0 Assigned ID: 2437 ----- Attempting Submission 1 (Fri Apr 09 23:19:26 GMT 2021) ----- HST Phase I Proposal 2437 successfully submitted. Receipt: # 2437-1 ----- Attempting Submission 2 (Fri Apr 09 23:34:44 GMT 2021) ----- HST Phase I Proposal 2437 successfully submitted. Receipt: # 2437-2 ----- Attempting Submission 3 (Fri Apr 09 23:48:34 GMT 2021) ----- HST Phase I Proposal 2437 successfully submitted. Receipt: # 2437-3 ----- Attempting Submission 4 (Fri Apr 09 23:54:34 GMT 2021) ----- 0 ----- Attempting Submission 1 (Wed Jul 28 20:09:40 GMT 2021) ----- ----- Attempting Submission 2 (Thu Jul 14 03:38:33 GMT 2022) ----- Advancing a decades long experiment at the Galactic center Observations of the orbits of stars around the supermassive black hole at the center of our Galaxy have brought us new and exciting insights into the physics and astrophysics of supermassive black holes. These observations have not only provided the best evidence so far for the existence of supermassive black holes, but they have also advanced our understanding of star formation and stellar dynamics in this region. With a time baseline of over 25 years, we can perform ever more powerful experiments, the latest of which are measurements of the effect of General Relativity on the orbit of the star S0-2/S2. As astrometry becomes more precise, the discovery of our Galaxy's dark cusp of compact objects is now within reach. However, there is a fundamental limitation: the stability of the reference frame. We propose to observe the Galactic center with WFC3-IR in the next three cycles, which when combined with Gaia data will create the most accurate reference frame to date. These measurements from 2022-2024 will (1) enable the detection of a dark cusp around the black hole (2) take advantage of the unique overlap in observations between HST, Gaia (ending in 2024), JWST (beginning in 2022), and the ground to cross-calibrate instrumental systematic uncertainties to build on the legacy of astrometry with HST; (3) use HST observations to double our yearly time baseline for monitoring the near-infrared activity of Sgr A* to determine the physical origins of its recent unprecedented increase in activity. This program has two main goals: Measure precise astrometry for a sample of stars in common between HST and Gaia at the Galactic center in order to build a stable reference frame for the measurements of orbits around the supermassive black hole at the Galactic center. The stability of the reference frame is driven by the goal of measuring the precession of the orbit of the star S0-2 from General Relativity and extended mass. As such, this program requires very high astrometric precision and minimal systematic errors. We have designed the Phase II observations to achieve these goals. Monitor the flux variation from the supermassive black hole Sgr A* in the near-infrared. Sgr A* is ideally monitored early and late in the year, when the Galactic center is not visible from the ground, doubling the yearly monitoring baseline. Astrometry and photometry: We request that observations in the F153M filter with a spiral dither pattern to subsample the PSF. We request that observations be made at an ORIENT=270 early in the year (Feb. to May) and ORIENT=90 later in the year (Jul. to Oct.). These two ORIENTS have been used in past astrometry programs at the Galactic center and keeping this orientation will help to minimize the effect of residual optical distortions. Timing constraints: We request 3 orbits be allocated early in the year and 3 later in the year. This in part, comes from the constraints on the ORIENT and also driven scientifically to maximize the baseline during the year that we can use to monitor the behavior of the near-infrared emission from Sgr A*. Our team consists of members who, together, have decades of astrometry and HST experience. PI Do, Co-I Lu, Co-I Anderson and Co-I Ghez have all been PI of previous successful HST astrometry programs with WFC3-IR. Our team has written over 100 papers about the Galactic center, many of which are focused on dynamics and accretion at the Galactic center. PI-Do has been studying the stellar population and infrared characteristics of Sgr A* for over 10 years. Co-I Ghez has led over 25 years of astrometric monitoring of orbits at the Galactic center with Keck Observatory. Co-I Becklin, and Co-I Morris have studying Sgr A* at multiple wavelengths for decades and have experience building and testing models of accretion. Co-I Lu is PI of a Cycle 1 JWST astrometry program at the Galactic center. Co-I Hosek has extensive experience with HST and Gaia and has constructed the first reference frame using HST and Gaia presented in this proposal. Co-I Sakai built the current astrometric reference frame in use for orbit measurements using observations of stellar masers. Co-I Hees is an expert on General Relativity and theories of gravity. Co-I Anderson is the world expert on HST imaging and astrometry. Co-I Gautam has experience on photometry in crowded fields. Co-I Chen has experience measuring the near infrared properties of Sgr A*. Co-I Lewis has experience comparing the reference frame from using radio masers to that from using Gaia stars. Stellar Populations and the Interstellar Medium Supermassive Black Holes and Active Galaxies 6 false false false false false false false SUPPORTED STELLAR CLUSTER NUCLEUS NO This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/IR Imaging No No F153M WFC3/IR Imaging No No F153M WFC3/IR Imaging No No F153M WFC3/IR Imaging No No F153M WFC3/IR Imaging No No F153M WFC3/IR Imaging No No F153M POS-TARG DITHER 0.42 LINE false false 21 11.0 SPIRAL 1 false The scheduling data was generated by CASM 30.3. 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