0 Assigned ID: 5797 ----- Attempting Submission 1 (Tue Mar 26 23:04:53 GMT 2024) ----- Small change in exposure times for existing exposures. All exposures for each orbit have been changed the same way. No other changes were made. -589763092 ----- Attempting Submission 1 (Fri Aug 09 13:46:56 GMT 2024) ----- HST Phase II Proposal 17745 successfully submitted. Receipt: # 17745-1 ----- Attempting Submission 2 (Thu Aug 15 22:52:51 GMT 2024) ----- It Takes Two Planets to Tango: Constraining the Orbit of a Planetary-Mass Binary We aim to measure the orbit of the plantary-mass (M < 13 M_Jup) binary WISE J135501.90-825838.9, the second-lowest mass (unresolved) binary currently known. This binary was first identified as a "spectral binary," a composite object of two unresolved near-infrared spectra which have the features of two ultracool (T < 1300 K) objects combined into one spectrum. A preliminary RV and parallax solution place WISE 1355-8258 within the AB Doardus kinematic association (150-200 Myr). The WISE parallax + proper motion solution also shows a periodic astrometric signal in the residuals, which is likely due to the orbital motion of the binary. However, higher precision astrometry is needed to constrain the orbital parameters, which has an estimated period of P ~ 1116 days. Using HST/WFC3-UVIS we will obtain a precise astrometric solution for the parallax and proper motion, and constrain the orbital solution in the periodic motion of the astrometric residuals using 10 orbits over the next 2 cycles (6+4 orbits). These measurements will provide the first ever direct mass measurement of a planetary-mass binary. This system will be a touchstone benchmark system for models of planet formation and evolution. Each orbit is spaced at least 2 months apart and is set to cover the longest time baseline possible within Cycle 32 while also offering higher coverage during the estimated maximum orbital separation of the two binary components. We are unable to cover the entire estimated orbital period of 1116 days, but we are able to cover approximately a third of the orbit which will allow us to put preliminary constraints on the orbital motion of the binary. We have 4 additional epochs in Cycle 33, which will allow us to constrain the orbital parameters to better than 1%. We have grouped epochs into 3 groups of 2 orbits, where each group of 2 orbits is taken within 1 month of each other, and each group is separated by 3 months, maximizing the time baseline of Cycle 32 (6 orbits total in Cycle 32). We will utilize an additional 4 orbits over the maximum time baseline of Cycle 33 to sample the majority of the estimated period. This strategy will also allow us to detect shorter and longer periods of orbital movement, between ~60--2000 days. The first epoch should be taken as early as possible (approximately November 28th, 2024), and the last epoch as late as possible (approximately October 31, 2025). We have created windows for each orgit that span ~20 days, offering flexibility in scheduling while also covering the requested time baseline for each epoch (orbit) and grouping of orbits. Each drift-scan image is taken along a principal axis of the detector. After each drift-scan image, we perform a large dither along the same axis and direction as the previous image before the next drift-scan image is taken along another principal axis. This effectively makes a box pattern which will help us reduce systematics. Each epoch contains two images in point source imaging mode, and four in spatial-scanning mode with WFC3/UVIS. In addition to the images taken in spatial scan mode using F775W which will be used for absolute astrometry, we will take two short exposure (100s) point-source images at the beginning and at the end of the single orbit visits in F775W and F606W. These snapshots will provide information about calibrator source color, which is paramount to discriminate object type and distance of the reference sources in the field. This project is supported by a team with the expertise and the resources to support each step to ensure its success, and to deliver the final products of the survey on a rapid timescale. All the data will be processed with dedicated tools already in-hand and currently being optimized for HST and delivered. Drs. Christopher Theissen (PI), Luigi Bedin, and Adam Burgasser, are experts in astrometry of ultracool objects. Their expertise includes WISE and HST to obtain precise astrometric solutions for the faintest, lowest-mass objects in our Galaxy. Drs. Daniella Carolina Bardalez Gagliuffi and Clemence Fontanive have extensive expertise in measuring the orbits of, and characterizing low-mass binaries using a variety of instrument and methodologies. Drs. Johanna Vos and Chih-Chun Hsu are experts in orbital soluations and high-resolution spectroscopy and will assist in radial velocity measurements using IGRINS and constraining the full 3-d orbit. 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