17785( 5) - 22-Apr-2026 15:56:22 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17785 Version: 5 Check-in Time: 22-Apr-2026 15:56:22 STScI Edit Number: 1 Title Anchoring the Diversity of Cold Worlds With HST Spectra ------------------------------------------------------------------------------------ Type Cycle GO 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Jacqueline Kelly Faherty American Museum of Natural History Y CoI: Dr. Ben Burningham University of Hertfordshire N CoI: Dr. Adam Schneider United States Naval Observatory Flagstaff N Station CoI: Dr. Aaron Meisner NOIRLab - (AZ) N CoI: Dr. J. Davy Kirkpatrick California Institute of Technology N CoI: Dr. Marc Jason Kuchner NASA Goddard Space Flight Center N CoI: Dan Caselden American Museum of Natural History N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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 17785( 5) - 22-Apr-2026 15:56:22 - [ 2] 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. ------------------------------------------------------------------------------------ 17785( 5) - 22-Apr-2026 15:56:22 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 W0503-5648 STAR, BROWN DWARF RA=05H03M6.9359S +/- 0.001", J2000 J=22.54 DEC=-56D48'29.44" +/- 0.001" Reference Frame: ICRS Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2022.6969 0.0924539368449424 0.2882 0.0983 ------------------------------------------------------------------------------------------------------------------------------------ 2 W1935-1546 STAR, BROWN DWARF RA=19H35M18.7380S +/- 0.001", J2000 J=23.93 DEC=-15D46'20.33" +/- 0.001" Reference Frame: ICRS Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2022.7168 0.020103577397385487 0.0431 0.0693 ------------------------------------------------------------------------------------------------------------------------------------ 17785( 5) - 22-Apr-2026 15:56:22 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-IR-DITHER-BOX-MIN Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.572 Line_Spacing 0.365 Coordinate_Frame POS-TARG Pattern_Orient 18.528 Angle_Between_Sides 74.653 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.572 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 0 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 17785( 5) - 22-Apr-2026 15:56:22 - [ 5] Visit: 01 (W0503 Pre-Image) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 W0503-5648 WFC3/IR MULTIAC GRISM1024 F125W NSAMP=13, SAMP-SEQ=SPARS50 1 DEF PATTERN 1 1 CUM (2411.751 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ W0503-5648 1 WFC3/IR MULTIAC GRISM102 F125W 1 none none none 1 N/A CUM 4 W0503-5648 1 WFC3/IR MULTIAC GRISM102 F125W 2 none none none 1 N/A CUM 4 W0503-5648 1 WFC3/IR MULTIAC GRISM102 F125W 3 none none none 1 N/A CUM 4 W0503-5648 1 WFC3/IR MULTIAC GRISM102 F125W 4 none none none 1 N/A CUM 4 17785( 5) - 22-Apr-2026 15:56:22 - [ 6] Visit: 02 (W1935 Pre-Image) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 W1935-1546 WFC3/IR MULTIAC GRISM1024 F125W NSAMP=13, SAMP-SEQ=SPARS50 1 DEF PATTERN 1 1 CUM (2411.751 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ W1935-1546 1 WFC3/IR MULTIAC GRISM102 F125W 1 none none none 1 N/A CUM 4 W1935-1546 1 WFC3/IR MULTIAC GRISM102 F125W 2 none none none 1 N/A CUM 4 W1935-1546 1 WFC3/IR MULTIAC GRISM102 F125W 3 none none none 1 N/A CUM 4 W1935-1546 1 WFC3/IR MULTIAC GRISM102 F125W 4 none none none 1 N/A CUM 4 17785( 5) - 22-Apr-2026 15:56:22 - [ 7] Visit: 03 (G141 Grism for W0503) Visit Requirements: SCHED 100%; ORIENT 15D TO 25 D; ORIENT 195D TO 205 D; AFTER 01 BY 60.0 D TO 365.0 D; ON HOLD FOR 01 On Hold Comments: Pre-image of W0503 is required to determine roll angle for G141 spectroscopic observations. 60 days allows for the determination of this angle and an update to the Phase II. Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 W0503-5648 WFC3/IR MULTIAC GRISM1024 F125W NSAMP=8, SAMP-SEQ=SPARS25 1 DEF CUM (177.936 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 W0503-5648 WFC3/IR MULTIAC GRISM1024 G141 SAMP-SEQ=SPARS100, NSAMP=12 1 DEF PATTERN 2 2 CUM (2205.872 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 W0503-5648 WFC3/IR MULTIAC GRISM1024 F125W NSAMP=8, SAMP-SEQ=SPARS25 1 DEF POS TARG 0.542, 0.182 CUM (177.936 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 W0503-5648 WFC3/IR MULTIAC GRISM1024 G141 SAMP-SEQ=SPARS100, NSAMP=12 1 DEF POS TARG 0.542, 0.182; CUM (2205.872 PATTERN 2 4 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ W0503-5648 1 WFC3/IR MULTIAC GRISM102 F125W none none none none 1 N/A CUM 4 W0503-5648 2 WFC3/IR MULTIAC GRISM102 G141 1 none none none 1 N/A CUM 4 W0503-5648 2 WFC3/IR MULTIAC GRISM102 G141 2 none none none 1 N/A CUM 4 W0503-5648 3 WFC3/IR MULTIAC GRISM102 F125W none none none none 2 N/A CUM 4 W0503-5648 4 WFC3/IR MULTIAC GRISM102 G141 1 none none none 2 N/A CUM 4 W0503-5648 4 WFC3/IR MULTIAC GRISM102 G141 2 none none none 2 N/A CUM 4 17785( 5) - 22-Apr-2026 15:56:22 - [ 8] Visit: 04 (G141 Grism for W1935) Visit Requirements: SCHED 100%; ORIENT 85D TO 95 D; ORIENT 265D TO 275 D; AFTER 02 BY 60.0 D TO 365.0 D; ON HOLD FOR 02 On Hold Comments: Pre-image of W1935 is required to determine roll angle for G141 spectroscopic observations. 60 days allows for the determination of this angle and an update to the Phase II. Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 W1935-1546 WFC3/IR MULTIAC GRISM1024 F125W NSAMP=8, SAMP-SEQ=SPARS25 1 DEF CUM (177.936 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 W1935-1546 WFC3/IR MULTIAC GRISM1024 G141 SAMP-SEQ=SPARS100, NSAMP=11 1 DEF PATTERN 2 2 CUM (2005.871 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 W1935-1546 WFC3/IR MULTIAC GRISM1024 F125W NSAMP=8, SAMP-SEQ=SPARS25 1 DEF POS TARG 0.542, 0.182 CUM (177.936 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 W1935-1546 WFC3/IR MULTIAC GRISM1024 G141 SAMP-SEQ=SPARS100, NSAMP=11 1 DEF POS TARG 0.542, 0.182; CUM (2005.871 PATTERN 2 4 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 W1935-1546 WFC3/IR MULTIAC GRISM1024 F125W NSAMP=8, SAMP-SEQ=SPARS25 1 DEF POS TARG 0.542, 0.182 CUM (177.936 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 W1935-1546 WFC3/IR MULTIAC GRISM1024 G141 SAMP-SEQ=SPARS100, NSAMP=11 1 DEF POS TARG 0.542, 0.182; CUM (2005.871 PATTERN 2 6 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 W1935-1546 WFC3/IR MULTIAC GRISM1024 F125W NSAMP=8, SAMP-SEQ=SPARS25 1 DEF POS TARG 0.542, 0.182 CUM (177.936 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 W1935-1546 WFC3/IR MULTIAC GRISM1024 G141 SAMP-SEQ=SPARS100, NSAMP=11 1 DEF POS TARG 0.542, 0.182; CUM (2005.871 PATTERN 2 8 S) ------------------------------------------------------------------------------------------------------------------------------------ 9 W1935-1546 WFC3/IR MULTIAC GRISM1024 F125W NSAMP=8, SAMP-SEQ=SPARS25 1 DEF POS TARG 0.542, 0.182 CUM (177.936 S) ------------------------------------------------------------------------------------------------------------------------------------ 10 W1935-1546 WFC3/IR MULTIAC GRISM1024 G141 SAMP-SEQ=SPARS100, NSAMP=11 1 DEF POS TARG 0.542, 0.182; CUM (2005.871 PATTERN 2 10 S) ------------------------------------------------------------------------------------------------------------------------------------ 17785( 5) - 22-Apr-2026 15:56:22 - [ 9] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ W1935-1546 1 WFC3/IR MULTIAC GRISM102 F125W none none none none 1 N/A CUM 4 W1935-1546 2 WFC3/IR MULTIAC GRISM102 G141 1 none none none 1 N/A CUM 4 W1935-1546 2 WFC3/IR MULTIAC GRISM102 G141 2 none none none 1 N/A CUM 4 W1935-1546 3 WFC3/IR MULTIAC GRISM102 F125W none none none none 2 N/A CUM 4 W1935-1546 4 WFC3/IR MULTIAC GRISM102 G141 1 none none none 2 N/A CUM 4 W1935-1546 4 WFC3/IR MULTIAC GRISM102 G141 2 none none none 2 N/A CUM 4 W1935-1546 5 WFC3/IR MULTIAC GRISM102 F125W none none none none 3 N/A CUM 4 W1935-1546 6 WFC3/IR MULTIAC GRISM102 G141 1 none none none 3 N/A CUM 4 W1935-1546 6 WFC3/IR MULTIAC GRISM102 G141 2 none none none 3 N/A CUM 4 W1935-1546 7 WFC3/IR MULTIAC GRISM102 F125W none none none none 4 N/A CUM 4 W1935-1546 8 WFC3/IR MULTIAC GRISM102 G141 1 none none none 4 N/A CUM 4 W1935-1546 8 WFC3/IR MULTIAC GRISM102 G141 2 none none none 4 N/A CUM 4 W1935-1546 9 WFC3/IR MULTIAC GRISM102 F125W none none none none 5 N/A CUM 4 W1935-1546 10 WFC3/IR MULTIAC GRISM102 G141 1 none none none 5 N/A CUM 4 W1935-1546 10 WFC3/IR MULTIAC GRISM102 G141 2 none none none 5 N/A CUM 4 17785( 5) - 22-Apr-2026 15:56:22 - [ 10] Summary Form for Proposal 17785 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures GRISM1024 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/IR ------------------------------------------------------------------------------------------------------------------------------------ Opmodes MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters NSAMP=11, NSAMP=12, NSAMP=13, NSAMP=8, SAMP-SEQ=SPARS100, SAMP-SEQ=SPARS25, SAMP-SEQ=SPARS50 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 60.0 D TO 365.0 D, AFTER 02 BY 60.0 D TO 365.0 D, ON HOLD FOR 01, ON HOLD FOR 02, ORIENT 15D TO 25 D, ORIENT 195D TO 205 D, ORIENT 265D TO 275 D, ORIENT 85D TO 95 D, PATTERN 1 1, PATTERN 2 10, PATTERN 2 2, PATTERN 2 4, PATTERN 2 6, PATTERN 2 8, POS TARG 0.542, 0.182, SCHED 100% ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F125W, G141 ------------------------------------------------------------------------------------------------------------------------------------ Target Names W0503-5648, W1935-1546 ------------------------------------------------------------------------------------------------------------------------------------