Molecular cloud distances based on the MWISP CO survey and Gaia DR2

We present a new method of calculating distances of molecular clouds in the Galactic plane, using CO observations and the Gaia DR2 parallax and G-band extinction ($A_G$) measurements. Due to the complexity of dust environments in the Galactic plane, $A_G$ contains irregular variations, which is difficult to model. To overcome this difficulty, we propose that the $A_G$ of off-cloud stars (Gaia stars around molecular clouds) can be used as a baseline to calibrate the $A_G$ of on-cloud stars (Gaia stars toward molecular clouds), which removes the $A_G$ components that are unrelated to molecular clouds. The distance is subsequently inferred from the jump point in on-cloud $A_G$ with Bayesian analysis and Markov chain Monte Carlo (MCMC) sampling. We applied this baseline subtraction method to a 100 square degree region (209.75{\deg} $\leq$ l $\leq$ 219.75{\deg} and |b| $\leq$ 5{\deg}) in the third Galactic quadrant, which was mapped as part of the Milky Way Imaging Scroll Painting (MWISP) project, covering three CO isotopologue lines, and derived distances and masses for 11 molecular clouds, including the Maddalena molecular cloud and Sh 2-287. The results indicate that the distance of the Perseus Arm in this region is about 2.4 kpc and molecular clouds are present in the interarm regions.