This is identical to visit 2 of 13058, which is already executed. The only change w.r.t. the previous submission is that we have added an ORIENT restriction of 275 to 281, to avoid the brigher star in the scan direction. -856897749 ----- Attempting Submission 2 (Mon Jun 24 17:08:27 GMT 2013) ----- ----- Attempting Submission 3 (Tue Sep 17 22:09:35 GMT 2013) ----- checked. We propose to determine the mass of the cool, nearby, high-proper-motion white dwarf (WD) G 105-30 (LHS 1838) through astrometric microlensing. In a reprise of the famous 1919 solar eclipse that verified general relativity, G 105-30 will pass very close in front of a 19.5-mag background star in June 2012, with an impact parameter of only ~0.08 arcsec. As it passes in front, it will cause a relativistic deflection of the background star's image by >2 milliarcsec, an amount easily detectable 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 distance to G 105-30 is already known from an accurate parallax, and the relative 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%, potentially <1% for favorable circumstances). One key astrophysical prediction for WDs is the existence of a mass-radius relation (MRR), which depends on the WD's core composition. Since the luminosity and distance of G 105-30 are known, its radius is known. Our measurements will thus provide a new, precise point in the MRR. The mass of G 105-30 is of special interest because it is an old and relatively massive WD, which would provide new constraints near the bottom of the WD cooling curve, currently being used to age-date stellar populations. First, we need to know accurate locations and proper motions for G105-30 and the source star, in order to predict the precise circumstances of the event, including the date and impact parameter. We will use direct imaging with WFC3 to determine the astrometric parameters of G105-30, the source star, and the surrounding reference field. We need to know the time of closest approach as accurately as possible in order to schedule subsequent critical observations. We have a four-phase observing program: (1) In the first phase, we will obtain the HST observations needed to make the final predictions of the circumstances of the event (impact parameter and time of closest approach) with high accuracy. At the beginning of Cycle~19 we will image the field twice at about a 3-month separation for this purpose. (2) Based on the early Cycle~19 astrometry, we will refine the predicted dates between which the images will be blended, so that we will know the optimal observation dates just before and after those times. (3) We will obtain one more observation in cycle 19 just before the onset of blending (and before the peak of the microlensing), when the astrometric deflection is at its largest observable value. (If this epoch turns turns out to lie in the solar-avoidance interval, May--August 2012, we would make sure to obtain this observation just before the solar-avoidance interval.) (4) We will obtain 4 more observations after the peak of the microlensing, beginning just after the time when the source and the lens are no more blended. COOL STARS 12 false false 0.0 0.0 false false false SUPPORTED STAR GRAVITATIONAL LENS The coordinates were calculated using POSS2 red image. The coordinates are that of the source that will be lensed by the WD. 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