Mapping the Complex Kinematic Substructure in the TW Hya Disk

We present ALMA observations of CO J = 2 - 1 and CS J = 5 - 4 emission from the disk around TW Hydrae. Both molecules trace a predominantly Keplerian velocity structure, although a slowing of the rotation velocity is detected at the outer edge of the disk beyond approximate to 140 au in CO emission. This was attributed to the enhanced pressure support from the gas density taper near the outer edge of the disk. Subtraction of an azimuthally symmetric background velocity structure reveals localized deviations in the gas kinematics traced by each of the molecules. Both CO and CS exhibit a Doppler flip feature, centered nearly along the minor axis of the disk (PA similar to 60 degrees) at a radius of 1.'' 35, coinciding with the large gap observed in scattered light and mm continuum. In addition, the CO emission, both through changes in intensity and its kinematics, traces a tightly wound spiral, previously seen with higher-frequency CO J = 3 - 2 observations. Through comparison with linear models of the spiral wakes generated by embedded planets, we interpret these features in the context of interactions with a Saturn-mass planet within the gap at a position angle of PA = 60 degrees, consistent with the theoretical predictions of Mentiplay et al. The lack of a corresponding spiral in the CS emission is attributed to the strong vertical dependence on the buoyancy spirals, which are believed to only grow in the atmospheric of the disk, rather than those traced by CS emission.

Automated summary

We present ALMA observations of the CO J = 2 - 1 and CS J = 5 - 4 emission from the disk around TW Hydrae, which shows a predominantly Keplerian velocity structure. The rotation velocity slows down at the outer edge of the disk due to enhanced pressure support from the gas density taper. Localized deviations in the gas kinematics traced by CO and CS are revealed, and both molecules exhibit a Doppler flip feature centered along the minor axis of the disk. The features are interpreted as interactions with a Saturn-mass planet within the observed gap, consistent with theoretical predictions.