0 Assigned ID: 619 ----- Attempting Submission 1 (Fri Apr 05 18:39:37 GMT 2019) ----- HST Phase I Proposal 619 (j1305_1.apt) successfully submitted. Receipt: # 619-1 ----- Attempting Submission 2 (Fri Apr 05 20:28:46 GMT 2019) ----- HST Phase I Proposal 619 (j1305_1.apt) successfully submitted. Receipt: # 619-2 ----- Attempting Submission 3 (Fri Apr 05 21:05:23 GMT 2019) ----- HST Phase I Proposal 619 (j1305_1.apt) successfully submitted. Receipt: # 619-3 ----- Attempting Submission 4 (Fri Apr 05 21:51:06 GMT 2019) ----- ----- Attempting Submission 5 (Sun Apr 07 13:50:26 GMT 2019) ----- FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser We are currently waiting for the cycle-27 last visit of this target to execute. We need to analyze the cycle-27 data to finalize the strategy and timing of this observation. So this vist is on "HOLD". 2071253161 ----- Attempting Submission 1 (Tue Jul 23 13:23:35 GMT 2019) ----- ----- Attempting Submission 2 (Mon Feb 03 19:42:57 GMT 2020) ----- ----- Attempting Submission 3 (Mon Jun 22 18:30:23 GMT 2020) ----- ----- Attempting Submission 4 (Fri Sep 11 18:01:35 GMT 2020) ----- Visit is on HOLD as explained. 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 theoretical models for metal poor, low-mass stars. 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. Dr. Sahu and collaborators recently measured the mass of the nearby white dwarf, Stein 2051, through astrometric microlensing using HST/WFC3 observations (Sahu et al. 2017, Science, 356, 1046). This was the first observation of astrometric microlensing by a star outside the solar system, and the first mass measurement of a star using this technique. Our group has demonstrated that HST/WFC3 has the required accuracy and sensitivity, and the group has the necessary expertise to carry out such a project. We plan to repeat such mass measurements for a low-mass star through this proposal. Our team includes several experts on microlensing (e.g. Casertano, Dominik, Kains, Sahu and Wambsganss), experts on M-dwarf physics (e.g. Bond, Smart, Nielsen), experts on stellar models (Brown), experts on astrometry (Smart, Kains) and experts on precision photometry (Anderson, Bramich). Dr. Anderson and Dr. Bellini are the world-experts on characterizing HST/PSFs, and astrometry using HST observations. Our team includes authors who have published papers on prediction of upcoming microlensing events using Gaia DR2 data (e.g. Bramich, Belokurov, Klueter, McGill, Nielsen, Smith, Wamsganss). Our group has already developed all of the algorithms necessary for the data analysis, including the modeling required for interpretation of the data. Stellar Physics 6 false false false false false false false false SUPPORTED EXT-STAR K V-IV J2000 J2000 WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F555W The scheduling data was generated by CASM 27.2. CASM was run in Phase II mode. 1599846717633 55.4 (Tic-Visit :proposal-id 16206 :visit-id "01" :proposal-version -1 :trans-version "20200619" :date "11-Sep-2020 13:51:57.0" :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 703872 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 16206 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16206 :visit-id "01" :orbit-number 1 :pcs-time 383 :si-vis-utilization 3129 :al-overhead-at-end 177 :internal-dur-at-end 190 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3525 :pcs-gap 0 ) true 1 0