2001ApJ...560..792I

In order to clarify the formation mechanisms of ethylene oxide (cyclic-C₂H₄O, hereafter c-C₂H₄O) and its structural isomer acetaldehyde (CH₃CHO), we carried out survey observations of these two molecules toward 20 massive star-forming regions and two dark clouds. CH₃CHO and c-C₂H₄O were detected in 10 massive star-forming regions, and CH₃CHO was also detected in five others. The column densities and the rotational temperatures were derived using the rotation diagram method. The column densities of these molecules were derived to be (0.1-3.3)x10¹⁴ and (0.2-5.0)x10¹⁴ cm^–2 for c-C₂H₄O and CH₃CHO, respectively. The fractional abundances with respect to H₂ are X(c-C₂H₄O)=4x10^–11 to 6x10^–10 and X(CH₃CHO)=7x10^–12 to 3x10^–9. We also detected several transitions of methanol (CH₃OH), ethanol (C₂H₅OH), dimethyl ether [(CH₃)₂O], methyl formate (HCOOCH₃), formic acid (HCOOH), vinyl cyanide (C₂H₃CN), and ethyl cyanide (C₂H₅CN). Comparing the abundances of the detected molecules with physical conditions of each source, we found that the abundances of most of the molecules except for c-C₂H₄O and CH₃CHO increase along with the dust temperature of each source. On the other hand, the abundances of c-C₂H₄O and CH₃CHO show little correlation with the dust temperature. The rotation temperatures of c-C₂H₄O, CH₃CHO, and HCOOH are low (10-40 K) in all sources in spite of the fact that the gas kinetic temperature greatly varies from cloud to cloud. This may indicate that the line emission from each molecular species is excited in regions with different physical conditions. We performed pseudo-time-dependent chemical reaction simulations based on pure gas-phase reactions and found that the calculated abundances of observed molecules decreased when the gas kinetic temperature was raised. We investigated the relationship between the column density of C₂H₅OH and that of the C₂H₄O group (c-C₂H₄O+CH₃CHO) because C₂H₅OH is believed to be a precursor of c-C₂H₄O and CH₃CHO in the gas-phase chemistry scheme. If this hypothesis is correct, it is expected that the column density of C₂H₅OH is related to that of the C₂H₄O group. We found that the column density of the C₂H₄O group is high in sources where the column density of C₂H₅OH is high. This result is consistent with the above-mentioned hypothesis. We also investigated the relationships between the column densities of several organic species [CH₃OH, C₂H₅OH, (CH₃)₂O, HCOOCH₃, C₂H₃CN, and C₂H₅CN] and the luminosity-to-mass ratio, L_IR/M, in OMC-1, W51A, and Sgr B2(N). We found that the column densities of these molecules are high in sources where L_IR/M is high. Since L_IR/M is believed to be a measure of the star formation rate per unit mass, it indicates that the column densities of these molecules become higher in sources where high star formation activity leads to a higher dust temperature. This strongly suggests that the formation of these molecules involves processes on the dust grains and subsequent sublimation to the gas phase, where they can be observed.