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We present ALMA Cycle~2 observations at $0.5^{\prime\prime}$ resolution of TWHya of CS $J=5-4$ emission. The radial profile of the integrated line emissiondisplays oscillatory features outwards of $1.5^{\prime\prime}$ ($\approx 90$au). A dip-like feature at $1.6^{\prime\prime}$ is coincident in location,depth and width with features observed in dust scattered light at near-infraredwavelengths. Using a thermochemical model indicative of TW Hya, gas-grainchemical modelling and non-LTE radiative transfer, we demonstrate that such afeature can be reproduced with a surface density depression, consistent withthe modelling performed for scattered light observations of TW Hya. We furtherdemonstrate that a gap in the dust distribution and dust opacity only cannotreproduce the observed CS feature. The outer enhancement at$3.1^{\prime\prime}$ is identified as a region of intensified desorption due toenhanced penetration of the interstellar FUV radiation at the exponential edgeof the disk surface density, which intensifies the photochemical processing ofgas and ices.

A Surface Density Perturbation in the TW Hydrae Disk at 95 au Traced by Molecular Emission