0 Assigned ID: 4142 ----- Attempting Submission 1 (Tue May 23 18:11:02 GMT 2023) ----- HST Phase I Proposal 4142 successfully submitted. Receipt: # 4142-1 ----- Attempting Submission 2 (Tue May 23 18:48:21 GMT 2023) ----- HST Phase I Proposal 4142 successfully submitted. Receipt: # 4142-2 edited to be <50 min 42689210 ----- Attempting Submission 1 (Mon Sep 11 20:23:30 GMT 2023) ----- Your proposal could not be saved. Submission aborted. ----- Attempting Submission 3 (Tue Jul 02 17:13:27 GMT 2024) ----- time constraints were removed We propose a comprehensive astrometric follow-up of the first Y+Y brown dwarf binary, WISE 0336-0143 AB, a <1-AU separation and low mass ratio system detected with JWST. We aim to follow the ~50-mas astrometric wobble of the photocentre resulting from the binary orbital motion, by exploiting HST's high astrometric precision with WFC3/IR. Our program will accurately measure the system's proper motion, trigonometric parallax, and orbital configuration, at an unprecedented accuracy of <0.5 mas through asbolute astrometric calibrations using Gaia. Capitalising on the three archival HST epochs already available and the recent JWST results, we have a unique opportunity to simultaneously refine the system's distance and to measure the individual components' dynamical masses, by acquiring only four new HST epochs over the next two years. These additional carefully timed datasets are required to demarcate the absolute astrometric displacement of the barycentre and remaining on-sky offsets due to the orbital motion of the binary components around the centre of mass. This work will be crucial to anchor current and future observed characteristics of the unique WISE 0336-0143 benchmark, significantly contributing to our understanding of its fundamental and binary properties, and an important step forward in the characterisation and atmospheric modelling of Y-temperature giant exoplanets and brown dwarfs. In addition, this program will also pave the way for future investigations of sub-AU binary systems around faint Y dwarfs, and will provide us with an extensive dataset to develop a framework for joint astrometric analyses linking HST, JWST and Gaia data. This multi-cycle program consists of 2 orbits of WFC3 IR channel imaging in Cycle 31 (and 2 more orbits in C32) for precise astrometric monitoring of a newly discovered Y+Y brown dwarf binary. All orbits are planned as single-orbit visits. We apply large and fractional-pixel dithering patterns to allow both removal of hot pixels and increase the spatial resolution of the datasets. All observations are taken in full frame mode, in order to maximise the number of reference sources in the fields of view. Following the strategy from GO 16229 for the same target, we will acquire 2 F105W exposures and a set of 6 F160W images. The F105W images will be placed in the middle of the sequences, to avoid problems associated with contaminating scattered Earthlight at low limb angles at those wavekengths. We will use 278 s exposures in MULTIACCUM mode, with suitable sampling times and sequence to maximise to total exposure times. We request specific time constraints for each visit, in order to optimise our science results in the derivation of the system's parallax, proper motion, and orbital motion. Based on phases of the year already covered by existing HST datasets for this system, and the allowed visibility windows in Cycle 31, we propose to obtain our 2 visits around early/mid Dec 2023 and late Jun/mid Aug 2024. With these new epochs, we will be able to start separating offsets from the barycentric motion from additional perturbations due to the binary, enabling the derivation of preliminary constraints for the orbit. We will then determine the optimal timing for the last two epochs in Cycle 32, based on remaining degeneracies in the derived solutions. Dr. Cemence Fontanive (PI, University of Montreal) is an expert on studies of free-floating brown dwarfs. She has extensive experience with astrometric and photometric HST observations of nearby late-type T and Y dwarfs (Fontanive et al. 2018, MNRAS, 479, 2702; Fontanive et al. MNRAS, 494, 206; Fontanive et al. 2023, submitted) and has three succesful HST programs targeting low-mass brown dwarfs with WFC3 (GO 14686, GO 15201, GO 16229). Dr. Fontanive is also an expert in orbital determinations from combinations of high-resolution imaging and absolute astrometry information (Fontanive et al. 2019, MNRAS, 490, 1120), and was part of the JWST team that led the NIRCam discovery of WISE0336. Dr. Fontanive will manage the overall program, coordinate observations, contribute to the analysis and interpretation of the data, and lead publications of results. Dr. Luigi Bedin (Co-I, INAF-OAPD) is an expert on imaging photometry and astrometry with HST under-sampled images, and has contributed to state of the art calibrations and corrections of HST data. Dr. Bedin, with Dr. Fontanive, has led the development and application of precision astrometric techniques with HST/WFC3 for low-temperature brown dwarfs (Bedin et al. 2017, MNRAS, 470, 1140), in particular to link HST astrometry to the Gaia reference system at sub-mas levels (Bedin & Fontanive 2018, MNRAS, 481, 5339; Bedin & Fontanive, Bedin et al. 2020, MNRAS, 494, 2068). He was also recently involved in state of-the-art astrometry and PSFs photometry with NIRCam (Papers I,II,III). Dr. Bedin will lead the data reduction and technical analyses of the observations, and conduct with Dr. Fontanive the extraction of photometric and astrometric measurements for the derivation of astrometric parameters. Mr. Matthew De Furio (Co-I, University of Michigan) has six years experience in the direct detection of multiple systems using space-based imaging. His PSF-fitting technique have been used successfully to detect companions at half the diffraction limit on HST and JWST (De Furio et al. 2019, 2022, 2023), and led to the discovery of the companion to WISE0336 (Calissendorff, De Furio et al. 2023, ApJ 947 30). He has also led surveys of brown dwarf multiplicity in young star forming regions, and is co-leading a JWST/NIRCam GTO program studying the low-mass brown dwarf population in NGC 2024 and its companion population. He will lead the effort to attempt to resolve the companion to WISE0336. Dr. Loic Albert (Co-I, University of Montreal) is an experienced brown dwarf observer having contributed in a range of surveys (CFBDS - Delorme et al. 2008, A&A 482 961, Albert et al. 2011, AJ 141 203) to study ultracool brown dwarfs as well as brown dwarfs in star forming regions (Jose et al. 2020, ApJ 892 122, Dubber et al. 2021, MNRAS 505 4215). Dr. Albert is an expert in infrared instrumentation and is an instrument scientist for NIRISS onboard JWST (Albert et al. 2023, PASP accepted). He is leading the team conducting a survey of 20 Y dwarfs (JWST GO 2473) to look for companions with JWST/NIRCam which led to the discovery of the first Y+Y brown dwarf binary, WISE 0336 (Calissendorff et al. 2023, ApJ 947 30). He will contribute to the JWST parts of the analysis. 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