| Molecular gas plays a crucial role in the evolution and formation of stars.It is within the giant molecular clouds that interstellar gas is cycled into the next generation of stars,and the most massive of these young stars produce a major part of the galactic luminosity.In addition,the dense interstellar medium,as it is highly dissipative,probably plays a fundamental part in determining the outcome of galactic interactions.The studies of molecules in galaxies include both detailed analyses of nearby galaxies and comparisons of the global properties of selected samples of galaxies.These two approaches are complementary,and both are necessary to improve our understanding of the large-scale processes that govern star formation and molecular cloud evolution.Furthermore,the systematic study of star formation regions by the lines of molecules with high dipole moments provides important information about the physical properties of the dense core of the newborn stars and helps reveal the general properties of the sample under study.In recent years,scientists have used molecular lines such as HCO+,NH3,HCN,and CS to trace samples of numerous sources in the Milky Way,and these observations have yielded important information about temperatures,masses,mean densities and velocity dispersions in the cores studied.It is well known,however,that there is a significant difference between mean densities and densities derived from multiline excitation analysis,which are 1-2 orders of magnitude higher in high-mass star forming regions.This fact indicates that emitting regions in many of these objects have inhomogeneous structure.To investigate further the density structure of the cores,one needs to perform multiline observations and analysis in lines of various density tracers.For that purpose,the HC3N molecule has been chosen.Due to its high dipole moment(3.6 D),the HC3N molecule is an indicator of denser gas than many other high density tracers,including CS(2-1).Comparing with other molecules,HC3N has important advantages,namely,large number of rotational transitions in the millimeter band and low optical depth of these transitions.On the other hand,HC3N molecule might be destroyed by ultraviolet(UV)photons from central massive young stars and thus it was thought that HC3N lines trace young star formation in galaxies.Observations of several selected massive star forming regions show than HC3N lines are rather strong and can be readily detected with modern single dish telescopes.We have made a multiple HC3N(J=17-16,J=11-10,J=8-7,J=24-23)survey toward a sample of massive star forming regions with the ARO 12m telescope and the CSO 10.4m telescope,which detected HC3N emission in above 90%sources.With different transitions of HC3N,we can see the high transition lines have broader line width than the lower transition in some sources.This might give us some spectral broadening mechanisms about the star forming regions.Suchdifference may also give us the feedback information of young stars to the inner dense hot molecular gas region.But since the observing efficiency is pretty low,single dish observations of HC3N high J transition lines are necessary to study the different velocity profile of different HC3N transition,which traces gas with different volume density,and also the feedback of young star to the surrounding gas.These are 19 sources that were observed with both two telescopes,and these are 18 sources that detected HC3N emission lines.The lines of J=24-23 and J=8-7 were not detected in the source of W44,but the other two lines were detected in the W44.Of course,not only the line of HC3N was detected but also many other molecular emission lines were detected with the CSO telescope,which also provides us with a large number of data for our systematic researches on the star formation process in the Milky Way.Meanwhile,we also compare the fluxes of these several emission lines,and we can obtain some excitation information of molecular lines in these sources,such as some information of the excitation temperature,to further understand some of the physical conditions in the process of star formation. |
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