18444( 3) - 04-Aug-2026 15:53:15 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18444 Version: 3 Check-in Time: 04-Aug-2026 15:53:15 STScI Edit Number: 0 Title Characterizing the hot companions of the first identified Yellow supergiant binary population ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Anna O'Grady Carnegie Mellon University Y CoI: Maria Drout University of Toronto N CoI: Dr. Ylva Gotberg Institute of Science and Technology N Austria CoI: Dr. Bethany Ann Ludwig Institute of Astronomy, KU Leuven N CoI: Dr. Katelyn Breivik Carnegie Mellon University N CoI: Shannon Bowes University of Toronto N ------------------------------------------------------------------------------------ Abstract Binary evolution theory predicts that the Hertzsprung Gap is home to multiple populations of binary systems with varied evolutionary histories. Recent works have constrained the binary fraction of evolved populations of massive stars in local galaxies such as red supergiants and Wolf-Rayet stars, but the binary fraction of yellow supergiants (YSGs) in the Hertzsprung Gap remains unconstrained. This is a critical gap in our knowledge of binary evolution, as YSGs in the Gap are expected to be the site of many post-main sequence binary interactions, the precise physical details of whose mechanisms are not well constrained. We have identified a population of likely YSG+OB star binary candidates in the Magellanic Clouds using UV/optical photometry, and confirmed the binary nature of a subset using optical spectroscopy. Here we propose to use HST/STIS to observe 6 of these YSG+OB binary systems in order to fully characterize the hot OB companion in the system. These observations will also us the constrain the the luminosity, temperature, and mass loss rates of the companions. These results will provide vital constraints on models of binary evolution, impacting our understanding of stellar populations, supernovae, and gravitational wave sources. ------------------------------------------------------------------------------------ Observations Description ------------------------ In this study we will observe hot (F spectral type) yellow supergiants (YSGs) with OB main sequence binary companions in the Large and Small Magellanic Clouds (L/SMC) with the goal of characterizing the OB companion star. Targets were chosen through a combination of UV-optical photometry and optical spectroscopy. Each target will be observed with the G140L grism on STIS/MAMA with the 52x0.2 slit. The wavelength coverage will allow us to access the diversity of lines needed to fully characterize the OB companion. We aim for a SNR of at least 30 to ensure weaker UV lines have enough flux. We use the Swift-UVOT UVM2 band magnitude (effective wavelength of 2245 Angstroms) as a proxy for WFC3/F225W band magnitudes for the ETC, choosing a Kurucz OB model with an effective temperature closest to the best-fit OB model for each target system. 18444( 3) - 04-Aug-2026 15:53:15 - [ 2] For 5 of our 6 targets, additional integration time is available beyond what is needed for good SNR in the G140L exposures. Here, we will additionally perform a short exposure with the G230L girsm, which will provide an even better continuum fit for the OB companion. Here, a SNR of at least 10 is sufficient. Each target requires 1 orbit to achieve these goals. Target acquisition will center on the YSG using the F28X50LP filter. The YSGs are bright, so the minimum exposure time for acquisition is sufficient. Any additional time left after meeting the minimum requirements for SNR will be added to the G140L exposures. ------------------------------------------------------------------------------------ 18444( 3) - 04-Aug-2026 15:53:15 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 2MASS-J00563 STAR, F3-F9, MAIN RA=00H56M30.8805S, J2000 V = 14.04 +/- 0.07 Swift-UVM2(AB) = 089-7146212 SEQUENCE O DEC=-71D46'21.23" 15.93, E(B-V) = 0.08 Reference Frame: ICRS Extended: NO Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 1.5196575077014348E-4 -0.001237000083165185 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 2 2MASS-J00480 STAR, F3-F9, MAIN RA=00H48M6.3126S, J2000 V = 12.53 +/- 0.06 Swift-UVM2(AB) = 629-7306379 SEQUENCE O DEC=-73D06'37.99" 15.90, E(B-V) = 0.08 Reference Frame: ICRS Extended: NO Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 1.8311616498394316E-4 -0.0013280000075610587 3.04E-5 ------------------------------------------------------------------------------------------------------------------------------------ 3 2MASS-J01050 STAR, B3-B5 V-IV, F3-F9 RA=01H05M3.5496S, J2000 V = 11.848 Swift-UVM2(AB) = 16.87, 354-7206321 DEC=-72D06'32.17" E(B-V) = 0.08 Reference Frame: ICRS Extended: NO Comments: This object was generated by the targetselector and retrieved from the 2MASS database; PI changed to Gaia DR3 coordinates. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 1.6421002701511248E-4 -0.001178 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18444( 3) - 04-Aug-2026 15:53:15 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 4 2MASS-J05370 STAR, F3-F9, MAIN RA=05H37M3.9150S, J2000 V = 12.95 Swift-UVM2(AB) = 14.82, 391-6910035 SEQUENCE O DEC=-69D10'3.65" E(B-V) = 0.11 Reference Frame: ICRS Extended: NO Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 2.8362474738532703E-4 7.3E-4 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 5 2MASS-J05263 STAR, F3-F9, MAIN RA=05H26M38.8978S, J2000 V = 13.3 Swift-UVM2(AB) = 15.58, E(B-V) 889-6925253 SEQUENCE O DEC=-69D25'25.37" = 0.11 Reference Frame: ICRS Extended: NO Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 4.87498327976217E-4 -3.9100002595660044E-4 3.837E-4 ------------------------------------------------------------------------------------------------------------------------------------ 6 2MASS-J05390 STAR, F3-F9, MAIN RA=05H39M5.9300S, J2000 V = 12.14 Swift-UVM2(AB) = 15.64, 594-6916267 SEQUENCE O DEC=-69D16'26.77" E(B-V) = 0.11 Reference Frame: ICRS Extended: NO Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 3.643238207816772E-4 7.46E-4 1.66E-5 ------------------------------------------------------------------------------------------------------------------------------------ 18444( 3) - 04-Aug-2026 15:53:15 - [ 5] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type STIS-ALONG-SLIT Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.35 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 90.0 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18444( 3) - 04-Aug-2026 15:53:15 - [ 6] Visit: 01 (2MASS-J00563089-7146212) 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 2MASS-J0056 STIS/CCD ACQ F28X50LP MIRROR 1 0.2 S (0.2 3089-714621 S) 2 ------------------------------------------------------------------------------------------------------------------------------------ 2 2MASS-J0056 STIS/FUV ACCUM 52X0.2 G140L 1425 1 707 S PATTERN 1 2 3089-714621 -MAMA see below 2 (1664 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 2MASS-J0056 STIS/NUV ACCUM 52X0.2 G230L 2376 1 100 S (100 3089-714621 -MAMA S) 2 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2MASS-J00563089-714 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A 6212 2MASS-J00563089-714 2 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none none none 1 832.0 S 6212 -MAMA 2MASS-J00563089-714 2 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none none none 1 832.0 S 6212 -MAMA 2MASS-J00563089-714 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A 6212 -MAMA 18444( 3) - 04-Aug-2026 15:53:15 - [ 7] Visit: 02 (2MASS-J00480629-7306379) Visit Requirements: ORIENT 20D TO 160 D; ORIENT 200D TO 340 D 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 2MASS-J0048 STIS/CCD ACQ F28X50LP MIRROR 1 0.1 S (0.1 0629-730637 S) 9 ------------------------------------------------------------------------------------------------------------------------------------ 2 2MASS-J0048 STIS/FUV ACCUM 52X0.2 G140L 1425 1 686 S PATTERN 1 2 0629-730637 -MAMA see below 9 (1652 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 2MASS-J0048 STIS/NUV ACCUM 52X0.2 G230L 2376 1 112 S (112 0629-730637 -MAMA S) 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2MASS-J00480629-730 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A 6379 2MASS-J00480629-730 2 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none none none 1 826.0 S 6379 -MAMA 2MASS-J00480629-730 2 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none none none 1 826.0 S 6379 -MAMA 2MASS-J00480629-730 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A 6379 -MAMA 18444( 3) - 04-Aug-2026 15:53:15 - [ 8] Visit: 03 (2MASS-J01050354-7206321) Visit Requirements: ORIENT 310D TO 290 D 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 2MASS-J0105 STIS/CCD ACQ F28X50LP MIRROR 1 0.1 S (0.1 0354-720632 S) 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 2MASS-J0105 STIS/FUV ACCUM 52X0.2 G140L 1425 1 1065 S PATTERN 1 2 0354-720632 -MAMA (2130 S) 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2MASS-J01050354-720 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A 6321 2MASS-J01050354-720 2 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none none none 1 N/A 6321 -MAMA 2MASS-J01050354-720 2 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none none none 1 N/A 6321 -MAMA 18444( 3) - 04-Aug-2026 15:53:15 - [ 9] Visit: 04 (2MASS-J05370391-6910035) Visit Requirements: ORIENT 320D TO 100 D; ORIENT 140D TO 280 D 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 2MASS-J0537 STIS/CCD ACQ F28X50LP MIRROR 1 0.1 S (0.1 0391-691003 S) 5 ------------------------------------------------------------------------------------------------------------------------------------ 2 2MASS-J0537 STIS/FUV ACCUM 52X0.2 G140L 1425 1 201 S PATTERN 1 2 0391-691003 -MAMA see below 5 (1700 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 2MASS-J0537 STIS/NUV ACCUM 52X0.2 G230L 2376 1 34 S (34 0391-691003 -MAMA S) 5 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2MASS-J05370391-691 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A 0035 2MASS-J05370391-691 2 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none none none 1 850.0 S 0035 -MAMA 2MASS-J05370391-691 2 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none none none 1 850.0 S 0035 -MAMA 2MASS-J05370391-691 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A 0035 -MAMA 18444( 3) - 04-Aug-2026 15:53:15 - [ 10] Visit: 05 (2MASS-J05263889-6925253) 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 2MASS-J0526 STIS/CCD ACQ F28X50LP MIRROR 1 0.1 S (0.1 3889-692525 S) 3 ------------------------------------------------------------------------------------------------------------------------------------ 2 2MASS-J0526 STIS/FUV ACCUM 52X0.2 G140L 1425 1 330 S PATTERN 1 2 3889-692525 -MAMA see below 3 (1666 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 2MASS-J0526 STIS/NUV ACCUM 52X0.2 G230L 2376 1 68 S (68 3889-692525 -MAMA S) 3 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2MASS-J05263889-692 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A 5253 2MASS-J05263889-692 2 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none none none 1 833.0 S 5253 -MAMA 2MASS-J05263889-692 2 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none none none 1 833.0 S 5253 -MAMA 2MASS-J05263889-692 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A 5253 -MAMA 18444( 3) - 04-Aug-2026 15:53:15 - [ 11] Visit: 06 (2MASS-J05390594-6916267) 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 2MASS-J0539 STIS/CCD ACQ F28X50LP MIRROR 1 0.1 S (0.1 0594-691626 S) 7 ------------------------------------------------------------------------------------------------------------------------------------ 2 2MASS-J0539 STIS/FUV ACCUM 52X0.2 G140L 1425 1 273 S PATTERN 1 2 0594-691626 -MAMA see below 7 (1668 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 2MASS-J0539 STIS/NUV ACCUM 52X0.2 G230L 2376 1 67 S (67 0594-691626 -MAMA S) 7 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2MASS-J05390594-691 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A 6267 2MASS-J05390594-691 2 STIS/FUV ACCUM 52X0.2 G140L 1425 1 none none none 1 834.0 S 6267 -MAMA 2MASS-J05390594-691 2 STIS/FUV ACCUM 52X0.2 G140L 1425 2 none none none 1 834.0 S 6267 -MAMA 2MASS-J05390594-691 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A 6267 -MAMA 18444( 3) - 04-Aug-2026 15:53:15 - [ 12] Summary Form for Proposal 18444 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.2, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 140D TO 280 D, ORIENT 200D TO 340 D, ORIENT 20D TO 160 D, ORIENT 310D TO 290 D, ORIENT 320D TO 100 D, PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, G230L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names 2MASS-J00480629-7306379, 2MASS-J00563089-7146212, 2MASS-J01050354-7206321, 2MASS-J05263889-6925253, 2MASS-J05370391-6910035, 2MASS-J05390594-6916267 ------------------------------------------------------------------------------------------------------------------------------------