0 Assigned ID: 2367 ----- Attempting Submission 1 (Fri Apr 09 22:32:16 GMT 2021) ----- HST Phase I Proposal 2367 successfully submitted. Receipt: # 2367-1 ----- Attempting Submission 2 (Fri Apr 09 23:14:47 GMT 2021) ----- HST Phase I Proposal 2367 successfully submitted. Receipt: # 2367-2 ----- Attempting Submission 3 (Fri Apr 09 23:21:55 GMT 2021) ----- At sub-solar metallicities, recent estimates of the CO/H2 abundance dependence differ by almost two orders of magnitude. While this dependence has been thoroughly calibrated in the Milky Way, its empirical estimation is incredibly difficult in other environments due to the lack of high-quality UV sightlines that pierce molecular gas clouds in low-metallicity galaxies. Such absorption-line measurements allow for the direct estimation of both NH2 and NCO. The recent execution of the ULYSSES Director's Discretionary program presents a unique and singular opportunity to independently constrain XCO in the Large and Small Magellanic Clouds, whose metallicities are 1/2 and 1/5 solar, respectively. These are the only two other galaxies in which UV-absorption-line measurements of both CO and H2 column densities are possible toward multiple sources. We propose to analyze the high-quality UV spectra for 43 targets behind dense H2 regions, 21 in the LMC, and 22 in the SMC, and thus increase the number of measurements of CO and H2 column densities in sub-solar metallicity environments by nearly a factor of 10. We will carefully model the continuum and blended absorption features in the tightly grouped CO bands to derive CO column densities along each line of sight, and compare them with previously-derived H2 column densities along the same lines of sight. These measurements will provide a strong constraint on XCO that does not depend on astronomical calibration factors and will serve as a new benchmark for theories and simulations of low-metallicity molecular gas. The PI, Dr. Kirill Tchernyshyov, is an expert in computational statistics and the analysis of interstellar absorption in COS, STIS, and FUSE spectra. He has developed sophisticated statistical tools for robustly measuring gas properties from absorption spectra. Dr. Julia Roman-Duval is an expert in COS and STIS data reduction and interstellar medium research, particularly dust properties and interstellar depletions (Roman-Duval+2010, 2014, 2017, 2019, 2021). She is the PI of the METAL (GO-14675, (Roman-Duval+2019, 2021) and METAL-Z (GO-15880) large HST programs to obtain depletions in the LMC, IC 1613, and Sextans-A. Dr. Jessica Werk is an expert in COS observations, COS data reduction, COS spectral analysis, and inferring column densities and physical properties from COS and other spectroscopic absorption line data. She has been a PI or Co-I on more than 20 HST/COS programs with a variety of science drivers over the last decade. Stellar Populations and the Interstellar Medium 0 Regular false false true false false SUPPORTED