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

Abstract

Recent observations show that the CO gas abundance, relative to H 2 , in many 1-10 Myr old protoplanetary disks may be heavily depleted, by a factor of 10-100 compared to the canonical interstellar medium value of 10 −4 . When and how this depletion happens can significantly affect compositions of planetesimals and atmospheres of giant planets. It is therefore important to constrain if the depletion occurs already at the earliest protostellar disk stage. Here we present spatially resolved observations of C 18 O, C 17 O, and 13 C 18 O J=2-1 lines in three protostellar disks. We show that the C 18 O line emits from both the disk and the inner envelope, while C 17 O and 13 C 18 O lines are consistent with a disk origin. The line ratios indicate that both C 18 O and C 17 O lines are optically thick in the disk region, and only 13 C 18 O line is optically thin. The line profiles of the 13 C 18 O emissions are best reproduced by Keplerian gaseous disks at similar sizes as their mm-continuum emissions, suggesting small radial separations between the gas and mm-sized grains in these disks, in contrast to the large separation commonly seen in protoplanetary disks. Assuming a gas-to-dust ratio of 100, we find that the CO gas abundance in these protostellar disks are consistent with the ISM abundance within a factor of 2, nearly one order of magnitude higher than the average value of 1-10 Myr old disks. These results suggest that there is a fast, ∼1 Myr, evolution of the abundance of CO gas from the protostellar disk stage to the protoplanetary disk stage.

An Evolutionary Study of Volatile Chemistry in Protoplanetary Disks

“…It is likely that these sources are depleted in CO by about an order of magnitude compared to ISM levels. We note that Zhang et al (2020) recently found evidence for ISM-like CO abundances in three protostellar disks, which may signify that a range of depletion timescales and outcomes are possible. Indeed, chemical models by Drozdovskaya et al (2014) demonstrate that differences in infall physics can strongly influence the chemical processing that occurs in embedded disks.…”

“…C 18 O emission has been found to be optically thick in several embedded disks (van 't Hoff et al 2018;Zhang et al 2020). The C 18 O 2-1 optical depths in the embedded disks in Zhang et al (2020) are likely around ∼4-8, based on the reported C 18 O and 13 C 18 O fluxes and assuming a 12 C/ 13 C ratio around 70. We therefore expect that moderate optical depth effects may affect the protostellar sources in our sample.…”

“…Observations of a rarer isotopologue would help to isolate emission from the inner disk-forming region (e.g. Zhang et al 2020).…”

“…Indeed, chemical models by Drozdovskaya et al (2014) demonstrate that differences in infall physics can strongly influence the chemical processing that occurs in embedded disks. The sources in Zhang et al (2020) are among the most massive protostellar disks known, and their higher CO abundances may reflect a different physical/chemical environment compared to the Serpens sources. Still, the results from this work and Zhang et al (2020) both indicate that CO depletion is a rapid process taking place on a ∼0.5-1 Myr timescale.…”

“…Anderl et al (2016) see evidence for ∼1 order of magnitude CO depletion in a sample of Class 0 envelopes, and Harsono et al (2020) find low water abundances in embedded Class I disks. Meanwhile, van 't Hoff et al (2018) and Zhang et al (2020) find evidence for an ISM CO abundance in the embedded disks L1527, TMC 1A, HL Tau, and DG Tau. Additional observations of embedded sources, particularly those that resolve the protostellar core or disk from the envelope, are needed to provide insight into the chemistry and depletion of volatile molecules in protostellar disks.…”