2017ApJ...835..228T

We present ALMA Cycle 2 observations at 0.''5 resolution of TW Hya of CS J=5-4 emission. The radial profile of the integrated line emission displays oscillatory features outward of 1.''5 (~90 au). A dip-like feature at 1.''6 is coincident in location, depth, and width with features observed in dust scattered light at near-infrared wavelengths. Using a thermochemical model indicative of TW Hya, gas-grain chemical modeling, and non-LTE radiative transfer, we demonstrate that such a feature can be reproduced with a surface density depression, consistent with the modeling performed for scattered-light observations of TW Hya. We further demonstrate that a gap in the dust distribution and dust opacity only cannot reproduce the observed CS feature. The outer enhancement at 3.''1 is identified as a region of intensified desorption due to enhanced penetration of the interstellar far-UV radiation at the exponential edge of the disk surface density, which intensifies the photochemical processing of gas and ices.