Searching For Open Clusters In Gaia DR2 And Study Of Galactic Spiral Structure

This paper will introduce three aspects:the search for open clusters in Gaia DR2 astrometric data,and the study of the spiral arm structure of the Milky Way using open clusters,and using stellar parallax and extinction data to study the distribution of interstellar dust.We used an unsupervised machine learning algorithm D BSCA N to cluster the stellar astrometric data.The difficulty of this clustering algorithm lies in the selection of minimum radius parameters.On the basis of the previous methods,we present a set of original clustering parameter acquisition method:kthNND fitting bimodal Gaussian function intersection,and we use this method to blindly search for open clusters in 170 million stars in the galactic plane |b|<20°.Finally,in addition to 2080 known clusters,986 new clustered groups are found,and 74 of them are most probable candidates with relatively large members.In this work,we present the position,apparent radius,parallax,proper motion and member stars information of these candidates.Meanwhile,in order to obtain the physical parameters of each candidate,the isochrone was fitted using its photometric data.The results show that the apparent radii and the observed proper motion dis-persion values of these new candidates are consistent with those of open clusters previously identified in Gaia DR2.It is the first time that we found open clusters present the age pattern across spiral arms,this is one of the key observational feature to study the dynamic nature of Galaxy spiral structure.With the most updated samples of high-mass star formation region(HMSFR)masers,O stars,and open clusters,we studied their distributions and kinematic properties in the vicinity of the Sun.We found that the Sagittarius-Carina Arm traced by HMSFR masers,O stars(? 10 Myr)and young open clusters(<30 Myr)seem to deviate gradually towards the anti-Galactic Center(GC)direction.The Local Arm traced by HMSFRs,O stars,young clusters and also medium-young clusters(30-100 Myr)are inclined to gradually deviate toward the GC direction.The properties for the Local Arm are supported by a simplified simulation of cluster motions in the Galaxy.Indications of systematic motions in the circular and radial velocities are noticed for the old open clusters(>200 Myr).These results are consistent with the idea that star formation can be triggered by spiral shocks of density waves,also indicate that the corotation radius of the Galaxy is located between the Sagittarius-Carina Arm and the Local Arm,close to the Solar circle.Gaia DR2 provides 88 million samples with measurements of the line-of-sight G band extinction,it gives us an opportunity to measure the precise distance and in-site extinction of the dust clouds,and also meaningful to determine their associated interstellar mediums.We made a census with a beam size of 30' to the whole sky,and got 1.03 million extinction spectra.Due to the training grid boundaries in machine learning for extinction,most of the clouds found in this work have an line-in-sight extinction saturation of AG<2.5-3 mag,and located within 2 kpc.Those dust clouds often associated with molecular clouds,it could be seen on 12CO radiatiobs.We can see the integrated extinction distribution of the dust clouds at Perseus,Local and Sagittarius spiral arms,as well as at the inter-arm regions.One can use the spectra to measure the distances to any chosen areas.In this work,we also found>20 masers which located at high mass star formation regions within AG saturation are associated with the dense dust clouds,the results of those distance calculation are comparable to VLBA/VERA maser trigonometric measurements.