A Study Of Methanol Masers And Large-scale Star Formation In Galaxies

Although there is some understanding of the formation of massive stars,the detailed process of their formation remains to be explored.The research of galactic spiral arms using massive star-forming regions is also one of the hottest areas in modern astronomy.In addition,the search for methanol maser in external galaxies has become increasingly popular in recent years.In this paper,we use methanol maser,a tracer of massive young stars,as the research method,and use the latest 6.7 GHz methanol maser catalogue to carry out a large sample statistical study to explore the distribution characteristics of methanol maser and its related massive star forming regions in the Milky Way on a large scale.In addition,we observed the 36.2 GHz class I and 6.7GHz class II methanol transitions in the nearby starburst galaxy Maffei 2 with VLA to obtain the excitation environment of class I and class II methanol masers in the external galaxy and their correlation with the galactic large-scale environment.In chapter 1,we explain the importance of studying star formation,especially for massive stars,and introduce existing models of massive star formation and the difficulties in studying them.Next,the maser,which can trace the process of mas-sive star formation,with emphasis on methanol maser,and the ammonia molecule,a probe that traces the physical environment of the massive star formation region,are introduced.Then,the structure of the Milky Way and two common methods for measuring distances of the Galactic objects are summarized,and the Be SSe L project is introduced.Finally,the research status of methanol maser in external galaxies is summarized.In chapter 2,we present the latest and most complete catalogue of 6.7 GHz class II methanol masers,and obtained the more accurate kinematics distance through the latest Galactic kinematic model provided by the international Be SSe L project,then analyzed the distribution of these sources on the Galactic plane and their luminosity.The distribution of 6.7 GHz methanol maser shows that massive star-forming regions are generally evenly distributed across the Milky Way,but analysis shows that the closer we are to the solar system,the more dense the maser is,suggesting that the detections of 6.7 GHz methanol maser are influenced by observational effects.After removing some of the distant sources that are heavily influenced by observational effects,we find some valuable results:(1)the number of 6.7 GHz methanol masers on the Perseus arm is less than half that of the other three arms,suggesting that the Perseus arm may not be a spiral dominated by massive stars.(2)The luminosity of 6.7 GHz methanol maser follows log-normal distribution,which indicates that the existing 6.7 GHz methanol maser source catalogue in the Milky Way galaxy is complete in some certain sense.(3)The smaller the radius of the Milky Way,the greater the luminosity,indicating that star formation in the inner Milky Way is more active and more abundant than in the outer Milky Way.In chapter 3,we describe the observation of NH₃(1,1),NH₃(2,2)and NH₃(3,3)in 216 ammonia sources with Shanghai Tianma 65m Telescope.The samples were selected from a number of observational papers on ammonia molecular.The number of sources with NH₃(1,1),NH₃(2,2)and NH₃(3,3)emission was 122,122 and 93 respec-tively.We obtain the temperature and density distribution characteristics of ammonia molecular clouds on large scale in the Milky Way.The results are as follows:(1)we find that the column density of the Perseus arm is low,which reflects that the massive star formation activity on the Perseus arm may be less active,which is consistent with the statistical results of 6.7 GHz methanol maser in Chapter 2.(2)The absence of a clear trend in the excitation temperature of ammonia with the galactocentric distance indicates that the excitation temperature is not related to the overall structure of the Galaxy,but to the small-scale environment of the molecular cloud.(3)The column density of ammonia decreases with increasing galactocentric distance,again reflecting the fact that matter is denser in the inner galaxy than in the outer galaxy,and that massive star-forming regions are more likely to be found at the head of the spiral arms.(4)We compared the ammonia column density on the spiral arms with that between the spiral arms and found that the latter is much lower,suggesting that the region on the spiral arms is more prone to massive star birth.In chapter 4,we describe the detections of the 36.2 GHz class I and 6.7 GHz class II methanol transitions in the nearby starburst galaxy Maffei 2 by using VLA.The observations show that the class I methanol masers in Maffei 2 locates a few hundred pc away from the galaxy center,possibly associated with the edge of the galaxy bar in Maffei 2.Narrow spectral line profiles with only a few km s^-1indicate that the detected emission are masers,of which the 36.2 GHz methanol maser was identified as the first detected class I methanol megamasers due to its high luminosity.The 36.2GHz methanol maser was found to be distributed in the gas flows in the galaxy bar region,suggesting that the class I methanol maser is related to the large scale shock generated by the rotation of the galaxy bar.In addition,6.7 GHz class II methanol maser has been tentatively detected,and its luminosity is similar to that of the 6.7GHz methanol maser in the Milky Way,suggesting that the 6.7 GHz methanol maser is only produced in small-scale star forming regions.Chapter 5 is a summary of the above three works on the properties of star for-mation at the galactic scale by using methanol masers or ammonia molecular,and prospects for a complete search of 6.7 GHz methanol masers in the Milky Way and further detection of class I and II methanol masers outside the Milky Way.