2018ApJ...864..133T

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 ∼0.''5 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_turb <= 0.1c_s can be placed across the entire radius of the disk. We are able to place strict limits of the local H₂ density due to the collisional excitations of the observed transitions. From these we find that a minimum disk mass of 3 x 10^–4 M_Sun is required to be consistent with the CS excitation conditions and can uniquely constrain the gas surface density profile in the outer disk.