Gas Mass Tracers in Protoplanetary Disks: CO is Still the Best

Protoplanetary disk mass is a key parameter controlling the process of planetary system formation. CO molecular emission is often used as a tracer of gas mass in the disk. In this study, we consider the ability of CO to trace the gas mass over a wide range of disk structural parameters, and we search for chemical species that could possibly be used as alternative mass tracers to CO. Specifically, we apply detailed astrochemical modeling to a large set of models of protoplanetary disks around low-mass stars to select molecules with abundances correlated with the disk mass and being relatively insensitive to other disk properties. We do not consider sophisticated dust evolution models, restricting ourselves to the standard astrochemical assumption of 0.1 mu m dust. We find that CO is indeed the best molecular tracer for total gas mass, despite the fact that it is not the main carbon carrier, provided reasonable assumptions about CO abundance in the disk are used. Typically, chemical reprocessing lowers the abundance of CO by a factor of 3, compared to the case where photodissociation and freeze-out are the only ways of CO depletion. On average, only 13% C atoms reside in gas-phase CO, albeit with variations from 2% to 30%. CO2, H2O, and H2CO can potentially serve as alternative mass tracers, with the latter two only applicable if disk structural parameters are known.