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 first week in May, 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 cataclysmic variable SDSS J151413.72+454911.9, the fifth of six targets in their campaign series. These observations serve two purposes: to look for time-variability in the ultraviolet spectrum associated 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 this target safely, the controllers at the Space Telescope Science Institute must know that SDSS J151413.72+454911.9 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 SDSS J151413.72+454911.9 in the two weeks prior to the HST observing window, and intensive observations during the window. SDSS J151413.72+454911.9 is at V=19.5 in quiescence, and so positive observations of this star will likely not be possible without long integrations. For this reason, "fainter-than" observations as deep as may be obtained in reasonable time are requested. The image below is a 7.0x7.0 arcminute unfiltered deep exposure (V ~ 23.0) taken with the USNO 1.0-meter. This object has never been observed in outburst before, so its maximum brightness is unknown. If you cannot detect SDSS J151413.72+454911.9 itself in reasonable time, please follow the observing guidelines below.
|Figure 2. A 7.0'x7.0' deep, unfiltered image of SDSS J151413.72+454911.9 obtained with the USNO-1.0m telescope with a limiting magnitude of about 23. Due to the faintness of the target, detection of the comparisons but not SDSS J151413.72+454911.9 will be sufficient proof for HST to initiate observations of this star. If necessary, take multiple short exposures and stack them to reach the required S/N in SDSS J151413.72+454911.9, or the comparison star used for fainter-than limits.|||
Observers are asked to monitor SDSS J151413.72+454911.9 (RA: 15h 14m 13.720s, Dec: +45d 49m 11.80s, J2000) beginning immediately (2008 April 23 UT). Visual observers please observe this object as normal, and report the faintest comparison star magnitude you can detect if unable to reach SDSS J151413.72+454911.9 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. The AAVSO will issue a Special Notice announcing when HST observations have been scheduled; intensive observations will be required during the 24 hour period prior to the HST observations. In the event of an outburst, please contact the AAVSO immediately.
For visual observers, any of the comparison stars may be reported for fainter than estimates, but please report the faintest star you can reliably detect.
For CCD observers, please expose so that you can make a "fainter-than" determination with a S/N of 10 in at least one of the comparison stars fainter than magnitude 15.0. All of the comparison stars may be used as fainter-than estimators for this campaign, but we recommend using at least the V=15.023 comparison (RA=15h 14m 17.96s, Dec=+45d 50m 23.9s), which is safely below the HST limits. If you can go fainter and still reach S/N of 10 in reasonable time, please do so.
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 SDSS J151413.72+454911.9 itself, please try to do so as we can then detect the start of any outburst; but fainter-thans are also very important constraints for the HST observation planners.
Please promptly submit all observations to the AAVSO via WebObs using the name "SDSS J151413.72+454911.9" or the AUID "000-BDB-386".
This campaign is being coordinated by Matthew Templeton at AAVSO headquarters.