LPVs for CCD observers
Dear LPV observers,
I would like to suggest two exciting objects for the CCD observers among you: RU Vul and T UMi. Both strongly decreased their pulsation period and amplitude in the past decades, and are therefore candidates for having undergone a helium shell flash (aka thermal pulse) in the recent past.
RU Vul used to be an SRa variable with P~155d and a full amplitude of 2mag when it was discovered, but the period and amplitude decreased substaintaly in the past decades. Nowadays the amplitude is too low to be detected by visual observations, so it would be required to be observed with a CCD camera to safely detect its light variation (pulsation). The last detected period was 108 days, so one observation every five days would be required to get a good light curve. At this time of the year, RU Vul rises in the East in the evening, so it is entering a phase of good observabilty. There is the possibility that the pulsation period and amplitude recover again in the near future, it would be very interesting to see the onset of period and amplitude increase.
The other star, T UMi, used to be a Mira variable with a period of 315d that started to sharply decrease in the 1980ies, for no apparent reason. Today, the period is about 113d (!), and the amplitude is below the classical definition limit for Miras of 2.5 mag. At the moment, its light variation can still be detected by visual observations, but if the amplitude decreases further, it could become a difficult target for visual observers. Hence, CCD observers are also encouraged to follow the exciting evolution of that star. As for RU Vul, there is the possibility that the period and amplitude start to increase again in the next few years, continued monitoring is very important.
Just to illustrate for what kind of science these observations are useful, I would like to point you to a recent paper by me, where these two stars are discussed amongst others: http://arxiv.org/abs/1105.2198 RU Vul and T UMi are candidates for a recent helium shell flash, so it is possible that we are witnessing real-time stellar evolution in these cases! Continued monitoring of these stars is of paramount importance to understand these objects.
Please let me know if there is someone among you who is going to observe these stars.