THE PHYSICS AND CHEMISTRY OF SMALL TRANSLUCENT MOLECULAR CLOUDS. Xll. MORE COMPLEX SPECIES EXPLAINABLE BY GAS-PHASE PROCESSES

摘要

We have observed 10 molecular species of ``intermediate'' complexity (four to seven atoms) in three translucent clouds and in TMC-1 and L183. Of these species HNCO, HOCO+ H2CCO, CCH, and CH3CCH are detected in all five objects, while HCOOH, CH3CHO, CH2CN, CH2NH, and VyCN are detected in fewer objects. These species are chosen because they are expected to mark the transition between simpler species that are formed in the gas phase and complex hydrogenated species [NH2CHO, . EtOH, EtCN, (CH3)2O, and CH3OCHO] that are believed to form by grain chemistry. The C/O ratio and the metal abundances are determining factors in the abundances of most of the species. The gas- phase chemistry of HNCO is set out for the first time. HNCO, H2CCO, CCH, and CH,CCH are suc- cessfully modeled for a wide range of C/O, metals, and cloud conditions. HOCO+, HCOOH, CH,CN, and VyCN are well modeled under fewer conditions, while CH3CHO and CH2NH are fitted in only a small range of conditions. For most species, the models tend to underestimate the observed abundances By introducing or modifying a number of gas-phase reactions in the New Standard Model we are able to explain the abundances of most species for specific sets of physical conditions (density, temperature, extinction), abundances (metals, C/O), and time epoch. The parameter sets for the different species are largely non overlapping. However, we have been able to find a single, unique set of parameters including a specific epoch for each of TMC-1, L183 and the translucent clouds that explains eight of the 10 species. The exceptions are CH2NH,