On November 2nd, a new AAVSO Special Notice (#221) arrived in my email inbox. HT Cas is a dwarf nova star in the constellation of Cassiopeia (the one that looks like a “W” and is currently well placed in our night sky).
This star infrequently has “outbursts” in its brightness. HT Cas actually is a binary star system with a red giant star as its companion. Material from the red giant is being pulled away by the gravity of the white dwarf forming an accretion disk. (read more about these at the following link: http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/001106a.html ) As the material spirals into the white dwarf it heats up to the point where X-rays are emitted. The white dwarf of this system is also an X-ray source. With the added mass, the white dwarf occasionally goes through a nova outburst and that’s what is happening now. HT Cas has brightened from magnitude 16 up to perhaps mag 12. While this is interesting, why write about it? Just another faint star? – Nope!
The red giant orbits the white dwarf in such a way that it eclipses it. The red giant completes a full orbit of the white dwarf in just 106 minutes! (That’s less than 2 hours!) High speed photometric measurements can tell us about the size and structure of the binary star system and the accretion disk. These measurements are well within the capability of the equipment we have at the Harken Observatory. I had to try this!
Dwarf Nova HT Cas in Outburst
As we are temporarily experiencing some trouble with the computerized pointing of the telescope, the target was found the old-fashioned way using the finder scope and “star-hopping”. Clouds were occasionally passing through the field of view. A first series of 250 images (10 sec exposures) were made at 30 second intervals to find out when the eclipse was at maximum. A second series of 600 images, one every 10 seconds, was then taken under cloudless skies. With over 500 megabytes of image data, a detailed analysis of the light curve is planned. In a subsequent blog post these results will be shared. In the mean time, a movie was put together to show the star “winking out”. The deepest part of the eclipse lasts only 12 minutes and repeats every 106 minutes. Please enjoy this!
Animated GIF of HT Cas eclipse
Clear Skies!
Randy B
Randy, awesome work on your part! When did you manage to get the observations in, given the problems with the telescope we are having? The animated “winking” of the star is really impressive. Kudos to you. Mike