Revised with the following 2 changes: 1. Added a 1.0sec exposure in F814W to add one more dither point in F814 short exposures 2. Shortened all 270 sec exposures to 240 sec to make the exposures fit within the orbit. These changes are minor, and hence should not need any further CS reviews. I talked with the CS Linda, and she agrees. Hope this revised version can be considered. Thanks. -1257343207 ----- Attempting Submission 14 (Mon Aug 04 19:21:57 GMT 2014) ----- ----- Attempting Submission 15 (Wed Sep 17 20:50:47 GMT 2014) ----- As explained above. The very nearby and well-known cool white dwarf (WD) Stein 2051B will pass very close to a 19.5-mag background star in March 2014, with an impact parameter of <0.2 arcsec. This affords a unique opportunity for a direct determination of its mass, through measurement of the gravitational deflection of the background star's image. As it passes in front, Stein 2051B will cause a deflection of the background star's image by ~3 milliarcsec, an amount detectable at the ~10-sigma level with HST/WFC3. The gravitational deflection angle depends only on the distances and relative positions of the stars, and on the mass of the WD. Since the distances and positions can be determined precisely before the event, the astrometric measurement offers a unique and direct method to measure the mass of the WD to high accuracy (<5%). One key astrophysical prediction for WDs is the existence of a mass-radius relation, which depends primarily on the core composition of the WD. Since the radius of Stein 2051B is known (from its distance, luminosity, and effective temperature), our mass measurement will provide an important addition to the very small number of WDs with well-determined radii and masses. The mass of Stein 2051B is of special interest because it is an old and relatively massive WD. We need to know accurate locations and proper motions for Stein 2051B and the source star, in order to predict the precise circumstances of the event, including the date and impact parameter. The existing ground-based measurements do not have the 1 mas accuracy needed for these purposes, especially given the 6.5 mas/day proper motion of Stein 2051B. Therefore, we will use direct imaging with WFC3 to determine the astrometric parameters of Stein 2051B, the source star, and the surrounding reference field. These determinations need to be done well in advance of the closest encounter in March 2014, beginning in Cycle 21. The maximum deflection of the source position that is actually observable occurs just before the images of the two stars become blended (at HST resolution), which happens about 30 days before closest approach (and 30 days after closest approach). So, we need to know the time of closest approach as accurately as possible in order to schedule these two critical observations. Additional observations should be scheduled before and after those critical times, when the deflection is changing most rapidly with time. COOL STARS 12 false false 0.0 0.0 false false false AVAILABLE STAR DA This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F475W The scheduling data was generated by CASM 22.2. CASM was run in Phase II mode. 1410986782005 /private/var/folders/xx/kgq1b0qs5jj32f6tg4vg0m5r00067j/T/OPVisit2459603582023887718.xml 54.8 ORIENT constraints are used to: (i) avoid diffraction spikes and bleed columns from Stein 2051A and B falling near the faint source star, and (ii) 0bserve at an orient which is close to (within 25 degrees of) visit 1 of HST program 13457 (Tic-Visit :proposal-id 13850 :visit-id "01" :proposal-version -1 :trans-version "20140605" :date "17-Sep-2014 16:46:21.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 703888 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13850 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13850 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2944 :al-overhead-at-end 170 :internal-dur-at-end 575 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3288.0 :pcs-gap 0 ) true 1 0