16206( 4) - 09-Aug-2021 15:43:23 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 16206 Version: 4 Check-in Time: 09-Aug-2021 15:43:23 STScI Edit Number: 2 Title Mass determination of an extreme halo M subdwarf through astrometric and photometric microlensing ------------------------------------------------------------------------------------ Type Cycle GO 28 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Kailash C. Sahu Space Telescope Science Institute Y CoI: Dr. Jay Anderson Space Telescope Science Institute N CoI: Dr. Andrea Bellini Space Telescope Science Institute N CoI: Dr. Vasily Belokurov University of Cambridge N CoI: Dr. Howard E. Bond The Pennsylvania State University N CoI: Dr. Thomas M. Brown Space Telescope Science Institute N CoI: Dr. Stefano Casertano Space Telescope Science Institute N CoI: Dr. Martin Dominik University of St. Andrews N CoI: Dr. Noe Kains Barnard College N CoI: Dr. Jonas Klueter Louisiana State University and A & M N College CoI: Mr. Peter McGill University of Cambridge N CoI: Dr. Martin Bo Nielsen New York University Abu Dhabi N CoI: Dr. Richard Smart Osservatorio Astronomico di Torino N CoI: Dr. Leigh Smith University of Cambridge N CoI: Dr. Joachim Wambsganss Astronomisches Rechen-Institut Heidelberg N CoI: Dr. Daniel Bramich New York University Abu Dhabi N ------------------------------------------------------------------------------------ Abstract We propose to measure the mass of the isolated M subdwarf 2MASS J13055171-7218081 (hereafter J1305), using both photometric and astrometric microlensing techniques. Searches of the recent Gaia DR2 have led to predictions of over 100 close passages of nearby stars in front of background sources that will occur in the next 20 years. Among these, J1305 is the closest of all, predicted to have a remarkably small impact parameter of ~5.8 mas, occuring in 2019 November. As J1305 passes in front, it will cause a relativistic deflection of the background star by ~0.9 milliarcsec, and a photometric brightening by ~5.5%. The deflections and the amplifications, measured with HST's WFC3, will provide the first opportunity to derive a lens mass through BOTH photometric and astrometric microlensing methods. The estimated accuracy of our mass measurement is ~4%. J1305 is of special interest as an extreme halo M subdwarf, having a transverse velocity of 236 km/s, and lying 3 magnitudes below the Population I main sequence. Spectroscopic observations show it to be an M3 dwarf with a very low metallicity of [Fe/H] = -1.8. Its estimated mass is ~0.11 Msun, but this is exquisitely sensitive to luminosity, effective temperature, and metallicity. To date, there are only a small number of mass measurements, based on binary stars, for metal-deficient M dwarfs, and none at all for masses less than 0.17 Msun or for [Fe/H] < -0.9. Our program will provide the first direct, model-independent mass determination for an extremely metal-deficient M dwarf, and a critical test of 16206( 4) - 09-Aug-2021 15:43:23 - [ 2] theoretical models for metal poor, low-mass stars. ------------------------------------------------------------------------------------ Observations Description ------------------------ We will image the field at 10 epochs as shown in Fig. 4, to monitor the brightening of the source during the photometric event; and to measure its astrometric position to detect the relativistic deflection, and to precisely determine the trajectory of the lens with respect to the source. At all the 10 epochs, we will obtain dithered images using the WFC3/UVIS F555W (V - band) filter. (We use direct imaging, since the background source is too faint for us to use the technique of spatial scanning introduced by Riess et al. 2014.) F555W is the bluest filter for which the PSF is well characterized for precision astrometry and photometry. Anderson private communications); a blue filter is desirable because the lens star is brighter and redder than the source. (The magnitude difference in the Gaia blue filter is only 0.6 mag, vs. 1.3 mag in the red filter.) We will use a 2k x 2k subarray to ensure that there are enough reference stars within the field, yet interruptions for buffer dumps will be unnecessary. At one epoch we will also obtain a few frames in F814W (I-band) in order to estimate the spectral types and distances of the reference stars, some of which are too faint for Gaia. Our strategy is to obtain 8 epochs during the astrometric event, which can also be used to measure photometric amplifications at 6 epochs, and the baseline at 2 epochs (see Fig. 4). Two additional epochs at late times provide the proper motions of the source and reference stars in the HST astrometric frame, and are taken 1 year apart at identical parallax factors. They will provide an accurate knowledge of the trajectory of the lens/source in the HST astrometric frame, which is necessary for both the photometric and the astrometric techniques. The additional observations at late times will also provide a check on any possible intrinsic variability of lens or source at millimag level (expected to be minimal for a very old dM star and the background K dwarf). These considerations make it necessary to observe the event over two HST Cycles. Scheduling: The desired epochs of observations close to the peak amplification and deflection are indicated in Figure 4 of our phase-I proposal, and require scheduling tolerances of a few days. The observations must begin as soon as possible after the start of Cycle 27. Away from the peak, the tolerances are considerably relaxed. Orientation: It will be necessary to use spacecraft orientations that will avoid diffraction spikes from the lens affecting the astrometry of the background star. However, the range of angles to avoid is small, and easily schedulable. ------------------------------------------------------------------------------------ 16206( 4) - 09-Aug-2021 15:43:23 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 2 J1305-SOURCE EXT-STAR, K V-IV RA=13H05M50.1680S, J2000 V = 18.15 DEC=-72D18'2.26" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 16206( 4) - 09-Aug-2021 15:43:23 - [ 4] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: 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 J1305-SOURC WFC3/UVI ACCUM UVIS2-2K2 F555W CR-SPLIT=NO, FLASH=19 1 75 S (75 POS TARG 10.4, 9 E S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 2 J1305-SOURC WFC3/UVI ACCUM UVIS2-2K2 F555W CR-SPLIT=NO, FLASH=19 1 75 S (75 POS TARG 15.34, 9 E S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 3 J1305-SOURC WFC3/UVI ACCUM UVIS2-2K2 F555W CR-SPLIT=NO, FLASH=19 1 75 S (75 POS TARG 14.18, 12.18 E S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 4 J1305-SOURC WFC3/UVI ACCUM UVIS2-2K2 F555W CR-SPLIT=NO, FLASH=19 1 75 S (75 POS TARG 11.25, 13.86 E S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 5 J1305-SOURC WFC3/UVI ACCUM UVIS2-2K2 F555W CR-SPLIT=NO, FLASH=19 1 75 S (75 POS TARG 7.92, 13.27 E S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 6 J1305-SOURC WFC3/UVI ACCUM UVIS2-2K2 F555W CR-SPLIT=NO, FLASH=19 1 75 S (75 POS TARG 5.75, 10.67 E S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 7 J1305-SOURC WFC3/UVI ACCUM UVIS2-2K2 F555W CR-SPLIT=NO, FLASH=19 1 75 S (75 POS TARG 5.77, 7.29 E S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 8 J1305-SOURC WFC3/UVI ACCUM UVIS2-2K2 F555W CR-SPLIT=NO, FLASH=19 1 75 S (75 POS TARG 7.96, 4.71 E S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 9 J1305-SOURC WFC3/UVI ACCUM UVIS2-2K2 F555W CR-SPLIT=NO, FLASH=19 1 75 S (75 POS TARG 11.29, 4.14 E S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 10 J1305-SOURC WFC3/UVI ACCUM UVIS2-2K2 F555W CR-SPLIT=NO, FLASH=19 1 75 S (75 POS TARG 14.21, 5.86 E 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 | ------------------------------------------------------------------------------------------------------------------------------------ J1305-SOURCE 1 WFC3/UVI ACCUM UVIS2-2K F555W none none none none 1 N/A S 2C-SUB J1305-SOURCE 2 WFC3/UVI ACCUM UVIS2-2K F555W none none none none 1 N/A S 2C-SUB J1305-SOURCE 3 WFC3/UVI ACCUM UVIS2-2K F555W none none none none 1 N/A S 2C-SUB 16206( 4) - 09-Aug-2021 15:43:23 - [ 5] 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 | ------------------------------------------------------------------------------------------------------------------------------------ J1305-SOURCE 4 WFC3/UVI ACCUM UVIS2-2K F555W none none none none 1 N/A S 2C-SUB J1305-SOURCE 5 WFC3/UVI ACCUM UVIS2-2K F555W none none none none 1 N/A S 2C-SUB J1305-SOURCE 6 WFC3/UVI ACCUM UVIS2-2K F555W none none none none 1 N/A S 2C-SUB J1305-SOURCE 7 WFC3/UVI ACCUM UVIS2-2K F555W none none none none 1 N/A S 2C-SUB J1305-SOURCE 8 WFC3/UVI ACCUM UVIS2-2K F555W none none none none 1 N/A S 2C-SUB J1305-SOURCE 9 WFC3/UVI ACCUM UVIS2-2K F555W none none none none 1 N/A S 2C-SUB J1305-SOURCE 10 WFC3/UVI ACCUM UVIS2-2K F555W none none none none 1 N/A S 2C-SUB 16206( 4) - 09-Aug-2021 15:43:23 - [ 6] Summary Form for Proposal 16206 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-2K2C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO, FLASH=19 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements POS TARG 10.4, 9, POS TARG 11.25, 13.86, POS TARG 11.29, 4.14, POS TARG 14.18, 12.18, POS TARG 14.21, 5.86, POS TARG 15.34, 9, POS TARG 5.75, 10.67, POS TARG 5.77, 7.29, POS TARG 7.92, 13.27, POS TARG 7.96, 4.71 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W ------------------------------------------------------------------------------------------------------------------------------------ Target Names J1305-SOURCE ------------------------------------------------------------------------------------------------------------------------------------