----- Attempting Submission 1 (Wed Jun 25 14:44:07 GMT 2008) ----- Assigned ID: 731 ----- Attempting Submission 1 (Fri Mar 07 23:12:16 GMT 2008) ----- The low mass white dwarf (WD) companion to the 3.26 ms pulsar PSR J1911-5958A offers an unprecedented opportunity for seismological study of the interior of a helium core WD. While much more massive carbon/oxygen core WDs are observed to pulsate in normal modes of oscillation called g-modes (known as ZZ Ceti stars), no helium core pulsator is known. By extrapolating the boundaries of the ZZ Ceti instability strip downward in surface gravity by a factor of 20 below any known pulsator, we find that the effective temperature of this WD makes it an excellent candidate to search for pulsation. Detection of g-mode pulsations in the lightcurve would have a transformative effect on the field of WD pulsations, as this would allow the first seismological study of the interior of a helium core WD, and the low gravity strongly constrains theories for the driving and amplitudes of pulsations. We show that with 3 orbits of HST, we will detect photometric variations with amplitudes of 1%, lower than typically seen in other hydrogen-dominated ZZ Ceti stars. A set of measured mode periods would also constrain the thickness of the presumed stably hydrogen burning shell, and help us determine its age more securely.

We require rapid and regular sampling to try to measure the oscillation modes of the white dwarf companion to PSR J1911-5958A. We have chosen WFC3 with the UVIS channel and the F200LP filter to maximize our throughput. We are using the UVIS1-C512A-SUB aperture so that we can cut down on the readout time and overhead. With 60-s exposures we have 55-s of overhead, making for a repetition time of 115s. This is fast enough to sample the expected oscillation modes (fastest is ~200 s). We are using CVZ observations to (1) get more exposure time per orbit and (2) get a much more continuous observing regiment without the gaps imposed by occultation that would harm our periodogram. With this strategy, we are using ~12 repetitions of each exposure for a total time of 1370s between exposure series. This is long enough that we will not affect detection of the longest expected periodicities (~600s). Each set of 12 exposures will be taken at the same position (not dithering) to preserve stability. Between each set of 12 we will dither, replicating the UVIS-DITHER-BOX pattern. In each orbit we have 4 sets of 12, and the dither positions will be shifted by (0.5,0.5) arcsec (orbit 2) and (-0.5, -0.5) arcsec (orbit 3) relative to orbit 1 to get better spatial sampling and average over flatfield errors. HOT STARS 12 false 0.0 0.0 false false false false false SUPPORTED STAR ZZ CETI STAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 19 11 42.7600 -59 58 26.90 J2000 WFC3/UVIS Imaging No No F200LP The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. /Users/dlk/TeX/proposals/hst08/psrj1911/phaseII/OP-cache/psrj1911-v01.xml 1213647501363 (Tic-Visit :proposal-id 11581 :visit-id "01" :proposal-version -1 :trans-version "20080609" :date "16-Jun-2008 16:18:13.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 654528 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11581 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :cvz T :proposal-id 11581 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 16256 :al-overhead-at-end 40 :internal-dur-at-end 529 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 0 :low-sky "N" :pcs-visibility 0 :pcs-gap 0 ) true 3 0