The Three Dimensional Distribution Of The Interstellar Dust In The Milky Way And Distance Measurements Of Molecular Clouds

The interstellar dust grains widely distributed in the Milky Way,especially in the Galactic disk,absorb and scatter light in the ultraviolet,visible and near-infrared bands,producing the extinction and reddening effects.The latter pose a serious obstacle for the study of the structure and the stellar populations of the Milky Way galaxy.In order to obtain the intrinsic luminosities and colors of the celestial objects,one needs to correct for the dust effects of extinction and reddening.Extinction maps are useful tools for this purpose.In addition,from the deduced extinction maps,one can further explore the distribution,structure and properties of the interstellar dust grains in the Milky Way,and trace the distribution of the Galactic molecular clouds,accurately measure their distances and study their physical properties.In the current thesis,firstly,based on the multi-band photometric data from the SkyMapper Southern Survey(SMSS)DR1,and from the 2MASS,WISE and Gaia surveys,we have estimated the extinction values of over 19 million stars in the Milky Way.Together with the distances calculated from the Gaia Data Release 2(DR2)parallaxes,we have constructed a 3D extinction map of the southern sky.The map cover the SMSS survey area of～14,000 deg 2 and has an angular resolution of 6.9 arcmin for Galactic latitude-10°<b<10°,13.7 arcmin for-30°<b<-10°,and 27.5 arcmin for b<-30°.By combining our 3D extinction map with those from the literature,we present an all-sky 3D extinction map,and use it to explore the 3D distribution of the Galactic dust grains.We use two different models,one consisting of a single disk and another of two disks,to fit the 3D distribution of the Galactic dust grains.The data is better fitted by the two-disk model,yielding a smaller value of the Bayesian Information Criterion which is used to select the best model.The best-fit model has scale heights of 73 and 225 pc for the "thin"and "thick" dust disks,respectively.One can accurately measure the distance of molecular cloud by using the dust grains as tracers.Accurate determination of the distances to the Planck Galactic cold clumps(PGCCs)are crucial for the measurements of their physical parameters and for the study of their Galactic distribution.Based on two large catalogues of stars with robust distance and reddening estimates from the literature,we have estimated accurate distances to 61 PGCCs in the second Galactic quadrant.For this purpose,we have selected stars along the sightlines overlapping with the cores of the sample clumps and fitted the reddening profiles with a simple reddening model.The typical uncertainties of the resultant distances of these PGCCs are less than 8 per cent.The new estimates differ significantly from the kinematic values,well known to suffer from large errors.With the new distances,we have updated the physical properties including the radii,masses and virial parameters of the cores of those PGCCs.In addition,based on the three dimensional dust extinction map and the best-fitting extinction and distance information of over 17 million stars,we have identified 250 dust/molecular clouds in the Southern sky using a hierarchical structure identification algorithm.Amongst those 71 clouds are located at high Galactic latitudes(b<-10°).We have estimated the distances to the clouds by fitting the extinction versus distance profiles of stars along the lines of sight overlapping with the clouds,using a simple Gaussian dust distribution model.The typical uncertainties of the resultant distances are less than 7 percent.We also provide the measurements of physical properties of the individual clouds,including the linear radius,mass and surface mass density.Finally,based on the previously identified molecular clouds,the CO gas observation and the large Young Stellar Object(YSO)catalogue,we have explored the kinematic correlation between the YSOs and their associated molecular clouds.We have selected 27 giant molecular clouds in the Galactic disk,including 1093 YSOs.We have collected their associated YSOs,which have spectra observation from Apache Point Observatory Galactic Evolution Experiment(APOGEE),Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST),Gaia Early Data Release 3(Gaia EDR3)and the Xinglong 2.16 m and the Lijiang 2.4 m telescopes.We obtained the radial velocities of the individual YSOs and calculated the average velocities of these YSOs in the individual molecular clouds.On the other hand,we have obtained the radial velocities of the molecular clouds from the observation of the CO gas.We find that the average velocities of the YSOs are in good agreement with the velocities from the observation of their associated molecular clouds.Our results will be useful for the further study of the kinematic properties of molecular clouds.