LARGE-SCALE KINEMATICS, ASTROCHEMISTRY, AND MAGNETIC FIELD STUDIES OF MASSIVE STAR-FORMING REGIONS THROUGH HC₃N, HNC, AND C₂H MAPPINGS

Abstract

We have mapped 27 massive star-forming regions associated with water masers\nusing three dense gas tracers: HC3N 10-9, HNC 1-0 and C2H 1-0. The FWHM sizes\nof HNC clumps and C2H clumps are about 1.5 and 1.6 times higher than those of\nHC3N, respectively, which can be explained by the fact that HC3N traces more\ndense gas than HNC and C2H. We found evidence for increase in optical depth of\nC2H with `radius' from center to outer regions in some targets, supporting the\nchemical model of C2H. The C2H optical depth is found to decline as molecular\nclouds evolve to later stage, suggesting that C2H might be used as "chemical\nclock" for molecular clouds. Large-scale kinematic structure of clouds was\ninvestigated with three molecular lines. All these sources show significant\nvelocity gradients. The magnitudes of gradient are found to increase towards\nthe inner region, indicating differential rotation of clouds. Both the ratio of\nrotational to gravitational energy and specific angular momentum seem to\ndecrease toward the inner region, implying obvious angular momentum transfer,\nwhich might be caused by magnetic braking. The average magnetic field strength\nand number density of molecular clouds is derived using the uniformly magnetic\nsphere model. The derived magnetic field strengths range from 3 to 88 \\mu G,\nwith a median value of 13 \\mu G. The mass-to-flux ratio of molecular cloud is\ncalculated to be much higher than critical value with derived parameters, which\nagrees well with numerical simulations.\n