The American Association of Variable Star Observers (AAVSO) is to begin a new campaign. It is calling for people to help derive light curves for a neglected type of variable star – UGZs, otherwise known as Z-Cams after the first one known, Z Camelopardalis.
Z-Cams are a type of dwarf nova star. That is every now and then they brighten up, then fall off in brightness, then brighten up again and so on. The brightenings are related to material from one star falling onto another and igniting fusion on the surface. Above a certain limit, this causes the material to explode off the surface again, causing the brightenings. Z-Cams differ from normal dwarf novae because whereas normal dwarf novae always return to the same brightness after the outburst as they had before it, Z-Cams sometimes don’t. They undergo phases known as ‘standstills’ where the brightness they return to is brighter than their usual minimum, though dimmer than their maximum. They then slowly drop down to their normal minimum brightness.
As a result, drawing a light curve of a UGZ star will show the standstill, and prove it to be a UGZ. Light curves are simply graphs of brightness with time and can be derived visually (compare the star’s brightness at different points to the brightnesses of nearby nonvariable stars of known brightness and guess how bright it is) or using a CCD (take a digital photograph and do the same, but on your computer). CCDs are best for the dimmer stars as long exposures will reveal them, but visual observing can be used on some of the stars in this campaign. So all the AAVSO want is for people to go to certain stars, record their brightnesses and send in the observations.
As mentioned, Z-Cams tend to be fairly neglected as far as variable stars go. Current models have variable stars as always starting standstills after an outburst and always ending standstills by dropping down into minimum brightness without an outburst. However, three stars in the campaign haven’t ended their standstills that way. As there are only thirty-forty UGZ stars listed, these oddballs represent a large percentage of the total population.
The list of campaign stars is here. Those marked in yellow are confirmed and just require monitoring (visually even) to improve light curves. The ones in green need their standstills confirmed, but again can be done visually. Those without colour markings can have outbursts monitored visually, but standstills require CCD monitoring. Those in blue require intensive CCD only monitoring, as do those in red, except the red ones are the oddballs that require particular attention. The campaign will be looking at all of these stars and trying to do the following:
- Confirm the stars are Z-cams, expose imposters
- Get new and reliable light curves for the stars
- Determine whether or not some standstills end with outbursts, or whether the drop back to minimum brightness has just been missed
- Report any other unexpected results from observing these stars
- Publish the results in a peer reviewed publication such as the Journal of the AAVSO