  The total disk mass sets the formation potential for exoplanets.
Carbon-monoxide (CO) has been used as a gas mass tracer in T Tauri disks, but
was found to be less abundant than expected due to freeze-out and chemical
conversion of CO on the surfaces of cold dust grains. The disks around more
massive intermediate mass pre-main sequence stars called Herbig disks are
likely to be warmer, allowing for the possibility of using CO as a more
effective total gas mass tracer. Using ALMA archival data and new NOEMA data of
12CO, 13CO, and C18O transitions of 35 Herbig disks within 450 pc, the masses
are determined using the thermo-chemical code Dust And LInes (DALI). The
majority of Herbig disks for which 13CO and C18O are detected are optically
thick in both. Computing the gas mass using a simple optically thin relation
between line flux and column density results in an underestimate of the gas
mass of at least an order of magnitude compared to the masses obtained with
DALI. The inferred gas masses with DALI are consistent with a gas-to-dust ratio
of at least 100. These gas-to-dust ratios are two orders of magnitude higher
compared to those found for T Tauri disks using similar techniques, even over
multiple orders of magnitude in dust mass, illustrating the importance of
chemical conversion of CO in colder T Tauri disks. Similar high gas-to-dust
ratios are found for Herbig group I and II disks. Since group II disks have
dust masses comparable to T Tauri disks, their higher CO gas masses illustrate
the determining role of temperature. Compared to debris disks, Herbig disks
have four orders of magnitude higher gas masses. At least one Herbig disk, HD
163296, has a detected molecular disk wind, but our investigation has not
turned up other detections of the CO disk wind in spite of similar
sensitivities.
