Dr. Paula Szkody of the University of Washington and collaborators are performing a comprehensive study of pulsating white dwarfs in cataclysmic variable systems using both ground and space-based observations. In the second week in September, they will use the Hubble Space Telescope's functional component (the Solar Blind Channel) of the Advanced Camera for Surveys to obtain time-resolved ultraviolet spectroscopy of the nova PQ And. These observations serve two purposes: to look for time-variability in the ultraviolet spectrum assocation with the pulsations of the white dwarf, and to use the overall spectrum to fit a temperature to the white dwarf itself. Detection of time-variability would help to define the pulsation periods of the white dwarf, which in turn provide information about the white dwarf's structure; an accurate measurement of the temperature would help white dwarf seismologists to define the white dwarf instability strip -- the region of the Hertzsprung-Russell diagram where white dwarfs are capable of maintaining self-excited pulsations.
Figure 1. A theoretical H-R diagram showing the location of several types of pulsating variable. The white dwarfs follow the cooling sequence on the left-hand side of the diagram. (Image from J. Christensen-Dalsgaard, Lecture Notes on Stellar Oscillations, Fifth Edition, May 2003; available from www.phys.au.dk/~jcd/oscilnotes/)
In order to observe PQ Andromedae safely, the controllers at the Space Telescope Science Institute must know that PQ Andromedae is at its quiescent level, or is fainter than a certain limit. AAVSO observers are able to make these observations, and your help is urgently needed to facilitate these important HST observations.
Observers are requested to perform nightly observations of PQ Andromedae in the two weeks prior to the HST observing window, and intensive observations during the window. Since PQ Andromedae is typically at V=19.1 during quiescence, this observation may be a challenge. The image below is a 2x2 arcminute subframe centered on PQ And, from a V-band image with a 15-minute exposure time taken with the USNO 1.0-meter. PQ Andromedae is clearly detected, but there are several bright stars nearby that will likely saturate, notably a V ∼ 9.2 comparison star a few arcminutes southwest of the star. Detection of PQ Andromedae is not required for a "successful" observation because the observational constraints are such that HST ground controllers only need to know it is fainter than a certain limit.
Figure 2. A 2'x2' subframe of a V-band image of PQ Andromedae obtained with the USNO-1.0m telescope in a 15-minute exposure. There is a bright star (V=9.2) at the bottom of the frame which will saturate when trying to reach PQ Andromedae itself (V=19.1 in quiescence). Take multiple short exposures and stack them to reach the required S/N in PQ And, or either of the two comparison stars used for fainter-than limits.
Observers are asked to monitor PQ Andromedae (RA: 02h 29m 29.61s, Dec: +40d 02m 40.0s, J2000) nightly for the two weeks beginning 2007 August 27 UT. Visual observers please observe this object as normal, and report the faintest comparison star magnitude you can detect if unable to reach PQ Andromedae itself. CCD observers are asked to use filters during observations if available; V filter is preferred, but B,Rc, and Ic may also be used. Beginning 2007 September 10, please observe this object as often as possible through September 16 UT using the observing procedure outlined below, and submit data as quickly as is possible. In the event of an (unlikely) outburst, please contact the AAVSO immediately.
For CCD observers, if you are capable of reaching V=19.1 in reasonable time by stacking or direct integration, please obtain a S/N of 10 in PQ And itself. Alternately, please expose so that you can make a "fainter-than" determination. The maximum useful brightness limit is that PQ And must be fainter than the V=14.5 comparison star (AUID 000-BBF-012; RA=02h 29m 41.98s, Dec=+40d 04m 20.7s) northeast of the variable. The preferred comparison star for a fainter-than measurement is the V=17.0 comparison star (AUID 000-BBF-008; RA=02h 29m 34.67s, Dec=+40d 04m 40.9s) north-northeast of the variable. Please expose so that you reach a minimum S/N of 10 for whichever comparison star you can detect, and report your observation as fainter than the comparison star magnitude. Again, we emphasize that both positive and fainter than observations are useful as long as the fainter-thans can reach the limits outlined above. If you are capable of detecting PQ Andromedae itself, please try to do so as we can then detect the start of any outburst, but fainter-thans are also important constraints for the HST observation planners.
This campaign is being coordinated by Matthew Templeton at AAVSO headquarters.
HST Program Information for this project (Szkody et al., GO 11163)
Szkody, P. et al., 2007, HST and Optical Observations of Three Pulsating Accreting White Dwarfs in Cataclysmic Variables, Astrophysical Journal 658, 1188
Recently submitted observations for PQ Andromedae
VSP-generated charts for PQ Andromedae
VSX entry for PQ Andromedae