13850( 5) - 18-Sep-2014 04:23:34 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 13850 Version: 5 Check-in Time: 18-Sep-2014 04:23:34 STScI Edit Number: 2 Title Accurate Mass Determination of the Nearby Old White Dwarf Stein 2051B through Astrometric Microlensing ------------------------------------------------------------------------------------ Type Cycle GO 22 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Kailash C. Sahu Space Telescope Science Institute Y CoI: Dr. Howard E. Bond The Pennsylvania State University N CoI: Dr. Jay Anderson Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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. ------------------------------------------------------------------------------------ 13850( 5) - 18-Sep-2014 04:23:34 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 STEIN-2051B- STAR, DA RA=04H31M15.0090S +/- 0.1", J2000 V = 12.45 SOURCE DEC=+58D58'13.17" +/- 0.1" Reference Frame: ICRS Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. ------------------------------------------------------------------------------------------------------------------------------------ 13850( 5) - 18-Sep-2014 04:23:34 - [ 3] Visit: 01 Visit Requirements: ORIENT 243D TO 243 D; BETWEEN 01-OCT-2014 AND 02-OCT-2014; VISIBILITY INTERVAL 54.8 M On Hold Comments: Additional Comments: 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 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F606W FLASH=12, BLADE=A 1 0.5 S (0.5 POS TARG 10.4, 9; SEQ -SOURCE S C-SUB S) 1-10 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F606W FLASH=1 1 240 S (240 SAME POS AS 1 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 3 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=12 1 1.0 S (1 SAME POS AS 1 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 4 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=6 1 240 S (240 SAME POS AS 1 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 5 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F606W FLASH=12 1 0.5 S (0.5 POS TARG 14.35, 9 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 6 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F606W FLASH=1 1 240 S (240 SAME POS AS 5 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 7 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=12 1 1.0 S (1 SAME POS AS 5 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 8 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=6 1 240 S (240 SAME POS AS 5 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 9 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=12 1 1 S (1 S) POS TARG 10.39, 12.95 -SOURCE S C-SUB ------------------------------------------------------------------------------------------------------------------------------------ 10 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=6 1 240 S (240 POS TARG 10.39, 12.95 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ STEIN-2051B-SOURCE 1 WFC3/UVI ACCUM UVIS2-2K F606W none none none none 1 N/A S 2C-SUB 13850( 5) - 18-Sep-2014 04:23:34 - [ 4] 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 | ------------------------------------------------------------------------------------------------------------------------------------ STEIN-2051B-SOURCE 2 WFC3/UVI ACCUM UVIS2-2K F606W none none none none 1 N/A S 2C-SUB STEIN-2051B-SOURCE 3 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB STEIN-2051B-SOURCE 4 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB STEIN-2051B-SOURCE 5 WFC3/UVI ACCUM UVIS2-2K F606W none none none none 1 N/A S 2C-SUB STEIN-2051B-SOURCE 6 WFC3/UVI ACCUM UVIS2-2K F606W none none none none 1 N/A S 2C-SUB STEIN-2051B-SOURCE 7 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB STEIN-2051B-SOURCE 8 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB STEIN-2051B-SOURCE 9 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB STEIN-2051B-SOURCE 10 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB 13850( 5) - 18-Sep-2014 04:23:34 - [ 5] Summary Form for Proposal 13850 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-2K2C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BLADE=A, FLASH=1, FLASH=12, FLASH=6 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 01-OCT-2014 AND 02-OCT-2014, ORIENT 243D TO 243 D, POS TARG 10.39, 12.95, POS TARG 10.4, 9, POS TARG 14.35, 9, SAME POS AS 1, SAME POS AS 5, SEQ 1-10 NON-INT, VISIBILITY INTERVAL 54.8 M ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names STEIN-2051B-SOURCE ------------------------------------------------------------------------------------------------------------------------------------