Molecular Observational Study Of Carbon Chains In Massive Star-forming Regions

Cyanopolyynes(HC2n+1N,n=1,2,3…)molecules are the most typical class of carbon-chain molecules in space.Due to their nitrile bonds,these molecules are considered as precursors to the prebiotic synthesis of simple amino acids and play an important role in the formation of PAH.Therefore,it is essential to study the physical and chemical properties of cyanopolyynes for understanding the origin of life.To investigate the evolution of HC3N in high-mass star-forming regions(HMSFRs)and its relation with shock tracers,and to study the existence of HC5N and HC7N in HMSFRs with a formed protostar,we carried out a cyanopolyynes line survey toward a large sample of HMSFRs by using the Shanghai Tian Ma 65m Radio Telescope(TMRT).Our sample consisted of 123 targets taken from the TMRT C band line survey.It included three kinds of sources,namely the sources with detection of the 6.7 GHz CH3OH maser alone,with detection of radio recombination line alone and with detection of both of them(hereafter Maser-only,RRL-only and Maser-RRL sources,respectively).We detected HC3N in 38 sources,HC5N in 11 ones and HC7N in G24.790+0.084 with the highest detection rate in Maser-RRL sources and a very low detection rate in RRL-only ones.Our detections support the idea that unsaturated complex organic molecules can exist in HMSFRs with a formed protostar.Since a transition line of HC3N cannot be used to calculate its rotational temperature by the rotation diagram method,the column densities of HC3N were derived with the rotational temperature Trot measured from the NH3 line instead of the Trot of HC3N.The mean column density of HC3N was found to be(1.75±0.42)×1013,(2.84±0.47)×1013 and(0.82±0.15)×1013 cm-2 for Maser-only,Maser-RRL and RRL-only sources,respectively.Based on a fit of the far-infrared spectral energy distributions(SED,obtained from the Herschel data and APEX data),we obtain their dust temperatures,H2 column densities and abundances of cyanopolyynes relative to H2.The mean relative abundance of HC3N was found to be(1.22±0.52)× 10-10 for Maser-only,(5.40±1.45)×10-10 for Maser-RRL and(1.65±1.50)×10-10 for RRL-only sources,respectively.The detection rate,the column density and the relative abundance of HC3N show a trend increasing from Maser-only to Maser-RRL sources and decreasing from Maser-RRL to RRL-only sources.This trend is consistent with the proposed evolutionary trend of HC3N,under assumption that our Maser-only,Maser-RRL and RRL-only sources possibly correspond to massive young stellar objects,ultra-compact H?(UC-H?)regions and normal classical H? regions,respectively.Shock model indicated that HC3N,SiO and H2CO associated with shock.Statistical analysis of the integrated line intensity and column density of HC3N and shock tracing molecules(H2CO,SiO),enabled to find positive correlations between them(correlation coefficients of 0.77 and 0.94).This suggests that HC3N may be another tracer of shock.Obtained results indirectly support the idea that the neutral-neutral reaction between C2H2 and CN is the dominant formation pathway of HC3N.Furthermore,we carried out observations of the HC3N(J=10-9 and 16-15)lines in a sample of UC-H? region(64 sources)using the IRAM 30-m telescope,to investigate further the physical and chemical properties of the HC3N molecule.Observational analysis shows that two spectral lines of HC3N,as well as the N2H+(J=1-0)spectral line,were detected simultaneously in 45 of these sources.The Trot and column densities of HC3N molecules for these 45 sources were obtained using rotation diagram method.Meanwhile,we used the line intensity ratio method to calculate the optical depth of N2H+and further deduced the excitation temperature and column density of N2H+in these sources.In UC-H? regions,the mean column density of HC3N is(1.97±0.27)×1013 cm-2,the mean column density of N2H+is(2.49±0.33)×1013 cm-2,and the mean N(HC3N)/N(N2H+)is 0.83±0.06.Combining previous data from the high-mass starless cores and high-mass protostellar objects,we find that the ratio N(HC3N)/N(N2H+)clearly increases with the evolution of HMSFRs,suggesting that it can serve as a chemical clock for HMSFRs.As well,we find that this trend may be due to the column density of HC3N molecule increasing with evolution until UC-H?regions,while the column density of N2H molecule remains essentially constant with evolution.This part of the work has been completed and will be submitted.And we will compare the result of chemical models with that of our observations.