third submission with small edits and fewer typos 1848423228 Submission failed due to error communicating with the submission server. Please retry in a few minutes. If this error persists please contact the help desk at help@stsci.edu Assigned ID: 4973 ----- Attempting Submission 1 (Tue Mar 12 17:47:28 GMT 2024) ----- HST Phase I Proposal 4973 successfully submitted. Receipt: # 4973-1 ----- Attempting Submission 2 (Tue Mar 26 00:01:18 GMT 2024) ----- HST Phase I Proposal 4973 successfully submitted. Receipt: # 4973-2 ----- Attempting Submission 3 (Tue Mar 26 19:57:55 GMT 2024) ----- How many low-mass, ultracool dwarfs (UCD) are in the Milky Way? And where are these low-mass stars and free-floating planet-like objects in our Galaxy? We propose to use the SuperBoRG collections of WFC3 Pure-Parallel Observations to search for Galactic ultracool dwarfs (UCD: M, L, and T) to accurately model the 3D structure of the Milky Way disk in UCDs. Recent studies of Milky Way interlopers in high-redshift observations have revealed a 300 pc thick disk of these cool stars with 7% of the M-dwarfs in an oblate stellar halo. We propose to use all Pure-Parallel WFC3 observations to quadruple the number of sight-lines (340 fields, an order of magnitude more volume) through the Milky Way and expand the search to L- and T-dwarfs. Accurate typing has been shown to work on HST grism data using machine learning techniques. Such an approach has yet to be applied on HST photometry but could produce accurate photometric typing to within two subtypes. With enough statistics and the power of random sampling and volume of the Milky Way structure, we will be able to constrain the scales of the structural components of the Milky Way (thin and thick disks and halo) using an MCMC approach for M-, L- and T-dwarfs. This approach has the benefit to allow us to constrain scale-lengths, -heights, and densities as well as the relative position of our Sun with respect to the disk of dwarf stars of our Milky Way. The total number of each UCD type can then be inferred from each model, constraining the low-end of the Galaxy-wide Initial Mass Function (GIMF) for the first time and the fraction of stars in the halo, a prediction from cold dark matter structure formation. Dr Holwera led the previous work on the BoRG pure-parallel observations and the catalog of M-dwarfs culled from this early data. He supervised van Vledder in her work on the MCMC model based on this catalog. He is an expert in low luminosity star/galaxy separation and the use of archival data in novel ways. As part of the CEERS team, he is developing the kNN photometric classification of M-L-T-Y dwarfs for JWST/NIRcam filters. Dr. Christian Aganze is the lead author on the recent effort on WISPS grism results on the low-mass population of Milky Way stars. This complementary data was enhanced with Machine Learning to maximize the typing of these stars. His first result provides a strong prior for the scale-height of substellar types, making the rest of the analysis practical. Dr. R. Russell is an expert in the identification of low-mass stars in WFC3 data. He is the author of an early M-L-T catalog using the WFC3 ERS data and leads the efforts in the identification in other fields. Dr. Chih-Chun Hsu is a postdoctoral researcher at Northwestern University, studying kinematics, rotation, and multiplicity of low-mass stars and brown dwarfs, using high-resolution near-infrared spectroscopy. Prof. Adam Burgasser is an expert in brown dwarf spectroscopy and population studies (Burgasser 2004, ApJS 155, 191) who has been PI/Admin-PI on several HST programs examining substellar multiplicity and globular cluster brown dwarfs. Galaxies Stellar Populations and the Interstellar Medium 0 false false false false false false false SUPPORTED