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Determining how galactic environment, especially the high gas densities andcomplex dynamics in bar-fed galaxy centers, alters the star formationefficiency (SFE) of molecular gas is critical to understanding galaxyevolution. However, these same physical or dynamical effects also alter theemissivity properties of CO, leading to variations in the CO-to-H$_2$conversion factor ($\alpha_\rm{CO}$) that impact the assessment of the gascolumn densities and thus of the SFE. To address such issues, we investigatethe dependence of $\alpha_\rm{CO}$ on local CO velocity dispersion at 150-pcscales using a new set of dust-based $\alpha_\rm{CO}$ measurements, and proposea new $\alpha_\rm{CO}$ prescription that accounts for CO emissivity variationsacross galaxies. Based on this prescription, we estimate the SFE in a sample of65 galaxies from the PHANGS-ALMA survey. We find increasing SFE towards highsurface density regions like galaxy centers, while using a constant ormetallicity-based $\alpha_\rm{CO}$ results in a more homogeneous SFE throughoutthe centers and disks. Our prescription further reveals a mean molecular gasdepletion time of 700 Myr in the centers of barred galaxies, which is overall3-4 times shorter than in non-barred galaxy centers or the disks. Across thegalaxy disks, the depletion time is consistently around 2-3 Gyr regardless ofthe choice of $\alpha_\rm{CO}$ prescription. All together, our results suggestthat the high level of star formation activity in barred centers is not simplydue to an increased amount of molecular gas but also an enhanced SFE comparedto non-barred centers or disk regions.

Star Formation Efficiency in Nearby Galaxies Revealed with a New CO-to-H2 Conversion Factor Prescription