Characterizing the hot companions of the first identified Yellow supergiant binary population

0 Assigned ID: 7215 ----- Attempting Submission 1 (Wed Apr 15 21:19:03 GMT 2026) ----- HST Phase I Proposal 7215 successfully submitted. Receipt: # 7215-1 ----- Attempting Submission 2 (Thu Apr 16 14:43:06 GMT 2026) ----- HST Phase I Proposal 7215 successfully submitted. Receipt: # 7215-2 ----- Attempting Submission 3 (Thu Apr 16 14:44:35 GMT 2026) ----- HST Phase I Proposal 7215 successfully submitted. Receipt: # 7215-3 ----- Attempting Submission 4 (Thu Apr 16 21:02:21 GMT 2026) ----- Second draft of Phase II. Adjusted dither pattern from 3 to 2, and updated exposure times and ETCs accordingly. Waiting on TTRB for allowability of ORIENT constraints. 35236149 ----- Attempting Submission 1 (Wed Jul 29 18:32:01 GMT 2026) ----- HST Phase II Proposal 18444 successfully submitted. Receipt: # 18444-1 ----- Attempting Submission 2 (Tue Aug 04 15:33:49 GMT 2026) ----- Characterizing the hot companions of the first identified Yellow supergiant binary population 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. 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. 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. All of the investigators on this proposal are experts in various aspects of massive star evolution and/or binary star populations. Co-Is Drout and Gotberg are experienced HST observers. PI Dr. Anna O'Grady is an expert in observational studies of evolved stellar populations in nearby galaxies. Dr. Maria Drout is an expert in observational studies of evolved stellar populations in nearby galaxies, a wide range of supernova explosions, and other transients. Dr. Ylva Gotberg is an expert in theoretical studies using stellar evolution models and radiative transfer models to investigate products of mass stripping in close binary evolution, their predicted observational signatures, and their contribution to the radiation budget of stellar populations. Dr. Bethany Ludwig is an expert at UV phototmetry in crowded stellar regions and binary stars. Dr. Katie Breivik is an expert in binary star evolution and population synthesis. Shannon Bowes is a graduate student and expert in massive star variability. Stellar Populations and the Interstellar Medium Stellar Physics and Stellar Types 6 false true false false false false false SUPPORTED STAR F3-F9 MAIN SEQUENCE O NO 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. GAIA-DR3-4689089820302699008 SMCOBS14.12-71.77 J2000 J2000 Swift-UVM2(AB) = 15.93, E(B-V) = 0.08 STAR F3-F9 MAIN SEQUENCE O NO 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. GAIA-DR3-4685948683034959232 SMCOBS12.02-73.11 J2000 J2000 Swift-UVM2(AB) = 15.90, E(B-V) = 0.08 STAR B3-B5 V-IV F3-F9 NO This object was generated by the targetselector and retrieved from the 2MASS database; PI changed to Gaia DR3 coordinates. GAIA-DR3-4687507554334608128 SMCOBS16.26-72.10 J2000 J2000 Swift-UVM2(AB) = 16.87, E(B-V) = 0.08 STAR F3-F9 MAIN SEQUENCE O NO 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. GAIA-DR3-4657690070314900224 LMCOBS84.26-69.16 J2000 J2000 Swift-UVM2(AB) = 14.82, E(B-V) = 0.11 STAR F3-F9 MAIN SEQUENCE O NO 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. GAIA-DR3-4658071424714429824 LMCOBS81.66-69.42 J2000 J2000 Swift-UVM2(AB) = 15.58, E(B-V) = 0.11 STAR F3-F9 MAIN SEQUENCE O NO 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. GAIA-DR3-4657676944892556416 LMCOBS84.77-69.27 J2000 J2000 Swift-UVM2(AB) = 15.64, E(B-V) = 0.11 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 POS-TARG DITHER 0.35 LINE false false 2 90.0 STIS-ALONG-SLIT 1

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