High resolution observations of molecular emission lines toward the CI Tau proto-planetary disc: planet-carved gaps or shadowing?

TL;DR

This study uses high-resolution ALMA observations of the CI Tau protoplanetary disc to investigate gas emission features and their relation to dust gaps, revealing effects of shadowing and chemical variations, and highlighting challenges in interpreting gas line data.

Contribution

The paper provides the first high-resolution gas emission line analysis of CI Tau's disc, demonstrating the impact of shadowing and chemical effects on observed gas structures, and discusses the complexities in deriving gas surface density.

Findings

The second continuum ring corresponds to a gas emission gap, likely caused by shadowing.

CS emission forms a ring due to chemical effects, not just density variations.

Foregound absorption prevents morphological analysis using $^{12}$CO lines.

Abstract

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.