from VSSC91

How low can you go?

John Toone

Recently whilst checking a sketch I made of M6 with a 6cm refractor from Manchester 20 years ago, I noticed that BM Sco (Mv 6.0-8.1, SRd, 850 days) was recorded and depicted as one of the brightest cluster members. Since BM Sco has a declination of -32 13' I asked myself how many other variable stars with declinations lower than 30 south could be observed from the UK. Moreover, I also wondered what is the lowest declination variable star that can and has been observed from the UK.
In the absence of finding any information within existing literature on this subject I searched my own variable star logbooks and the following estimates of far south variable stars from the UK were unearthed:

Star Location Observed From Estimated
Designation Declination Site Latitude
Y Scl-3008'Manchester5330'8.220-10-81
RY Sgr-3331'Sychtyn5239'[11.316-8-93
RR Sco-3035'Ongar5143'11.28-6-94
RY Sgr-3331'Newquay5024'8.214-6-94
T Cen-3336'Cressage5238'7.07-5-95

All of the above observations were made with a 20cm Schmidt Cassegrain which is very convenient to handle when set almost horizontally. With 12x50 binoculars the lowest declination variable star that I have an estimate logged for was that of T Mic (declination -2816') recorded at magnitude 7.9 on the 22-6-88 from Newquay. On the same night that T Cen was observed (7-5-95) I glimpsed V806 Cen (declination -3427') with 12x50 binoculars skimming the horizon. No magnitude estimate could be made due to the lack of comparison stars. Visual estimates of this star are probably worthless anyway as the extreme range is catalogued as just 0.1 magnitudes. V806 Cen by the way can never rise more than 3 above the horizon at the latitude of where the star was seen.
The observations of RY Sgr were confirmed to be accurate by Colin Henshaw observing from Botswana who reported that this star had faded from its normal maximum of magnitude 6 in June 1993 and wrote me a note stating that it was returning from minimum in June 1994 (his observation on 15-6-94 was 8.1).
Incidentally the 1993 negative observation of RY Sgr from Sychtyn (mid Wales) proved that the star was at a fainter level than could be determined by a southern hemisphere observer armed with a normal pair of binoculars.
If any observer intends to hunt down far south variable stars such as these then the following points are worthwhile noting:-

Setting circles can assist the observer but I have always relied upon star hopping or triangulation methods. To locate RY Sgr, for instance, I always draw an imaginary line between sigma and phi Sgr passing south eastwards through zeta Sgr. When I reach a position roughly equal to the separation between zeta Sgr and the other two stars I check the telescopic view. RY should then be inside the field of view forming a right-angled triangle with two 7.5 magnitude stars (this is dependent, however, upon RY being at or near maximum).
What is the lowest declination variable star that has been observed from Scotland? Perhaps VSS members in Scotland could write and advise us on this. Although Scottish observers have a minimum 5 latitude disadvantage over their southern English counterparts they have in some areas light pollution and altitude advantages over the rest of the UK.
In order to observe a lower declination variable star than either T Cen or RY Sgr from the UK I would recomment AI Sco (declination -3349') as a suitable target star. AI Sco lies just 1 NE M7 which is quite easy to see under good conditions with a small pair of binoculars from southern England (I have photographed it from Cornwall). This brings me back to where I began - a bright open cluster in Scorpio.
I would like to hear from anyone who has observed low declination variable stars before or anyone who is successful in observing AI Sco or any other -30 declination variable star from the UK. It would be particularly interesting to see how close variable star observers can get to within their own theoretical horizon limit. Any UK based observer can of course get involved in this.

Happy hunting and good luck.