0 Assigned ID: 6488 ----- Attempting Submission 1 (Thu Apr 10 15:43:01 GMT 2025) ----- Please let me know if there are any mistakes. 745590882 ----- Attempting Submission 1 (Sun Aug 10 07:41:05 GMT 2025) ----- Massive stars stripped of their hydrogen-rich envelopes are one of the most common products of binary interaction. They are the progenitors of stripped-envelope supernovae and binary compact object mergers. Uncertain assumptions in binary mass transfer, including the efficiency of mass and angular momentum transfer, affect our understanding of stripped-envelope supernovae and gravitational wave merger progenitors. Without empirical constraints from stripped stars, parameter-space degeneracies are likely to persist despite the anticipated growth in data in the coming decade. Observationally, only a few stripped stars have been detected because the light from their much brighter B- or O-star companions dominates the spectral signature. Recent simulations indicate that high-quality FUV spectra from HST/STIS are ideal for identifying low-luminosity stripped stars with O-star companions. We propose a pilot study requiring only six orbits to characterise the first discovered stripped star+O star binary, HD 96670, in the Milky Way. We will cross-correlate model spectra of stripped stars with the observed spectrum to derive the radial velocities of the stripped star. Combined with the O-star radial velocity curve from the literature, we will provide the first dynamical mass estimate for a stripped star+O star binary. This project will pave the way for more extensive programs to identify populations of stripped star+O star binaries with luminosity contrasts down to 1%. We demonstrate how our analysis of HD 96670 will serve as a cornerstone for providing robust empirical constraints on mass and angular momentum transfer efficiency in massive binary evolution. We request 2 epochs of high-resolution R ~ 45800 spectra (using the FUV-MAMA detector and the E140M echelle grating) of HD96670 (1144 - 1710 Angstrom) at a signal-to-noise ratio SNR ~ 120 at 1425 Angstrom (global SNR ~ 170 for 2 epochs). For the E140M grating with a 0.2 x 0.06 aperture, a Pickles O9 star spectrum with V = 7.43 and E(B - V ) = 0.483 (Gomez et al. 2021), the exposure time required to reach an SNR of 120 at 1425 Angstrom is ~ 5208 s or ~ 87 min, accounting for the minimum buffer time of 99 s (count rate > 20000/s). The target visibility is 52 min per orbit (APT). The total overhead is calculated as the sum of the initial acquisition overhead (6 min) + target acquisition overhead (6 min) + first scientific exposure in MAMA (8 min) ~ 20 min. This allows for 52-20 ~ 32 mins of science exposure time per orbit. As such, 88/32 ~ 3 orbits are required per epoch to reach SNR ~ 120. Koushik Sen is an expert in massive binary evolution theory. He is also the PI of an observing proposal at the ESO VLT, program ID: 114.27G8, focused on multi-epoch optical spectroscopy of three Wolf-Rayet + O star binaries in the LMC. Additionally, he serves as the PI of an observing program at the XMM-Newton telescope, proposal no. 096343, on the same target HD 96670, which we propose to study in the FUV band using HST. Yael Naze and Gregor Rauw are experts in observational astronomy of massive stars, concentrating on binary interactions: eclipsing binaries, stripped star binaries, Be star binaries, colliding wind binaries, and magnetic stars. They routinely combine multiwavelength data (X-ray, optical, and UV) to constrain stellar properties. One of their latest analyses in this field was on HD96670, which revealed the stripped star nature of the companion. Nikolay Britavskiy is an observational astronomer with expertise in eclipsing binaries, colliding wind systems, multi-epoch photometry and spectroscopy in the optical and infrared bands. Mathieu Renzo is an expert in massive binary evolution and the developer of the widely used detailed binary evolution software MESA, which will be utilised to study the observed binary in this proposal. Stellar Physics and Stellar Types Stellar Populations and the Interstellar Medium 6 false false false false false false false SUPPORTED STAR 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. J2000 J2000 U 6.86, B 7.60, R 8.38, G 7.35, J 7.056, H 7.07, K 7.00 STIS/FUV-MAMA Spectroscopic No No E140M 1425 STIS/FUV-MAMA Spectroscopic No No E140M 1425 1754811255215 The scheduling data was generated by CASM 33.1. 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