Cataclysmic Variables

Makoto Uemura

To many of the active observers present, Makoto Uemura is known to them through the very active VSNet group in Japan. His talk was most interesting and thorough, especially for somebody whose first language is not English!

Looking at Cataclysmic Variables generally, some observations can be made. For instance, the luminosity of the accretion disk is proportional to the rate at which mass is accreted. Those stars with very short orbital periods such as the SU UMa stars exhibit superhumps that is, smaller and shorter-term fluctuations imposed on the main outburst, since these are caused by tidal effects causing the accretion disk to precess.

At the opposite corner of the diagram above, we have the Nova-Like variables with longer orbital periods. These have a high accretion rate with little thermal instability and so less variation is observed, since the higher brightness of the disk tends to dampen magnitude variations. The outbursts of the NL stars are less frequent and extreme than those of the 'mainstream' CVs such as SS Cygni. NL stars that I observe include V Sagittae, Z Andromedae and YY Herculis.

Eclipsing systems such as U Geminorum and OY Carinae are also useful in that they can provide details of accretion disks. The eclipses of U Geminorum take place around magnitude 14 - 14.6 and can be observed with equipment in the hands of many amateurs. Accurate timing is of course necessary.