0 Assigned ID: 1046 ----- Attempting Submission 1 (Sat Mar 07 00:47:57 GMT 2020) ----- HST Phase I Proposal 1046 successfully submitted. Receipt: # 1046-1 ----- Attempting Submission 2 (Sat Mar 07 00:57:47 GMT 2020) ----- Over the last several years, surprising detections of strong high-ionization nebular emission lines in star-forming galaxies ranging in distance from a few megaparsecs to deep into the reionization era have challenged the accuracy of modern stellar population synthesis models. Meanwhile, deep spectroscopy of massive OB stars in the Magellanic Clouds has uncovered increasingly strong evidence that canonical models of stellar evolution may already break down at the moderately-low metallicity of the SMC (20% solar). Both lines of evidence suggest that yet more metal-poor massive stars may be far more abundant producers of hard ionizing radiation than predicted by the latest generation of stellar models; but no direct test of stellar physics in this regime has yet been possible. Fortunately, over the next few HST cycles, the ULLYSES DDT program will help amass the ultraviolet spectra of a statistical sample of massive OB stars at sub-SMC metallicities necessary to conduct such an experiment for the first time. We propose to complete the ULLYSES spectral atlas below 20% solar metallicity with additional UV and optical spectra, and build the accompanying theoretical stellar atmosphere library necessary to constrain fundamental stellar parameters with these data. Leveraging the wealth of UV data ULLYSES will provide in the Magellanic Clouds, we will derive the first robust measurement of the gradient in stellar wind strengths and CNO surface abundances at fixed stellar class down to 10% solar metallicity. This work will provide the crucial framework for calibrating stellar models into the metallicity regime relevant to the interpretation of reionization-era galaxies with JWST. Team expertise: PI Senchyna is a final-year graduate student under Co-I Stark (moving to Carnegie Observatories as a postdoctoral fellow in September) whose thesis work focuses on nearby star-forming regions dominated by massive stars. Co-I Stark (Associate Professor at Univeristy of Arizona) has worked extensively on spectroscopy of star-forming galaxies at all redshifts. Co-I Weisz (Assistant Professor at UC Berkeley) 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 nearby unresolved massive stellar populations in programs GO: 14168, 14679, 15185, 15646, and 15881 (see Senchyna et al. 2017, 2019) and sub-SMC massive stars in an in-progress program from Cycle 27 (GO: 15921). The team has access to state-of-the-art optical observing facilities in both hemispheres and the computing resources necessary to complete the described program. The analysis plan will be carried out jointly by the small team along with associated students. Stellar Physics and Stellar Types Galaxies 0 Regular false false true false false SUPPORTED