The First Detection of ¹³C¹⁷O in a Protoplanetary Disk: A Robust Tracer of Disk Gas Mass

Abstract

Measurements of the gas mass are necessary to determine the planet formation potential of protoplanetary disks. Observations of rare CO isotopologues are typically used to determine disk gas masses; however, if the line emission is optically thick this will result in an underestimated disk mass. With ALMA we have detected the rarest stable CO isotopologue, 13 C 17 O, in a protoplanetary disk for the first time. We compare our observations with the existing detections of 12 CO, 13 CO, C 18 O and C 17 O in the HD 163296 disk. Radiative transfer modelling using a previously benchmarked model, and assuming interstellar isotopic abundances, significantly underestimates the integrated intensity of the 13 C 17 O J=3-2 line. Reconciliation between the observations and the model requires a global increase in CO gas mass by a factor of 3.5. This is a factor of 2-6 larger than previous gas mass estimates using C 18 O. We find that C 18 O emission is optically thick within the snow line, while the 13 C 17 O emission is optically thin and is thus a robust tracer of the bulk disk CO gas mass.

Excess C/H in Protoplanetary Disk Gas from Icy Pebble Drift Across the CO Snowline

“…We note that there is a detection of 13 C 17 O (3-2) line in the HD 163296 disk with the match-filter method (Booth et al 2019). Although the line flux is uncertain, the detection suggests a high column of CO gas inside the CO snowline, which is consistent with our results.…”

13C17O suggests gravitational instability in the HL Tau disc

“…However, using our data and the data from the radial profile presented in (Wu et al 2018) for a radius of 100 au, we find that the ratio is only ∼ 4. This demonstrates that the C 18 O J = 2-1 emission is optically thick in the HL Tau disc, and is therefore not tracing the bulk gas mass in the molecular layer (as also found for HD 163296; Booth et al 2019).…”

“…We use the 13 C 17 O observations in order to indirectly measure the total gas mass of the disc. In our previous study of the HD 163296 disc we performed 2D radiative transfer models of multiple CO isotopologues and transitions using a well tested 2D physical structure (Booth et al 2019). Many molecular line observations toward HL Tau are significantly affected by absorption and complex velocity gradients from the surrounding cloud (see, e.g., ALMA Partnership et al 2015;Yen et al 2019), meaning a similar wealth of observational data is not readily available.…”

Rapid Evolution of Volatile CO from the Protostellar Disk Stage to the Protoplanetary Disk Stage

“…However, even C 18 O lines can be optically thick in protostellar disks (van 't Hoff et al 2018). A more robust way is to employ rarer CO isotopologue lines (e.g., C 17 O, 13 C 18 O, 13 C 17 O ) that have smaller optical depths (Zhang et al 2017;Artur de la Villarmois et al 2018;Booth et al 2019;Booth & Ilee 2020).…”