Observing Variable Stars Visually

There has been a lot of discussion lately about what visual observers can do to make an honest contribution to science. What variable stars are really interesting to astronomers, and what observations are likely to lead to new understanding of the properties of these and other stars?

It’s no secret that with CCDs being capable of higher precision and numerous surveys covering the sky, with more coming online in the future, visual observers will have to be more selective about what they observe if they want to make a meaningful contribution to science. But there is still a lot the visual observer can do.

No survey exists at the moment that covers the entire sky every night in any bandpass or to any limiting magnitude. ASAS-3, for example, has a cadence of about once every three nights or so. It has a useful range of magnitude coverage from around 8 to 13.5V, and it only covers the sky from the South celestial pole to around +28 degrees in declination.

Pretty much anything north of ASAS-3 sky coverage is still a viable target for visual observers, in spite of the fact that ASAS is running a similar survey in the northern sky right now. There is no guarantee that data will ever become public. There are a number of other surveys going, but the data is not publicly available, and until it is, these surveys do not impact the visual observers usefulness at all.

There are also plenty of targets for visual observers to monitor that require higher cadence of observations, or that a three day delay in notification of activity, like a rare outburst of a cataclysmic variable, a precipitous fade of an R CrB type star, or some other unusual rapid behavior, would cause valuable science to be lost.

Most surveys do not spend much time observing stars approaching conjunction with the Sun, nor do they adequately observe stars in the morning sky that are just coming out from behind the Sun. So observing stars that are setting soon after sunset or rising in the pre-dawn hours can also be fruitful territory for visual observers.

No surveys cover stars brighter than 8th magnitude, and individual CCD observers tend to avoid stars this bright also. Stars that get as bright as this, or are always brighter than this will remain good visual targets for a long time. Naked eye and binocular variables remain the domain of visual observers for now.

Similarly, visual observers with larger telescopes, able to observe stars fainter than 13th magnitude can still support science by covering the gap between the faint end of ASAS coverage and the bright limit of many of the new surveys coming online.

So with that in mind, here are some suggestions for visual targets for winter 2009.

Cataclysmic Variables
There are a number of CVs that rarely go into outburst. Catching one of these on the rise is not only useful, it can be a lot of fun.

First there are three recurrent novae. One visible in the southern sky early in the evening, T Pyx, is overdue for a dramatic outburst. Two more that are worth getting up early in the morning for as winter progresses are U Sco and T CrB. An outburst of any of these stars will be big news and very interesting.

Andromeda, Cassiopiea and Perseus in the north hold several treasures in the rare if ever seen category; LL And, PQ And, V630 Cas, UW Per, V336 Per. If you’ve never heard of these CVs before, it’s because no one has seen them in a while. Normally, I would have included UW Tri in this short list, but it just had a rare outburst in late October 2008. AL Com, IR Com and UZ Boo would be happy discoveries on a cold night.

Some circumpolar stars, easily observed on winter nights include DV UMa, an eclipsing UGSU, and KV Dra.

The ecliptic is well stocked with rarely outbursting cataclysmics. EI Psc, XY Psc, V701 Tau, EG Cnc, a UGWZ that exhibits post outburst re-brightenings as it fades, NSV 18241 in Leo, RZ Leo and HV Vir. A little further south you can monitor WX Cet, CG CMa, and EX Hya.

R Coronae Borealis stars
There are some northern RCBs that observers can and should monitor very clear night for evidence of sudden fading episodes from maximum, or the unpredictable way they brighten to maximum from a deep fade. These are DY Per, Z UMi, R CrB, and SU Tau. LT Dra is listed as RCB in VSX. That is one I didn’t know about. In the south RY Sgr is above the ASAS limit at maximum and V CrA is at or near the limit, hovering around 8.3 or so at maximum. Many RCBs get fainter than the ASAS limit, so following the deep fades of some of these when they occur may also be something worth pursuing, if you have the aperture and dark skies to do it.

Miras and SRs
Any Miras north of +28 degrees declination are still great candidates for the visual observer. You might also consider tracking down stars that are going into conjunction. In the winter these might include stars in Aql, Aqr, PsA, Sge, Vul, Lyr and Del. If you’re an early riser, stars emerging from conjunction in Cen, Lup, Sco, Sgr, Sct, Oph, Her and CrB provide an opportunity for you to cover stars not likely well observed by the surveys.

There are dozens of bright SR stars to observe that are generally not covered by any survey. For the winter months these include: RS And, TZ And, AQ And, V Ari, UU Aur, X Cnc, RS Cnc, RT Cnc, TU Gem, TV Gem, NQ Gem, Y Lyn, SV Lyn, CE Lyn, RV Mon, SX Mon,W Ori, BQ Ori, CK Ori, SU Per, AD Per, Y Tau and TT Tau.

There are also plenty northern circumpolar SRs you can observe year-round.
U Cam, RY Cam, ST Cam, UV Cam, WZ Cas, V393 Cas, V465 Cas, Rho Cas, W Cep,
RU Cep, RW Cep, SS Cep, AR Cep, FZ Cep, Mu Cep, RY Dra, TX Dra, UX Dra, AH Dra, Z UMa and RY UMa.

Eclipsing variables
Since making and sky checking the IYA training charts for Algol, I’ve found myself looking up and doing a quick estimate every time I go outside on a clear night. I even managed to catch an eclipse of Algol from Nantucket during the 2008 fall meeting. That Friday night was the only time the sky was clear during the whole trip, and it happily coincided with the planned observatory tours and star party, as well as an eclipse of Algol.

A very interesting eclipse will take place this winter. EE Cep is a poorly understood eclipsing binary, similar to Epsilon Aur, in that the secondary is probably an object surrounded by a disk of dust. Eclipses only happen once every 5.6 years, it is well placed for observation and the range of magnitudes is from 10-12th magnitude. Mid-eclipse should occur on or near January 14, 2009. The longest eclipse lasted 60 days, so observations from early December through the end of February are potentially useful. The most interesting action will take place between January 2 and 27.

As you can see, there are plenty of opportunities as a visual observer in the next few months to support science. Don’t let anyone tell you the age of visual observations is dead. You can chuckle to yourself when you are the one to catch the next outburst of T CrB while some of your friends were observing superhumps of yet another UGSU with their CCD only a few degrees away all night.


Anonymous said...

Hi Mike, very good arguments for keeping observing visual!

clear skies, Hubert

Anonymous said...

You mentioned LT Draconis - it is no longer thought to be an R CRB-type star - see: