Integrated spectra and moment 0 maps of LkCa 15

The transitions that constrain the gas mass and carbon abundance the most are C^17^O J=2-1, N_2_H^+^ J=3-2 and HD J=1-0. Using these three molecules, we find that the gas mass in the LkCa 15 disk is Mg=0.01^+0.01^_-0.004_M_{sun}_, which is a factor of 6 lower than previous estimations. This value is consistent with cosmic ray ionization rates between 10^-16^-10^-18^s^-1^, where 10^-18^s^-1^ is a lower limit based on the HD upper limit.

The carbon abundance is C/H=(3+/-1.5)x10^-5^, implying a moderate depletion of elemental carbon by a factor of 3-9. All other analyzed transitions also agree with these numbers, within a modeling uncertainty of a factor of 2. Using the resolved C_2_H image we find a C/O ratio of ~1, which is consistent with literature values of H_2_O depletion in this disk.

The absence of severe carbon depletion in the LkCa 15 disk is consistent with the young age of the disk, but stands in contrast to the higher levels of depletion seen in older cold transition disks. Combining optically thin CO isotopologue lines with N_2_H+ is promising with regard to breaking the degeneracy between gas mass and CO abundance. The moderate level of depletion for this source with a cold, but young disk, suggests that long carbon transformation timescales contribute to the evolutionary trend seen in the level of carbon depletion among disk populations, rather than evolving temperature effects and presence of dust traps alone.

HD observations remain important for determining the disk's gas mass.