0 Assigned ID: 5077 ----- Attempting Submission 1 (Sat Mar 23 10:45:27 GMT 2024) ----- Update to include ORIENT for Visit 04 1661060357 ----- Attempting Submission 1 (Tue Aug 13 20:58:46 GMT 2024) ----- ----- Attempting Submission 2 (Mon May 12 15:32:04 GMT 2025) ----- ----- Attempting Submission 3 (Thu Apr 16 16:49:05 GMT 2026) ----- Using cold brown dwarfs to understand Jupiter-like atmospheres hinges on defining and explaining their diverse properties. As such a recently accepted JWST cycle-1 GO proposal used the parallax sample of the reddest/faintest brown dwarfs to define 12 sources that share a common mid infrared color -- an excellent proxy for temperature -- but show meaningfully different Spitzer [4.5] micron absolute magnitudes. Each source is to be followed up with JWST NIRSPEC G395H high resolution (R~2700) spectra as well as MIR F1000W, F1280W, and F1800W photometry to measure bolometric luminosities and differentiate what causes the sample diversity in the mid-infrared. However two of the 12 sources in this legacy JWST proposal lack NIR information therefore can not be maximized for their scientific output. For this HST proposal we request 9 orbits to obtain WFC3 Grism spectra for 2 cold brown dwarfs that are already slated to be definitional for cold world science. This program will obtain a low-resolution near-infrared spectrum of two Y dwarfs: W1935- 1546 and W0503-5648. We will use the WFC3 G141 grism, which has a resolving power of 130 over the 1.1-1.7 micron wavelength range. In order to wavelength calibrate the slitless grism images, direct images of the field in the F125W filter will be obtained. The WFC3 grism observations are slitless and therefore roll angle constraints must be placed on the observations in order to avoid contamination of the target by other sources in the field. To ensure we are able to find a suitable roll angle free of contamination from nearby sources, we will also require a pre-image of the field around both targets. Given the J band magnitude of W1935-1546 and W0503-5648 at 23.93+/-0.33 and 22.54+/-| 0.09 mag respectively, we require one orbit each to obtain a pre-image for analysis before the spectrum can be taken. Based on published data of HST Y dwarfs as well as modeling of cold brown dwarf spectra, we require a minimum S/N of ~30 - 50 for the G141 grism observation. This S/N will allow for a high precision spectral retrieval that can be used in tandem with the soon to be acquired JWST data. We will use 4 dither positions for both the direct F125W images and the G141 grism observations. We used a model Y0 template with the HST WFC3 exposure time calculator to conclude that W0503-5648 requires 2 orbits and W1935-1546 requires 5 orbits to achieve the required S/N. In all we request 3 orbits for W0503-5648, 6 orbits for W1935-1546 or 9 orbits in total to complete the program. Dr. Jackie Faherty, the PI of this proposal, is an expert in brown dwarf characterization studies. She is also PI of the JWST proposal which this HST project will compliment. The co-investigators on this team consist of many of the Co-I's from the JWST cycle 1 proposal (1) Adam Schneider who has authored two of the benchmark papers for characterizing Y dwarfs using Hubble Space telescope data, (2) Davy Kirkpatrick who helped define the Y dwarf spectral classification scheme and has authored or co-authored numerous papers on these cold compact objects including a recent paper inclusive of parallaxes for all objects needed in the analysis of this work, (3) Dan Caselden, a citizen scientist who was critical in helped to establish the JWST sample through his discoveries in the Backyard Worlds project, (4) Federico Marocco who has papers characterizing multiple new Y dwarfs using WISE, Spitzer, and HST data, (5) Aaron Meisner who has published a significant number of new cold compact sources in the WISE and Spiter era, and (6) Ben Burningham who has written the spectral retrieval code called BREWSTER that will be used to analyze all data in this proposal. 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