0Assigned ID: 2424
----- Attempting Submission 1 (Fri Apr 09 23:10:52 GMT 2021) -----INEFFICIENT ORDERING OF FP-POS POSITIONSInitial submission.1881027430----- Attempting Submission 1 (Thu Jul 22 17:15:18 GMT 2021) -----The inefficient ordering warning does not impact on exposure times as the FP-POS change occurs during occultation; this ordering ensures equal total exposure time in each FP-POS setting.UV spectroscopy of a serendipitously-detected He star in Leo A: an unprecendented glimpse of binary mass transfer at extremely low metallicityBinary mass transfer likely plays an outsized role in the evolution of massive stars, yet direct empirical constraints on these processes remain elusive. In particular, stars stripped by a binary companion are among the hottest sources of ionizing radiation expected at low metallicity. Such stripped stars plausibly dominate the flux of composite stellar populations at the hardest EUV energies, and may be required to explain the puzzling high-ionization nebular emission routinely encountered in low-metallicity star-forming galaxies. But despite substantial interest and modeling effort, only one star unambiguously stripped by binary interaction has yet been uncovered and studied in-detail. Exploratory optical spectroscopy in the extremely metal-poor (<10% solar) dwarf galaxy Leo A has recently uncovered a peculiar FUV-bright star with extremely prominent optical He II emission embedded in a small ionized nebula. Preliminary NLTE atmosphere modeling confirms that this source is broadly consistent with an intermediate mass metal-poor He star stripped by binary mass transfer; yet its fundamental properties remain uncertain without spectroscopy at the FUV wavelengths where it is brightest. Here we propose COS/G160M follow-up to confirm and characterize this unique target. These observations will enable confident measurement of the luminosity, temperature, and mass of this likely stripped star as well as strong constraints to be placed on its wind velocity structure and mass loss rate. The serendipitous discovery of this object represents an unexpected and extremely timely opportunity for HST to directly observe a product of binary evolution at unprecendentedly-low metallicity.This program will obtain a deep FUV spectrum for a peculiar star in Leo A identified as a potential stripped binary product in optical spectroscopy and NUV HST imaging. We leverage both in designing a safe observing program with COS.
Coordinates: To determine accurate ICRS coordinates for the target star, we compare the centroids of 14 nearby optically-bright stars in the F475W image to their GAIA ICRS coordinates to derive a (small and consistent to <0.1") correction to the F475W image astrometry. We use this offset to correct the F475W coordinates of the target star to the ICRS frame. Comparison to a calibrated SDSS u-band image in which the target is detected at modest SNR and which was previously used successfully to derive ICRS coordinates for COS spectroscopy of two brighter Leo A targets suggests good agreement.
Acquisition: We have a calibrated HST WFC3/F275W image in-hand covering the target star (GO:15275), which is close to the COS NUV imaging bandpass. Since the shape of the target star SED is not yet constrained blueward of F275W, we assume conservatively a flat continum in Flambda and require a S/N of 30 to derive an acquisiton exposure time estimate.
Observing plan: In accordance with STScI recommendations to maximize the scheduling of observations in Cycle 29, we split our spectroscopic observations into 4 visits of 2 orbits each. After acqusition, each orbit is filled with a single spectroscopic exposure at fixed FP-POS, with full buffer dumps and grating shifts occurring during occultation. All four FP-POS positions are cycled through over the course of the four visits to minimize the impact of fixed-pattern noise on the final stacked spectra.
Safety: While the shape of the target SED is not yet constrained into the FUV, the faint F275W magnitude of 19.3 (Vega) provides a strong constraint. Even assuming an extraordinarily hot unextincted blackbody of Teff>~200 kK results in expected counts well below safety limits both for a Mirror A target acquisition and G160M spectroscopic observation (where it is not expected to approach filling the buffer).PI Senchyna and Co-I Stark are experienced HST/COS observers, having extensively employed FUV+NUV spectroscopy to observe both unresolved dwarf galaxies in the <100 Mpc local volume (GO: 14168, 14679, 15185, 15646, 15881, PI: Stark; Senchyna+2017,2019,2021) as well as resolved O dwarfs in Leo A (GO: 15921, results in-prep; PI: Senchyna, Stark, Weisz). Co-I Weisz and Gull are experts on resolved stellar populations, Local Group dwarf galaxies, and stellar models. Co-Is Goetberg, Smith, and Drout are experts in the observation and modeling of massive stars in the context of binary evolution, mass loss, and supernovae. The data reduction, analysis, and model comparison will be conducted jointly by the small team.Stellar Physics and Stellar TypesGalaxies6falsefalsetruefalsefalsefalsefalseSUPPORTEDEXT-STAREMISSION LINE STARSDOUNDESIGNATEDWINDNOJ2000J2000COS/FUVSpectroscopicNoNoG160M1589The scheduling data was generated by CASM 29.2.CASM was run in Phase II mode.1651908344000107.30000000000001(Tic-Visit
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