Dynamical Masses of Young Stars Inferred from Two Transitions of CO with ALMA

TL;DR

This study uses ALMA CO observations to measure the masses of young stars via disk kinematics, revealing inconsistencies between different CO transitions and highlighting the need for caution in such measurements.

Contribution

It demonstrates that dynamical mass estimates from different CO transitions can be inconsistent, emphasizing the importance of multi-line analysis for accurate stellar mass determination.

Findings

Mass estimates from CO(2-1) and CO(3-2) are inconsistent at 2-4 sigma.

Systematic errors may arise from disk model assumptions or resolution limitations.

Single-transition measurements may lead to biased stellar mass estimates.

Abstract

Stellar masses are fundamental but often difficult to measure. Thanks to the Atacama Large Millimeter Array (ALMA) and $Gaia$, dynamical masses of pre-main sequence stars can be precisely measured using the Keplerian rotation of protoplanetary disks. We used ALMA CO(2-1) and CO(3-2) observations of CT Cha and DS Tau to determine their masses by modeling the geometry, kinematics, and physical properties of their disks with a Bayesian-based radiative transfer modeling code $pdspy$. We found that the posterior distributions of the masses from the two transitions are inconsistent at the 2-4 $\sigma$ level. These systematic errors may originate from assumptions in the disk model, or perhaps the modest spatial or spectral resolutions used in this study. Regardless, this indicates that dynamical mass measurements using disk kinematics should be treated with caution when using only a single transition line because of these systematic errors.