Spectral Line Observations Of Massive Star Forming Regions And Circumstellar SiO Maser

In this dissertation, we report spectral line observations of CO molecularof massive star forming regions and circumstellar SiO masers, using the 13.7 mmillimeter wave telescope at Qinghai station of Purple Mountain Observatory,China and Very Long Baseline Array (VLBA) of National Radio AstronomyObservatory, USA, respectively.The study of massive star formation is very active at present. The under-standings of forming processes of massive stars are not clear relative to that oflow mass star formation. Now there are two models regarding the massive starformation: infall and accretion model as does the low-mass star formation, andthe collision or merging of low- or intermediate-mass stars model. If massive starscould form via accretion, their forming processes could be separated into follow-ing different phases: giant molecular clouds→prestellar cores→hot cores→UC Hâ…¡regions. Therefore, a direct comparison of the physical properties of molecularcores and outflows at different phases would be helpful to better understand theevolution of high-mass star formation. The ¹²CO (J=1-0), ¹³CO (J=1-0) andC¹⁸O (J=1-0) emissions in 9 massive star forming regions, which are believed tobe at different stages of massive star formation, were mapped with the 13.7 mmillimeter wave telescope at Qinghai Station of Purple Mountain Observatory.Of the observed 9 sources, ¹³CO cores were detected in seven of them, and C¹⁸Ocores in five of them. And only two sources associated with C¹⁸O cores and H₂Omasers showed the extended structures and strong outflows. This is the firstdetection of outflow associated with IRAS 22566+5828 in the observing filed ofS152/S153. The physical parameters of cores and outflows for these sources,derived from Local Thermal Equilibrium (LTE) analysis, are presented. Theseresults suggest that the C¹⁸O cores will appear only when the gas density is highenough, and the probability to have an outflow is very high when the clumpsshow the C¹⁸O core and H₂O maser simultaneously.As an extreme nonthermal equilibrium phenomena, astronomical masers provide the best tool for us to study physical and dynamical conditions for somepeculiar astronomical circumstance, especially for the smaller scale circumstance.Spectral line very long baseline interferometry (VLBI) technology being widelyapplied in the observations of astronomical masers offers a powerful means tostudy the spatial distributions, apparent size and kinematics of masers. We givean introduction to the newest research progress and its application in astro-physics, and summarize the fundamental theory as well as the data reduction ofthe spectral line VLBI in this dissertation.The 43 GHzÏ…=1, J=1-0 SiO maser emission toward M-type semi-regularvariable star VX Sagittarii (VX Sgr) was observed at 3 epochs during 1999 April-May using the VLBA. Our observations confirmed a persistent ringlike structureof SiO masers with a projected radius of about 3 stellar radii (R_*), which isconsistent with the typical 2-4 R, for late-type stars. Moreover, by comparingwith the previous VLBI observations, we find that the overall morphology haschanged significantly with the majority of masers appearing in North-East (NE)of the star in 1999, compared to that lying to the South-West (SW) directionin 1992 and 1994, suggesting that the direction of the major mass loss has alsochanged from the SW to NE over～5-7 years from 1992/1994 to 1999. The two-point correlation function of spots shows the maser spots are strongly clusteredon scales of 0.03-0.25 mas and 0.5-20 mas. The break of the power-law at 0.25 massuggests an angular diameter of 0.5 mas for clustering of spots to make a feature.We also find that the apparent size of maser features in 1999 is distinctly smallerthan that observed in 1992, by comparing their fractions of total power imaged.This may be related to stellar activity that caused a large SiO flare during ourobservations. Analysis of "pairwise separation" of 42 matched features appearingin all the three epochs suggests that the maser shell contracts toward VX Sgrwith a velocity of about 4 km s^-1 at a distance of 1.7 kpc to VX Sgr. Such avelocity is on the order of the sound speed, and can be easily explained by thegravitational infall of material from the circumstellar dust shell. We estimate adistance to VX Sgr of 1.58±0.15 kpc with statistical parallax analysis for SiOmasers proper motions, which is consistent with that based on proper motionsof H₂O masers, at which VX Sgr is indeed a red supergiant.