CH₃OCHO As A Probe Of Physical Property In Massive Protostar Cluster IRAS 15520-5234

In this paper,we report the observation results of methyl formate(CH3OCHO)in the region of massive star forming region IRAS 15520-5234 with Atacama millimeter/submillimeter wave array telescope(ALMA).The molecular spectra of four sidebands are obtained,and their frequency ranges are 354.277 to 354.739,356.613 to 357.074,344.254 to 346.098 and 342.372 to 344.216GHz,respectively.Based on these observation datas,we use XClASS software to identify methyl formate and related molecules,then use MAGIX to fit their parameters.Through the high-resolution observation of ALMA,IRAS 15520-5234 can be divided into seven dust cores,of which five dust cores have detected methyl formate.We have detected 178,65,76,45 and 37 methyl formate transition lines in the detected five cores respectively,and the upper energy level range of the detected transition is 41-721k.The spectral setup covers many methyl formate transitions,and these transitions span a large energy level range,allowing us to derive physical parameters accurately.The rotation temperature range of the obtained methyl formate is 80 to 120K,and the column density is 1.2 ×1015 to 3.2×1017cm-2.According to the information of continuum,we calculate the column densities of H2 from corel to core4 are 3.2×1023,4.8×1023,5.5×1023,4.1×1023cm-2,respectively.And the abundance ratio of methyl formate to H2 is calculated.We also judge the evolution stages of different dust cores through temperature,column density distribution and other relevant information.Core3 and core4 are relatively evolved protostars,corel is younger than these two,and core2 and core5 are the youngest protostars.By studying the gas distribution of methyl formate and its chemical related molecules,the peak position of methyl formate deviates slightly from the peak position of continuum.Our data also show that the gas distribution of methyl formate is very similar with dimethyl ether(CH3OCH3)and ketene(H2CCO).We also detected the vibrational transition of methyl formate CH3OCHO(v=18),but no two isomers of methyl formate were detected.According to the observation results,IRAS 15520-5234 is divided into seven dust cores in different evolution stages.The formation of methyl formate is more sensitive to temperature.Whether methyl formate can be detected is independent of evolutionary stage,but depends on the regional temperature.