0 Assigned ID: 347 ----- Attempting Submission 1 (Fri Aug 17 18:41:58 GMT 2018) ----- HST Phase I Proposal 347 (hst-cycle-26_v01.pdf.apt) successfully submitted. Receipt: # 347-1 ----- Attempting Submission 2 (Sat Aug 18 00:01:21 GMT 2018) ----- HST Phase I Proposal 347 (hst-cycle-26_v01.pdf.apt) successfully submitted. Receipt: # 347-2 ----- Attempting Submission 3 (Sat Aug 18 00:26:33 GMT 2018) ----- HST Phase I Proposal 347 (hst-cycle-26_v01.pdf.apt) successfully submitted. Receipt: # 347-3 ----- Attempting Submission 4 (Sat Aug 18 00:47:57 GMT 2018) ----- 0 Assuming assigned ID: 347 The exquisite astrometric capabilities of HST have allowed us to measure proper motions (PMs) for many stellar systems in the Local Group. However, PMs require two epochs of observation which are not always available. PM precision depends on the time baseline of the observations, and on the feasibility of background galaxies to define an absolute reference frame, which are unfortunately not optimal in many cases. The second Gaia data release (DR2) offers unprecedented spatial coverage by providing PMs across the whole sky. However, Gaia's relatively short time baseline limits its highest PM performance to relatively bright stars. The positional error of DR2 is the same as a single HST exposure at G~18 mag and rapidly increases for fainter magnitudes. Here, we propose to develop tools that will combine HST archival images with Gaia measurements to derive PMs for stars at G~18 mag and fainter. This will increase the scientific impact of both observatories beyond their individual capabilities by providing a second epoch observation for any HST archival image, and improving the PM accuracy for any faint source (G > 18) in the Gaia catalog observed by HST more than 5 years ago. As specific applications, we will obtain precise PMs for stars in at least 4 dSphs, and thereby constrain their mass profiles and internal 3D dynamics. We will measure precise PMs for stars in a sample of GCs drawn from the distant and low-mass portions of the population, which we will use to study various aspects of their internal kinematics and put them in context with other GCs. Our source catalogs and tools will be made public so they may benefit the whole astronomical community. Stellar Populations 0 Legacy false false false false SUPPORTED