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 third 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 RE J1255+266, the last 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 RE J1255+266 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 RE J1255+266 in the two weeks prior to the HST observing window, and intensive observations during the window. RE J1255+266 is at V=19.2 in quiescence, and so positive observations of this star will be difficult 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 7.5x7.5 arcminute image from SDSS. This object has never been observed in outburst before, so its maximum brightness is unknown. If you cannot detect RE J1255+266 itself in reasonable time, please follow the observing guidelines below.
|Figure 2. A 7.5'x7.5' deep, unfiltered image of RE J1255+266 from the Sloan Digital Sky Survey. Due to the faintness of the target, detection of the comparisons but not RE J1255+266 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 for the variable, or for the comparison star used for fainter-than limits.|||
Observers are asked to monitor RE J1255+266 (RA: 12h 55m 10.650s, Dec: +26d 42m 26.30s, J2000) beginning immediately (2008 May 5 UT). Visual observers please observe this object as normal, and report the faintest comparison star magnitude you can detect. CCD observers are asked to use filters during observations if available; V filter is preferred, but B,Rc, and Ic may also be used.
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 17.0. All of the comparison stars may be used as fainter-than estimators for this campaign, but we recommend using at least the V=16.998 comparison (RA=12h 54m 57.12s, Dec=+26d 41m 33.6s), 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 RE J1255+266 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 "RE J1255+266" or the AUID "000-BCY-206".
This campaign is being coordinated by Matthew Templeton at AAVSO headquarters.