0 Assigned ID: 960 ----- Attempting Submission 1 (Fri Apr 05 23:36:49 GMT 2019) ----- INEFFICIENT ORDERING OF FP-POS POSITIONS Resubmission with additional single-orbit repeat (Visit 23) of failed exposure in Visit 3, per HOPR 91757. 2004840630 ----- Attempting Submission 1 (Fri Jul 26 00:37:33 GMT 2019) ----- ----- Attempting Submission 2 (Fri May 01 01:04:18 GMT 2020) ----- Grating shifts occur during occultation, so changing the ordering of the FP-POS positions has no impact on the integration time. Ordering this way allows us to obtain identitical total integration times at each FP-POS. Uncovering Extremely Metal-Poor Massive Stars in Leo A Young, extremely metal-poor stellar populations are central to our quest to characterize the faintest star-forming galaxies in the Universe at all redshifts. Ongoing efforts to model the nebular and stellar wind lines in extremely metal-poor (< 10% Zsun) star-forming galaxies have revealed substantial disagreements with current stellar models. A primary source of this tension is a lack of calibration at low-metallicites: no UV spectrum of an individual O star at Z/Zsun < 0.1 has ever been taken. We propose to target the brightest two O stars in the extremely metal-poor (Z/Zsun<0.1) dwarf galaxy Leo A (D~700 kpc) with deep HST/COS FUV spectroscopy. Leo A is the nearest galaxy with a population of massive stars at these metallicities, making these the most efficient observations possible. With these COS observations, we will (i) directly constrain the stellar metallicity (independent of the gas phase) by accessing faint photospheric lines of iron and (ii) for the first time, make a precise measurement of the wind strengths these massive low-metallicity stars drive. Our results have significant implications for massive stellar models in this metallicity regime, and consequently for population synthesis techniques applied to galaxies throughout the universe. This experiment relies entirely on FUV spectra to access crucial massive stellar features inaccessible at other wavelengths, and cannot be conducted in the optical. This will constitute the first observational test of stellar models at the metallicities we expect to encounter regularly in reionization-era galaxies with JWST. We will obtain medium-resolution FUV COS spectroscopy for two O stars in the extremely metal-poor nearby dwarf galaxy Leo A. The G160M/1533 grating setting will be used to simultaneously target a set of both stellar wind (O V, Si IV, C IV, and He II) and photospheric absorption lines which are required to address our science goals of constraining the stellar iron abundance and wind strengths. The target stars were identified in HST imaging, and their ICRS coordinates were determined by centroiding the sources in SDSS u-band imaging. Both targets are relatively bright (F275W~17.7, 18.0), and can be successfully acquired in ACQ/IMAGE mode with Mirror A with 8 second exposures. In order to maximize the scheduling flexibility of our program, we split the 8-orbit integration time for each star into 4 visits of 2 orbits each. We will take spectroscopic exposures in TIME-TAG mode with each orbit devoted to a single FP-POS setting such that buffer readouts and grating shifts occur during target occultation. Although the estimated BUFFER-TIME required is larger than the exposure times, we set the BUFFER-TIME to the exposure time to ensure the entire buffer is read during occultation. We additionally swap the order of the FP-POS grating changes in subsequent visits to ensure each achieves the same amount of integration time. In 8 orbits per star, ETC estimates indicate that our required S/N of 20 per angstrom will be achieved for both stars, enabling confident measurement of both the stellar wind and photospheric features. PI Senchyna is a graduate student working with Co-I Stark whose work focuses on star-forming galaxies within ~100 Mpc. Co-I Stark has worked extensively on spectroscopy of star-forming galaxies at all redshifts. Co-I Weisz is an expert on resolved stellar populations, Local Group dwarf galaxies, and stellar models. PI Senchyna and Co-I Stark have extensive experience with HST/COS spectroscopy, having employed it to study unresolved stellar populations in programs GO: 14168, 14679, 15185, and 15646 (see Senchyna et al. 2017, 2019). The data reduction, analysis, and model comparison will be conducted jointly by the small team. Galaxies and the IGM 6 false false true false false false false SUPPORTED EXT-STAR MAIN SEQUENCE O NO J2000 J2000 EXT-STAR MAIN SEQUENCE O NO J2000 J2000 COS/FUV Spectroscopic No No G160M 1533 COS/FUV Spectroscopic No No G160M 1533 The scheduling data was generated by CASM 28.0. 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