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 November, 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 SDSS091908. 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 this target safely, the controllers at the Space Telescope Science Institute must know that SDSS091908 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 SDSS091908 in the two weeks prior to the HST observing window, and intensive observations during the window. SDSS091908 is at V=18.3, and so this observation will be slightly easier than those of the previous two HST targets, PQ Andromedae and SDSS074545. The image below is a 4.5x4.5 arcminute V-band image with a 5-minute exposure time 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 091908 in reasonable time, please follow the observing guidelines below.
|Figure 2. A 4.5'x4.5' V-band image of SDSS091908 obtained with the USNO-1.0m telescope in a 5-minute exposure. Depending upon your observing system, target either SDSS091908 itself, or any of the comparison stars. Detection of the comparisons but not SDSS091908 will be sufficient proof for HST to initiate observations of this target. If necessary, take multiple short exposures and stack them to reach the required S/N in SDSS091908, or the comparison star used for fainter-than limits.|||
Observers are asked to monitor SDSS091908 (full name: SDSS J091945.11+085710.0: RA: 09h 19m 45.11s, Dec: +08d 57m 10.0s, J2000) beginning immediately (2007 November 07 UT). Visual observers please observe this object as normal, and report the faintest comparison star magnitude you can detect if unable to reach SDSS091908 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 November 12, please observe this object as often as possible through November 15 UT using the observing procedure outlined below, and submit data as quickly as is possible. In the event of an outburst, please contact the AAVSO immediately.
For CCD observers, if you are capable of reaching V=18.3 in reasonable time by stacking or direct integration, please obtain a S/N of 10 in SDSS091908 itself. Alternately, 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. All of the comparison stars are useful as fainter-than estimators for this campaign, but we recommend using at least the V=14.536 comparison (AUID 000-BCY-471; RA=09h 19m 29.70s, Dec=+08d 55m 51.8s), 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.
Because of the position of this object at RA 09h and the early morning reporting cutoff on 2007 November 14, observers in Europe, Asia, and Oceania are strongly encouraged to submit observations on this campaign. European observers observing in the pre-dawn hours of November 14 are best-placed to provide confirming observations on the day itself, but any observations during the preceeding 12-24 hours will be useful to observation planners at STScI.
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 SDSS091908 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.
This campaign is being coordinated by Matthew Templeton at AAVSO headquarters.