Temperature, Mass and Turbulence: A Spatially Resolved Multi-Band Non-LTE Analysis of CS in TW~Hya

TL;DR

This study uses high-resolution multi-band observations of CS in TW Hya to derive detailed temperature, density, and turbulence profiles, providing new insights into the disk's physical conditions and mass distribution.

Contribution

It presents the first spatially resolved multi-transition analysis of CS in TW Hya, enabling precise constraints on temperature, turbulence, and disk mass in the outer regions.

Findings

Turbulent velocities are constrained to be less than 0.1 times the sound speed.

A minimum disk mass of 3 x 10^-4 solar masses is established.

The temperature and density profiles extend out to 230 au, beyond previous measurements.

Abstract

Observations of multiple rotational transitions from a single molecule allow for unparalleled constraints on the physical conditions of the emitting region. We present an analysis of CS in TW~Hya using the $J=7-6$, $5-4$ and $3-2$ transitions imaged at $\sim 0.5^{\prime\prime}$ spatial resolution, resulting in a temperature and column density profile of the CS emission region extending out to 230~au, far beyond previous measurements. In addition, the 15~kHz resolution of the observations and the ability to directly estimate the temperature of the CS emitting gas, allow for one of the most sensitive searches for turbulent broadening in a disk to date. Limits of $v_{\rm turb} \lesssim 0.1 c_s$ can be placed across the entire radius of the disk. We are able to place strict limits of the local H$_2$ density due to the collisional excitations of the observed transitions. From these we find that a minimum disk mass of $3 \times 10^{-4}~M_{\rm sun}$ is required to be consistent with the CS excitation conditions and can uniquely constrain the gas surface density profile in the outer disk.