  We present molecular line observations of 13CO and C18O J=3-2, CN N = 3 - 2,
and CS J=7-6 lines in the protoplanetary disk around TW Hya at a high spatial
resolution of ~9 au (angular resolution of 0.15''), using the Atacama Large
Millimeter/Submillimeter Array. A possible gas gap is found in the deprojected
radial intensity profile of the integrated C18O line around a disk radius of
~58 au, slightly beyond the location of the au-scale dust clump at ~52 au,
which resembles predictions from hydrodynamic simulations of planet-disk
interaction. In addition, we construct models for the physical and chemical
structure of the TW Hya disk, taking account of the dust surface density
profile obtained from high spatial resolution dust continuum observations. As a
result, the observed flat radial profile of the CN line intensities is
reproduced due to a high dust-to-gas surface density ratio inside ~20 au.
Meanwhile, the CO isotopologue line intensities trace high temperature gas and
increase rapidly inside a disk radius of ~30 au. A model with either CO gas
depletion or depletion of gas-phase oxygen elemental abundance is required to
reproduce the relatively weak CO isotopologue line intensities observed in the
outer disk, consistent with previous atomic and molecular line observations
towards the TW Hya disk. {Further observations of line emission of
carbon-bearing species, such as atomic carbon and HCN, with high spatial
resolution would help to better constrain the distribution of elemental carbon
abundance in the disk gas.
