11290( 4) - 07/18/07 09:06 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 11290 Version: 4 Check-in Time: 18-Jul-2007 13:06:18 STScI Edit Number: 2 Title Dynamical Masses and Third Bodies in the Sirius System ------------------------------------------------------------------------------------ Type Cycle Parallel Pointing Tolerance GO 16 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Howard E. Bond Space Telescope Science Institute CoI: Dr. Martin Barstow University of Leicester N CoI: Dr. Matthew Burleigh University of Leicester N CoI: Dr. Jay B. Holberg University of Arizona N CoI: Dr. Edmund Nelan Space Telescope Science Institute N CoI: Dr. Gail Schaefer Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ 11290( 4) - 07/18/07 09:06 - [ 2] Observations Description ------------------------ 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 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. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 11290( 4) - 07/18/07 09:06 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SIRIUS STAR, A0-A3 V-IV, DA RA=06H45M8.7900S +/- 0.3S, DEC=- J2000 V = -1.47 16D43'6.30" +/- 0.3" Reference Frame: Other: HIPPARCOS/TYCHO_CATALOGUE Comments: 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. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2006.988 -0.038011 -1.22308 0.379 11290( 4) - 07/18/07 09:06 - [ 4] Visit: 01 Visit Priority: Visit Requirements: PCS MODE FINE; SCHED 90%; ORIENT 241.0D TO 274.8 D; ORIENT 281.8D TO 4.2 D; ORIENT 11.8D TO 45.0 D; BETWEEN 25-DEC-2007:00:00:00 AND 15-MAY- 2008:00:00:00 On Hold Comments: Additional Comments: ORIENT requirement is done so that companion star will not lie near diffraction spikes or bleeding columns from the very bright primary star. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 SIRIUS WFPC2 IMAGE PC1 F1042M ATD-GAIN=15,CLOCKS=YES 1 8 S POS TARG 0, 0 ------------------------------------------------------------------------------------------------------------------------------------ 2 SIRIUS WFPC2 IMAGE PC1 F1042M ATD-GAIN=15,CLOCKS=YES 1 60 S SAME POS AS 1 ------------------------------------------------------------------------------------------------------------------------------------ 3 SIRIUS WFPC2 IMAGE PC1 F1042M ATD-GAIN=15,CLOCKS=YES 1 8 S POS TARG -0.661,-0.661 ------------------------------------------------------------------------------------------------------------------------------------ 4 SIRIUS WFPC2 IMAGE PC1 F1042M ATD-GAIN=15,CLOCKS=YES 1 60 S SAME POS AS 3 ------------------------------------------------------------------------------------------------------------------------------------ 5 SIRIUS WFPC2 IMAGE PC1 F1042M ATD-GAIN=15,CLOCKS=YES 1 8 S POS TARG -0.422,-0.376 ------------------------------------------------------------------------------------------------------------------------------------ 6 SIRIUS WFPC2 IMAGE PC1 F1042M ATD-GAIN=15,CLOCKS=YES 1 60 S SAME POS AS 5 ------------------------------------------------------------------------------------------------------------------------------------ 7 SIRIUS WFPC2 IMAGE PC1 F1042M ATD-GAIN=15,CLOCKS=YES 1 8 S POS TARG -0.319,-0.479 ------------------------------------------------------------------------------------------------------------------------------------ 8 SIRIUS WFPC2 IMAGE PC1 F1042M ATD-GAIN=15,CLOCKS=YES 2 60 S SAME POS AS 7 ------------------------------------------------------------------------------------------------------------------------------------ 9 SIRIUS WFPC2 IMAGE PC1 F1042M ATD-GAIN=15,CLOCKS=YES 1 8 S POS TARG -0.57,-0.273 ------------------------------------------------------------------------------------------------------------------------------------ 10 SIRIUS WFPC2 IMAGE PC1 F1042M ATD-GAIN=15,CLOCKS=YES 2 60 S SAME POS AS 9 ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ SIRIUS 1 WFPC2 IMAGE PC1 F1042M none none none none 1 N/A SIRIUS 2 WFPC2 IMAGE PC1 F1042M none none none none 1 N/A SIRIUS 3 WFPC2 IMAGE PC1 F1042M none none none none 1 N/A SIRIUS 4 WFPC2 IMAGE PC1 F1042M none none none none 1 N/A SIRIUS 5 WFPC2 IMAGE PC1 F1042M none none none none 1 N/A SIRIUS 6 WFPC2 IMAGE PC1 F1042M none none none none 1 N/A SIRIUS 7 WFPC2 IMAGE PC1 F1042M none none none none 1 N/A SIRIUS 8 WFPC2 IMAGE PC1 F1042M none none 1 none 1 N/A SIRIUS 8 WFPC2 IMAGE PC1 F1042M none none 2 none 1 N/A SIRIUS 9 WFPC2 IMAGE PC1 F1042M none none none none 1 N/A SIRIUS 10 WFPC2 IMAGE PC1 F1042M none none 1 none 1 N/A SIRIUS 10 WFPC2 IMAGE PC1 F1042M none none 2 none 1 N/A 11290( 4) - 07/18/07 09:06 - [ 5] Summary Form for Proposal 11290 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ATD-GAIN=15 CLOCKS=YES ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE FINE; SCHED 90%; ORIENT 241.0D TO 274.8 D; ORIENT 281.8D TO 4.2 D; ORIENT 11.8D TO 45.0 D; BETWEEN 25-DEC-2007 00 00 00 AND 15-MAY-2008 00 00 00; POS TARG 0, 0; SAME POS AS 1; POS TARG -0.661,-0.661; SAME POS AS 3; POS TARG -0.422,-0.376; SAME POS AS 5; POS TARG -0.319,-0.479; SAME POS AS 7; POS TARG -0.57,-0.273; SAME POS AS 9; ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F1042M ------------------------------------------------------------------------------------------------------------------------------------ Target Names SIRIUS ------------------------------------------------------------------------------------------------------------------------------------