Measuring the gas mass of protoplanetary disks, the reservoir available for giant planet formation, has proven to be difficult. We currently lack a far-infrared observatory capable of observing HD, and the most common gas mass tracer, CO, suffers from a poorly constrained CO-to-H2 ratio.
I will present recently published work where we show that N2H+, a chemical tracer of CO poor gas, can be used to observationally measure the CO-to-H2 ratio in disks and correct their CO-based gas masses.
To test this, we set up thermochemical models for the three disks (TW Hya, DM Tau and GM Aur) where HD 1-0 has been previously detected and used to independently measure the disk gas mass.
We show that the CO-to-H2 ratio and gas mass can be measured from N2H+ and C18O line fluxes and that these gas masses agree with values obtained from HD within their respective uncertainties. These results demonstrate the potential of using the combination of N2H+ and C18O to measure gas masses of protoplanetary disks.
I will also show how this method will be applied to measure gas masses of the well-defined sample of AGE-PRO, the ALMA survey of Gas Evolution in PROtoplanetary disks. AGE-PRO observed N2H+ and CO for 30 disks around similar spectral type stars with ages between 0.1 and 10 Myr. This ALMA Large Program that will provide a systematic survey of the evolution of gas throughout the lifetime of protoplanetary disks.