Sirius B is the nearest and brightest of all white dwarfs (WDs), but it is fiendishly difficult to observe from the ground because of the overwhelming brightness of Sirius A. We propose a continuation of our program of imaging observations of the Sirius system with WFPC2, which has been underway since 2001. The resulting astrometric data will not only greatly improve the precision of the binary orbit and the dynamical mass measurements for both the main-sequence and WD components, but will also test definitively for the claimed presence of a third body in this famous system, down to planetary masses. At present, there is a tantalizing suggestion in our data that there indeed may exist a substellar or planetary third body in the system. Our team has also obtained superb spectra of Sirius B using STIS, and we have achieved an excellent fit to the spectrum using model stellar atmospheres. However, the implied mass of the WD disagrees significantly with the dynamical mass implied by the existing visual-binary orbit (which still has to be based on a combination of low-accuracy ground-based astrometry plus the small number of existing HST astrometric observations). This is another critical motivation for improving the astrometry.

Our team has been making regular (semi-annual) observations of the separation and position angle of the Sirius system since 2001. In order to refine the visual orbit of Sirius and the implied dynamical masses of both stars, and to search for perturbations due to third bodies down to planetary mass, we propose to continue this program during Cycles~14, 15, and~16. Based on Fig.~2, it does not appear that there are significant perturbations on a timescale of less than about a year. We therefore propose to observe with WFPC2 once a year. We will continue to use the F1042M filter, which suppresses the light of Sirius~A to the extent that its image can be centroided using the diffraction spikes (this was done by Schroeder et~al.\ 2000, and we have found that it works well). The short exposures for Sirius~A will be alternated with longer exposures for Sirius~B at the same telescope pointing, with the process repeated at several different dither positions (a technique Bond is also using successfully in his ongoing companion program on Procyon). The photocenter motion of Sirius~A is well known from the ground, so we do not need to request any FGS observations (fortunately, since it is too bright anyway!). Total request: 1 WFPC2 orbit each in Cycles 14, 15, and 16. We prefer to use WFPC2 as long as it is available, for continuity of the program. However, it would be feasible to switch to ACS/HRC if necessary, or of course to WFC3 when it is installed. HOT STARS 12 false 0.0 false false false false false false SUPPORTED STAR A0-A3 V-IV DA Coordinates from Hipparcos Catalog. However, we actually list a position 1.5 arcsec south of the Hipparcos position, in order to point at the center of mass of the binary. July 17, 2007: position and epoch updated again to point halfway between the two components of the binary, based on December 2006 WFPC2 observations. J2000 06 45 8.9000 -16 42 58.00 J2000 WFPC2 Imaging No F1042M ORIENT requirement is done so that companion star will not lie near diffraction spikes or bleeding columns from the very bright primary star. (Tic-Visit :proposal-id 11290 :visit-id "01" :proposal-version -1 :trans-version "20070325" :date "29-Apr-2007 17:16:31.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 537792 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11290 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11290 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2640 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3041 :pcs-gap 0 ) false 1 0