Astrophys. J., 518, 699-732 (1999/June-3)
The physics and chemistry of small translucent molecular clouds. XII. More complex species explainable by gas-phase processes.
TURNER B.E., TERZIEVA R. and HERBST E.
Abstract (from CDS):
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, CH3CN, 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 CH3CCH are successfully modeled for a wide range of C/O, metals, and cloud conditions. HOCO+, HCOOH, CH3CN, 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, which requires higher C/O than other species, and CH3CHO, which is possibly modeled in dark clouds but not in translucent objects. For the eight well-fitted species, C/O=0.4 is favored, and the time epoch is 5(5)±4(5) yr for each type of object. In this exercise we cannot distinguish between high and low metals. Previously studied species (HCN, C3H2, CCS, and HC3N) fit the same set of conditions. It is possible that all 10 species may be explained by gas-phase chemistry, perhaps when suitable neutral-neutral reactions are found for species such as CH3CHO and CH2NH. Until then, grain processes cannot be ruled out for these and other species such as HOCO+ (formed from desorbed CO2) or VyCN (highly saturated). Ortho/para and E/A ratios are found to lie within the thermal limits for all cases except O/P (H2CCO), whose value of 5.9 in TMC-1 exceeds the limit of 3.0, which confirms an earlier result by Ohishi. This result cannot be explained by grain interactions. Nevertheless, in the species of intermediate complexity, we may be encountering the boundary between gas and grain chemistry for partly hydrogen-saturated species. The transition to grain chemistry seems clearly to occur at the next level of complexity, which includes NH2CHO, EtOH, EtCN, (CH3)2O, and CH3OCHO, species we have failed to detect after exhaustive searches in translucent and dark clouds.:
ISM: Abundances - ISM: Clouds
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