14448( 5) - 30-Sep-2015 18:03:23 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 14448 Version: 5 Check-in Time: 30-Sep-2015 18:03:23 STScI Edit Number: 2 Title Accurate Mass Determination of the Nearby Old White Dwarf Stein 2051B through Astrometric Microlensing ------------------------------------------------------------------------------------ Type Cycle GO/DD 23 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Kailash C. Sahu Space Telescope Science Institute Y CoI: Dr. Jay Anderson Space Telescope Science Institute N CoI: Dr. Howard E. Bond The Pennsylvania State University N CoI: Dr. Stefano Casertano Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract We request one orbit of WFC3 imaging of the nearby white dwarf (WD) Stein 2051B. In 2014 March, this WD passed very close to a faint background star, affording the opportunity to measure the relativistic deflection of its apparent position and thus determine the mass of the WD--the first application of this novel technique to the measurement of a stellar mass. We were awarded 7 orbits of HST time for WFC3 astrometry of the event. The deflection is clearly detected (at >3 sigma) in 3 of the epochs. Unfortunately, at the other 4 epochs, the background star (400 times fainter than the WD) was extremely close to the WD (inside the second Airy ring). In spite of strenuous efforts at PSF subtraction by our expert CoI, we have been unable to achieve high astrometric accuracy at these 4 epochs. Using only the 3 measured deflections, there is an ambiguity, resulting in two possible masses. One solution, at 0.71 Msun, is consistent with physical models for a CO-core WD, but the other, at 0.52 Msun, would imply an iron core. The latter would be inconsistent with most of our WD-formation theories, but oddly enough agrees with a ground-based mass derived by Strand in the 1980's based on an analysis of differential proper motions. Fortunately, the deflection is still detectable until ~2015 November, and as a bonus will now be in a different direction relative to the proper motion of the source. Thus one more observation, if taken before 2015 December, will both remove the ambiguity between the two mass solutions, and reduce the error bar on the correct mass. Hence we propose one orbit of DD time, to be obtained before 2015 December. ------------------------------------------------------------------------------------ Observations Description ------------------------ This is a continuation of our program 13850. Since (I) proper motions of the source and reference stars have the largest uncertainty at this point, (ii) there is a clear science gain if we measure the position of the red dwarf with respect to the WD, (iv) the source is ~90 pixels away from the WD, we plan to observe in 2kx2k mode in F814W, so that we can have the maximum number of reference stars. We will take ~2 additional short exposures in F763M to get the position of the red dwarf. ------------------------------------------------------------------------------------ 14448( 5) - 30-Sep-2015 18:03:23 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 2 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. ------------------------------------------------------------------------------------------------------------------------------------ 14448( 5) - 30-Sep-2015 18:03:23 - [ 3] Visit: 01 Visit Requirements: ORIENT 255D TO 255 D; BETWEEN 08-OCT-2015 AND 16-OCT-2015; VISIBILITY INTERVAL 56 M On Hold Comments: Additional Comments: ORIENT constraints are used to avoid diffraction spikes and bleed columns from Stein 2051A and B falling near the faint source star. 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 F814W FLASH=12, BLADE=A 1 1 S (1 S) POS TARG 10.4, 9; SEQ -SOURCE S C-SUB 1-10 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=6 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 POS TARG 15.4, 14 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 4 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=6 1 240 S (240 POS TARG 15.4, 14 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 5 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F763M FLASH=12 1 0.5 S (0.5 POS TARG 5.4, 14 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 6 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=6 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 POS TARG 5.4, 4 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 8 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=6 1 240 S (240 POS TARG 5.4, 4 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 9 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F763M FLASH=12 1 0.5 S (0.5 POS TARG 15.4, 4 -SOURCE S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 10 STEIN-2051B WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=6 1 240 S (240 POS TARG 15.4, 4 -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 F814W none none none none 1 N/A S 2C-SUB STEIN-2051B-SOURCE 2 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB 14448( 5) - 30-Sep-2015 18:03:23 - [ 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 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 F763M none none none none 1 N/A S 2C-SUB STEIN-2051B-SOURCE 6 WFC3/UVI ACCUM UVIS2-2K F814W 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 F763M 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 14448( 5) - 30-Sep-2015 18:03:23 - [ 5] Summary Form for Proposal 14448 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-2K2C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BLADE=A, FLASH=12, FLASH=6 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 08-OCT-2015 AND 16-OCT-2015, ORIENT 255D TO 255 D, POS TARG 10.4, 9, POS TARG 15.4, 14, POS TARG 15.4, 4, POS TARG 5.4, 14, POS TARG 5.4, 4, SAME POS AS 1, SAME POS AS 5, SEQ 1-10 NON-INT, VISIBILITY INTERVAL 56 M ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F763M, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names STEIN-2051B-SOURCE ------------------------------------------------------------------------------------------------------------------------------------