0 Assigned ID: 2700 ----- Attempting Submission 1 (Tue Feb 01 01:34:53 GMT 2022) ----- HST Phase I Proposal 2700 successfully submitted. Receipt: # 2700-1 0 ----- Attempting Submission 1 (Mon Mar 28 22:21:18 GMT 2022) ----- The Winds and Ionizing Spectra of the Hottest O Stars at Extremely Low Metallicity Fast-approaching JWST observations of high-redshift galaxies seek to determine the contribution of low-metallicity massive stars to reionization. Yet, the predicted ionizing fluxes of metal-poor O stars are sensitive to uncertain, theoretical mass-loss rates and their downstream effects on stellar rotation and evolution. New observational constraints are required to make progress, but mass-loss measurements from FUV spectra of O stars in nearby galaxies below the metallicity of the SMC (20% Z_sun) remain extremely rare, particularly for the hottest, earliest spectral types that dominate the ionizing photon budget. We propose G130M and G160M COS observations of a bright, early-O star in Sextans A (6% Z_sun) that was identified in NUV imaging obtained in January 2022. This is just the third known extremely metal-poor, early-O star in any galaxy close enough for detailed study; the other two will both be observed at low spectral resolution by ULLYSES. From these FUV spectra, we will: (1) measure the star's mass-loss rate to test extrapolation of widely used mass-loss recipes to low metallicity; (2) use weak metal line detections to determine which metal ions drive winds in the hottest metal-poor O stars; and (3) provide crucial constraints not possible at lower resolution to maximize the value of the ULLYSES spectra in Sextans A. Combined with archival and ULLYSES data, the proposed observations will form the first empirical benchmark for stellar models that predict the ionizing spectra of reionization-era galaxies. The proposed observations are urgently needed to interpret JWST observations of high-redshift galaxies coming this year and can only be done with HST/COS. Here we plan observations of a newly identified, hot O star in the nearby, metal-poor galaxy Sextans A with the Cosmic Origins Spectrograph. We will use both the G130M and G160M gratings to get continuous coverage of important stellar wind and photospheric features to determine the mass-loss rates and abundances of this O star to constrain its ionizing photon production. We obtained initial coordinates by locating the target star in HST imaging with updated WCS information aligned to Gaia DR2. We then identified the target star in the Gaia catalog and adopted the coordinates reported by Gaia for these observations. Gaia reports an astrometric uncertainty of approximately 1 mas, but we conservatively estimate an uncertainty of 0.05 arcsec to account for any possible offset due to the (very small) proper motion. We estimate exposure times using the NUV F225W magnitude. We adopt the Milky Way foreground interstellar reddening, since the target's very blue optical and NUV colors are consistent with the intrinsically blue colors of hot O stars. We use the Bruzual O5 V spectral template in the COS ETC, and experimented with similar templates to confirm that the estimated exposure times will achieve SNR ~ 10 per resolution element near key wind features. We use these estimates of exposure times for both target acquisition and spectroscopic observations. All G130M BUFFER-TIMEs are 2/3 times the values reported by the COS ETC, while we adopt 10,000s for the G160M BUFFER-TIME (which is much greater than the orbit-filling exposure time, but less than 20,000, as per the COS handbook). We will use the ACQ/IMAGE mode with the PSA and Mirror A for target acquisition. Our observations satisfy the BOT safety standards. We organize the observations into visits of at most 3 orbits, as strongly encouraged for Cycle 29 observations. For all spectroscopic observations, we use TIME-TAG mode with FLASH=YES and SEGMENT=BOTH. We use the G130M grating with the 1291 central wavelength to cover the N V 1240, O V 1371, and Si IV 1400 stellar wind profiles. The G130M configuration only uses FP=3, 4 configurations to satisfy the COS2025 restrictions. We also use the G160M grating with the 1600 central wavelength to cover the Fe V 1428, S V 1502, C IV 1550, He II 1640, and N IV 1719 stellar wind and photospheric profiles. We use FP=All for the G160M observations to extend the possible wavelength coverage. Our team is well-positioned to deliver high-quality and timely analysis of the proposed observations. Collectively, we have published over a dozen papers on COS data. PI Telford is an expert in spectroscopy and photometry of metal-poor massive stars. Telford et al. (2021) presented the analysis of COS spectra and HST NUV-NIR photometry of three metal-poor massive stars (GO-15967, PI Chisholm). Telford is currently leading a followup analysis of their FUV photospheric and wind profiles with new stellar atmosphere models, and is the PI of an ongoing HST program (GO-16767) to use new and archival COS observations of the O stars in Sextans A to calibrate stellar models at low metallicity. Co-Is Berg and Chisholm have a great deal of experience working with COS spectra and both are leaders in the CLASSY program (GO-15840, PI Berg) to create a treasury of COS spectra of metal-poor, star-forming galaxies in the local universe. Chisholm has an extensive background in stellar population synthesis modeling and spatially unresolved observations of massive stars at both high and low metallicity. Co-I McQuinn has deep expertise in stellar photometry and the stellar populations of nearby, metal-poor galaxies, including in-progress work on the galaxy hosting our target O star, Sextans A. McQuinn and Telford are both experts on stellar evolution models and modeling the resolved stellar populations in nearby galaxies. Stellar Physics and Stellar Types 3 false false true false false true false SUPPORTED EXT-STAR MAIN SEQUENCE O NO J101104.79-044220.9 J2000 J2000 F225W = 18.0, F275W = 18.3, F336W = 18.8, F475W = 20.6, F814W = 21.1 COS/FUV Spectroscopic No No G130M 1291 COS/FUV Spectroscopic No No G160M 1600 /Users/grace/Writing/Proposals/HST29-MidJan-SextansA/OP-cache/Telford_HST29_MidJan_SextansA_phaseII-v01.xml The scheduling data was generated by CASM 30.1. CASM was run in Phase II mode. 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