Survey observations of c-C2H4O and CH3CHO toward massive star-forming regions

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) × 1014 and (0.2-5.0) × 1014 cm-2 for c-C2H4O and CH3CHO, respectively. The fractional abundances with respect to H2 are X(c-C2H4O) = 4 × 10-11 to 6 × 10-10 and X(CH3CHO) = 7 × 10-12 to 3 × 10-9. 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, LIR/M, in OMC-1, W51A, and Sgr B2(N). We found that the column densities of these molecules are high in sources where LIR/M is high. Since LIR/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.
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