Evidence for a Sharp CO Snowline Transition in a Protoplanetary Disk and Implications for Millimeter-wave Observations of CO Isotopologues

TL;DR

This study demonstrates that the vertical thickness of an isothermal layer in protoplanetary disks causes a sharp CO snowline transition detectable via millimeter-wave observations, with implications for disk mass estimates and future observations.

Contribution

We show that the vertical structure influences the CO snowline sharpness and develop a method to detect it using ALMA and ngVLA data, improving understanding of disk chemistry.

Findings

Sharp CO snowline transition detected near 80 au in HD 163296

CO column density decreases by over a factor of 20 at the snowline

Thermal structure introduces systematic uncertainties in gas mass estimates

Abstract

Observations of CO isotopologue emission from protoplanetary disks at millimeter wavelengths are a powerful tool for probing the CO snowline, an important marker for disk chemistry, and also for estimating total disk gas mass, a key quantity for planet formation. We use simple models to demonstrate that the vertical thickness of an isothermal layer around the disk midplane has important effects on the CO column density radial profile, with a thick layer producing a sharp CO snowline transition. We simulate ngVLA and ALMA images to show that this sharp change in CO column density can be detected in the derivative of the radial profile of emission from optically thin CO isotopologue lines. We apply this method to archival ALMA observations of the disk around the Herbig Ae star HD 163296 in the C$^{17}$O and C$^{18}$O J=1-0 and J=2-1 lines to identify a sharp CO snowline transition near 80 au (0.8 arcsec at 101 pc), and show the CO column density decreases by more than a factor of 20. This finding is consistent with previous inferences from the steep rise of N$_2$H$^+$ emission, which marks the location where CO depletes. We also demonstrate that the disk's thermal structure introduces a significant systematic uncertainty to estimates of total disk gas mass derived from these lines. The substantial improvement in sensitivity envisioned for the ngVLA over ALMA for observations of ground-state lines of CO isotopologues has the potential to extend this approach to a much larger population of disks.