2025AJ....170..176T

How does molecular cloud chemistry change with metallicity? In this work, we study the relation between molecular hydrogen (H₂) and carbon monoxide (CO) at 1/2 and 1/5 solar metallicity using ultraviolet absorption spectroscopy obtained as part of the UV Legacy Library of Young Stars as Essential Standards Hubble Space Telescope program. We determine CO column densities or upper limits for a sample of 50 lines of sight through the Large and Small Magellanic Clouds (LMC and SMC). ¹²CO is detected along eight lines of sight and ¹³CO is detected along two. Combining our new CO column densities with ${N}_{{{\rm{H}}}_{2}}$ measurements from the literature, we find that the evolution of ${N}_{{\rm{CO}}}\,({N}_{{{\rm{H}}}_{2}})$ from the Milky Way to the LMC and SMC is a relatively shallow function of metallicity. Taking N_CO > 3 × 10¹⁵ cm^–2 as a threshold value above which CO emission is likely to be detectable at the distance of the Magellanic Clouds, the ${{\rm{log}}}_{10}\,{N}_{{{\rm{H}}}_{2}}$ at which a sightline has a 50% probability of having N_CO above this threshold is 20.8 in the Milky Way, 20.9 in the LMC, and 21.1 in the SMC. This is an 0.3 dex change in threshold ${{\rm{log}}}_{10}{N}_{{{\rm{H}}}_{2}}$ over an 0.7 dex change in metallicity. We compare our measurements with ${N}_{{\rm{CO}}}\,({N}_{{{\rm{H}}}_{2}})$ relations from literature chemical models and find that the measured relations agree best with models in which the dynamical timescale is longer than the chemical timescale for H₂ but shorter than the chemical timescale for CO.