Magic moments with HT Cas

David Strange

I must admit that I have never been much of a variable star observer. I had seen Algol go through its eclipses many times over a period of several hours, but I had never been attracted to observing variables over time spans of weeks or months in order to obtain those impressive light curves from many hundreds of observations.

There is, however, a particular class of variable star that does exhibit exciting fluctuations in light ouput measurable in a matter of seconds rather than hours – that of the eclipsing dwarf novae. A telephone call from Dr Tim Naylor, at Keele, alerted me to the fact that the dwarf nova HT Cas was undergoing a rare outburst, and he asked if I could obtain a series of CCD images of it when it undergoes its eclipse. This is an intriguing binary star – a red giant that rotates around a white dwarf companion in a period of a little under 2 hours. It seems hard to visualise the gravitational forces at work here, but from our vantage point the red giant actually eclipses the white dwarf companion, with each eclipse lasting just 12 minutes.

Unfortunately, these stars are normally very faint at mag. 16.4, but every so often the white dwarf undergoes an outburst caused by an instability of matter falling onto its accretion disk from its companion, the last such outburst occurring in 1985. At these outbursts the star brightens to mag. 13, which is an easy target for CCD cameras. The project involved obtaining images throughout the eclipse at a high time resolution of ideally 6-second intervals. By so doing, CCD photometry would enable us to obtain a light curve which may well shed light on the finer structure of the star and accretion disk in outburst; any humps in the curve would point to hotspots in the system known as ‘superhumps’. That sounds fine in theory, but at that rate of data collection you have to keep your wits about you!

The main problem, of course, lies with computer memory and storage. Each of my images requires 260k, so over a period of 12 minutes I would capture 120 images requiring 31 Mb of memory. Time did not allow me to do the necessary disk housekeeping, so I decided to monitor the eclipse at 40-second intervals. By keeping the camera on continuous mode I found that I had time to type in a time stamp and file name and save each image to disk before the next image arrived. Because the period of the binary is 2 hours there are several chances each night to observe the eclipse. The times given for the evening of 19/20 November 1995 were 2010, 2156, 2343, 0128, 0314, 0500 and 0647 UT.

Unfortunately I did not have an accurate finder chart, so I spent half an hour or so until I felt confident that I was on target. I carried out a practice run on the 2156 UT eclipse; all went well and, to use ‘HST speak’, no anomalies were noted! I then proceeded to dark-frame each image, and while reloading each image it was very apparent how quickly the light of HT Cas was changing! At 1-minute intervals the changing magnitude was easily visible in the raw image. To observe a star change by 3 magnitudes in a matter of 6 minutes is really quite an exciting event to witness, and adds a completely new dimension to the art of variable-star observing.

The table shows my magnitude estimates for the eclipse that took place at 2343 UT on 19 November 1995. They represent the star coming out of mid-eclipse at 2343 UT, brightening from mag. 14.71 to mag. 12.55 in a matter of 6 minutes. This outburst was short-lived, however, and the following night it had faded a further magnitude. The series of images – each obtained with a 40-second exposure – shows HT Cas at 2138, 2155, 2157 and 2200 UT. For these measurements I used a 50-cm f/4 reflector, a Starlight Xpress CCD and a V filter.

UT d Mag star A V Mag 2342 2.81 14.71 2343 2.86 14.76 2344 2.49 14.39 2345 1.05 12.95 2346 0.89 12.79 2347 0.78 12.68 2348 0.67 12.57 2349 0.65 12.55 2350 0.69 12.59 2351 0.73 12.63 2352 0.71 12.61 2353 0.73 12.63 2354 0.72 12.62