High resolution observations of molecular emission lines toward the CI
  Tau proto-planetary disc: planet-carved gaps or shadowing?
  Recent observations have revealed that most proto-planetary discs show a
pattern of bright rings and dark gaps. However, most of the high-resolution
observations have focused only on the continuum emission. In this Paper we
present high-resolution ALMA band 7 (0.89mm) observations of the disc around
the star CI Tau in the $^{12}$CO & $^{13}$CO $J=3$-2 and CS $J=7$-6 emission
lines. Our recent work demonstrated that the disc around CI Tau contains three
gaps and rings in continuum emission, and we look for their counterparts in the
gas emission. While we find no counterpart of the third gap and ring in
$^{13}$CO, the disc has a gap in emission at the location of the second
continuum ring (rather than gap). We demonstrate that this is mostly an
artefact of the continuum subtraction, although a residual gap still remains
after accounting for this effect. Through radiative transfer modelling we
propose this is due to the inner disc shadowing the outer parts of the disc and
making them colder. This raises a note of caution in mapping high-resolution
gas emission lines observations to the gas surface density - while possible,
this needs to be done carefully. In contrast to $^{13}$CO, CS emission shows
instead a ring morphology, most likely due to chemical effects. Finally, we
note that $^{12}$CO is heavily absorbed by the foreground preventing any
morphological study using this line.
