exoALMA. XIII. Gas Masses from N2H+ and C18O: A Comparison of Measurement Techniques for Protoplanetary Gas Disk Masses

The gas masses of protoplanetary disks are important but elusive quantities. In this work we present new Atacama Large Millimeter/submillimeter Array (ALMA) observations of N2H+ (3-2) for 11 exoALMA disks. N2H+ is a molecule sensitive to CO freeze-out and has recently been shown to significantly improve the accuracy of gas masses estimated from CO line emission. We combine these new observations with archival N2H+ and CO isotopologue observations to measure gas masses for 19 disks, predominantly from the exoALMA large program. For 15 of these disks the gas mass has also been measured using gas rotation curves. We show that the CO + N2H+ line emission-based gas masses typically agree with the kinematically measured ones within a factor of 3 (∼1σ-2σ). Gas disk masses from CO + N2H+ are on average a factor of 2.3 −1.0+0.7 × lower than the kinematic disk masses, which could suggest slightly lower N2 abundances and/or lower midplane ionization rates than typically assumed. Herbig disks are found to have CO gas abundances at the level of the interstellar medium based on their CO and N2H+ fluxes, which sets them apart from T Tauri disks, where abundances are typically ∼3−30× lower. The agreement between CO + N2H+-based and kinematically measured gas masses is promising and shows that multimolecule line fluxes are a robust tool to accurately measure disk masses at least for extended disks.