This is a continuation of proposal 10930. The FGS magnitudes of the target and reference stars have been updated based upon data acquired via proposal 10930. -EN ----- Attempting Submission 1 (Wed May 02 17:17:32 GMT 2007) ----- The proposal lists several target which are not used. The associated warnings can be ignored. Mass and Radius of a Near-Chandrasekhar-limit magnetic white dwarf REJ0317-853 is a unique object. According to our analyses it is the most massive white dwarf ever found, with a mass of 1.35 solar masses, approaching the Chandrasekhar limit. With a period of just 725 seconds it is the most rapidly rotating isolated white dwarf ever found. Moreover, RE J0317-853 is the hottest magnetic white dwarf discovered so far and has a strong magnetic field varying from about 180 to more than 700 MG over the stellar surface. Due to its strong polarization and high mass it has been used to test gravitational theories predicting gravitational birefringence. However, the existing mass and radius determination is indirect and still uncertain and would greatly profit from a high-precision parallax determination with the HST FGS.
We will obtain parallaxes of the unique magnetic white dwarf RE J0317-853 and its companion star, the non-magnetic DA white dwarf LB~09802, just 7 arcseconds away. The best current distance estimation based on a spectroscopic parallax of LB~09802 is about 35 pc. The mass determination of RE J0317-853 is entirely based on this measurement with all uncertainties of the spectroscopic parallax particularly the log g determination of LB~09802. Theoretically it is also possible although completely implausible that both stars are not physically associated. At 35pc the distance between both components is about 600 au, far too large for common-envelope evolution. The FGS measurements will allow us to measure the parallaxes of both stars with an uncertainty of about 1 mas ($\Delta \pi/\pi=3$\%). With the brightness of our two target stars of V=14.9 (RE J0317-853) and V=14.1 (LB~09802) we calculate that 6 orbits are necessary to achieve the final parallaxes. The observing scheme that we propose is the one suggested by the FGS handbook and used in practically all prior studies (e.g. Beuermann et al. 2004, A&A 419, 291; Beuermann et al. 2003, A&A 412, 821; McArthur et al. 1999, ApJ 520, L59; Harrison et al. 1999, ApJ 515, L93). To obtain a parallax measurement, a field must generally be observed at least three times, at the seasons of maximum parallax factor. These seasons are separated by six months. In order to obtain a proper parallax solution which involves solving for six unknowns, three visits spanning two HST cycles are necessary, two each in cycle 15 at the seasons of maximum parallax factor of the individual source and one in cycle 16. In the special case of RE J0317-853 and its companion (ecliptic latitude beta=62degrees, sin beta=0.88) the condition of maximum parallax factor is satisfied at all times, but still the second and third visits must be separated by about six and twelve months from the first one. Given the brightness of the targets, previous experience shows that two orbits per visit are required to achieve the targeted accuracy in the final parallaxes. Using the general approach of earlier observations, these orbits should preferably be scheduled a week apart. We will use the same reference stars for the two target white dwarfs, thus saving observation time and using no more HST orbits than would be necessary for a single parallax measurement. Using identical reference stars will make the parallax difference between the two putative companion stars more precise than their absolute parallaxes even. HOT STARS 12 false 0.0 false false false false false false SUPPORTED STAR DA Prime science target B-V=-0.16,U-B=-1.13,V-R=0.01,V-I=-0.11, Fmag=15.09 GSC2.010033026502 J2000 03 17 16.1750 -85 32 25.45 J2000 STAR DA White Dwarf star B-V=-0.16,U-B=-1.13,V-R=0.01,V-I=-0.11,Fmag=14.22 GSC2.0100330406 J2000 03 17 16.1750 -85 32 25.45 J2000 CALIBRATION ASTROMETRIC S0100330424 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC F8V (estimate spectral type) Fpg=11.8, Jpg=12.8 S0100330428 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC G8V (estimated spectral type) Fpg=14.1, Jpg=15.4 S0100330436 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC F6V (estimated spectral type) Fpg=14.57, Jpg=15.50 S010033043 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC G9V (estimated spectral type) Fpg=14.90, Jpg=16.2 S01003306643 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC K1V (estimated spectral type) Fpg = 13.7, Jpg=15.2 S010330376 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC G6V (estimated spectral type) Fpg = 14.5, Jpg = 15.7 S0100330378 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC F8V (estimated spectral type) Fpg = 13.9, Jpg = 14.9 S0100330374 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC F8V (estimated spectral type) Fpg = 13.89, Jpg = 14.89 S0100330440 J2000 00 00 0.0000 +00 00 0.00 J2000 FGS POS No F605W FGS POS No F605W FGS POS No F605W FGS POS No F605W /Users/nelan/Desktop/cycle16/LB9802/OP-cache/11300-v01.xml Two Guide Star FineLock Required. 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