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 deg ^ { 2 } region ( 209.75 { { } ^ { \circ } } \leq l \leq 219.75 { { } ^ { \circ } } and |b| \leq 5 { { } ^ { \circ } } ) 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 .