0 Overbeek AAVSO Circ. 250 1992.0314 48696 11.8 Overbeek AAVSO Circ. 258 1993.0220 49039 12.0 Overbeek AAVSO Circ. 269 1993.0707 49176 12.0 Williams, Overbeek [TAEC 758] [AAVSO Circ. 274] 1994.0529 49502 12.0 Overbeek AAVSO Circ. 284 1994.0915 49611 12.0 Overbeek AAVSO Circ. 288 1995.1021 50012 12.2 Stewart, Overbeek AAVSO Circ. 301 1996.0710 50275 12.2 Overbeek AAVSO Circ. 310
Note that V1830 Sgr has been as bright on each of these occasions as it is during the current outburst.
Dannie Overbeek might be able to comment more precisely on the outburst behaviour of this dwarf nova. He's probably the one who observed it most frequently.
Regards,
Tonny Vanmunster
Greetings,
Can anyone advise exactly where V1830 Sagittarii is and if it is still in outburst? I've used the Downes & Shara chart from PASP but this is a little ambiguous. Their stated limit is 14 on this chart and they identify a star about 40" south of a star of about magnitude 10. This star is mag 14 if their limit is accurate - the limit looks rather fainter to me.
My problem is that I have a star of mag 12 about 10" south of the tenth magnitude star and nothing where Downes & Shara put V1830. May 16 & 17 UT. They don't show this twelfth magnitude star. Is It V1830? Or has it now returned to normal? I'd like to make sense of the CCD frames I have.
Regards,
Stan Walker
The Downes and Shara chart shows V1830 a lottle too far south of a satr Stan Walker calls magnitude 10 and Kato-san calls 12.55. This star is actually GSC 6851 1802, V=12.03, B-V=1.62 (Vogt 1977). V1830 Sgr is 9.4" south of this star, at the following J2000 position:
V1830 Sgr 18h 13m 50.60s -27d 42' 22.4" (J2000)Bruce Sumner
Hi,
the RASNZ chart of V1830 Sgr shows this variable very close to a star of magnitude 12.0 (Vogt). I have questioned the accuracy of this magnitude on some earlier occasion and have used the VSNET sequences, which give this star 12.55. Having a limiting magnitude of around 12.6 I never saw this star 'with confidence'. ... Could it be V1830 Sgr? On the other hand, re my ignorance, if this star exists and be considered 12.0, previous outbursts of V1830 Sgr, reported at 12.2, would have actually been around 12.8. This could explain why I never saw such an outburst!
Regards,
Berto Monard
Greetings, All,
Thanks to Bruce Sumner and Berto I can now place this star as the fainter of the two semi-merged images on my CCD frames. It's about 9" from the above star for which Bruce gives GSC 6851 1802, V=12.03, B-V=1.62 (Vogt 1977). Thus both Downes & Shara and the VSNet charts place it rather too far south which, in this very crowded field (average separation at magnitude 16 is about 10"), is confusing. It's correct on Bill Gray's Guide charts.
After comments at PEP5 I allow the images to defocus a little to spread them over more pixels. But they spread out a little anyway. Hence the merging. Most visual observers should see this as a close double at maximum. Berto's comments about the magnitude are interesting. With a B-V of 1.62 this could be in the region where stars are usually low amplitude variables but in this field there would be a lot of reddening and this star is probably G or K. Did Vogt give a U-B which could allow this to be estimated?
I'm using unfiltered CCD with an ST6 which is normal for CVs. But the response is obviously red-biased which is why the 12.03 comes out at 10. Should V1830 still be around on the next clear night I'll take a couple of frames in B and V using the Bessell filters to get a good value for both stars.
Can I draw attention to Joe Patterson's call re V803 Cen? At present the minima are over NZ but are shifting eastward. It varies between 13.3 and 14.7 and a couple of visual measures each night would greatly assist in catching all cycles.
Regards,
Stan Walker
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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