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

In order to clarify the formation mechanisms of ethylene oxide (cyclic-C2H4 O, hereafter c-C2H4O) its structural isomer acetaldehyde (CH3CHO), we carri ed out survey observations of these two molecules toward 20 massive star-fo rming regions and two dark clouds. CH3CHO and c-C2H4O were detected in 10 m assive star-forming regions, and CH3CHO was also detected in five others. T he column densities and the rotational temperatures were derived using the rotation diagram method. The column densities of these molecules were deriv ed to be (0.1-3.3) x 10(14) and (0.2-5.0) x 10(14) cm(-2) for c-C2H2O and C H3CHO, respectively. The fractional abundances with respect to H-2 are X(c- C2H4O) = 4 x 10(-11) to 6 x 10(-10) and X(CH3CHO) = 7 x 10(-12) to 3 x 10(- 9). We also detected several transitions of methanol (CH3OH), ethanol (C2H5 OH), dimethyl ether [(CH3)(2)O], methyl formate (HCOOCH3), formic acid (HCO OH), vinyl cyanide (C2H3CN), and ethyl cyanide (C2H5CN). Comparing the abun dances of the detected molecules with physical conditions of each source, w e 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 oth er hand, the abundances of c-C2H4O and CH3CHO show little correlation with the dust temperature. The rotation temperatures of c-C2H4O, CH3CHO, and HCO OH are low (10-40 K) in all sources in spite of the fact that the gas kinet ic temperature greatly varies from cloud to cloud. This may indicate that t he line emission from each molecular species is excited in regions with dif ferent physical conditions. We performed pseudo-time-dependent chemical rea ction simulations based on pure gasphase reactions and found that the calcu lated abundances of observed molecules decreased when the gas kinetic tempe rature was raised. We investigated the relationship between the column dens ity 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 chemi stry 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 densi ty of C2H5OH is high. This result is consistent with the above-mentioned hy pothesis. We also investigated the relationships between the column densiti es of several organic species [CH3OH, C2H5OH, (CH3)(2)O, HCOOCH3, C2H3CN, a nd C2H5CN] and the luminosity-to-mass ratio, L-IR/M, in OMC-1, W51A, and Sg r 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 t he star formation rate per unit mass, it indicates that the column densitie s of these molecules become higher in sources where high star formation act ivity leads to a higher dust temperature. This strongly suggests that the formation of these molecules involves proce sses on the dust grains and subsequent sublimation to the gas phase, where they can be observed.