PubMed1984
The spectra of the Orion KL molecular cloud and the envelope of the carbon star IRC+10216 have been surveyed at 1 MHz resolution over the interval 72.2-91.1 GHz to a sensitivity (in terms of main beam brightness temperature) usually better than 0.1 K. Some complementary data have been obtained at selected higher and lower frequencies. Detected lines are reported in Table 1. Approximately 170 lines from 24 known interstellar molecules were detected in Orion (Table 2); the corresponding numbers for IRC+10216 are 45 and 12, respectively (Table 3). Some 15 recombination lines of hydrogen (alpha, beta, gamma, delta) and helium (alpha) are also identified in Orion (Table 2). More than 100 of the Orion lines can be attributed to astronomically new transitions, the vast majority of which belong to only five species: SO2 (in total 17 lines), CH3CH2CN (19), CH3OH (16), (CH3)2O (21) and CH3OOCH (32). The rare occurrence of unidentified lines (5 definite lines in each object) is significant, possibly indicating that, in the case of Orion, the dominant chemical constituents are recognized. For IRC+10216 our results are less conclusive in this respect, partly due to the less effective excitation conditions in the envelope. We report (or confirm) the existence in these sources of several molecules which previously had only been found elsewhere: methylformate (CH3OOCH), isocyanic acid (HNCO), cyanodiacetylene (HC5N), vinyl cyanide (CH2CHCN), ketene (H2CCO) and probably the formyl radical (HCO) in Orion; and in IRC+10216 the first species containing the methyl group, methyl cyanide (CH3CN). The first astronomical detections of the isotopically rare species 34SO2 (Orion), and 29SiS and, tentatively, Si34S (IRC+10216) are also reported, as well as a number of newly identified high energy methanol lines. Multi-transition analysis of several molecular species is presented and gives information on physical conditions and chemical abundances in the Orion KL region (Tables 5 and 6). Gaussian decomposition of 0.25 MHz resolution profiles into "ridge", "hot core" and "plateau" emissions is used in an attempt to individually characterize these subregions. Striking chemical differences among the subregions emerge: (i) The estimated abundance ratios [SO, SO2, SiO]/[CO] are enhanced by a factor approximately > 100 in the plateau relative to the ridge, [HCN, HDO]/[CO] by a factor approximately > 10, while [HCO+, OCS, HNC, HC3N]/[CO] seem to be less enhanced in the plateau (by a factor approximately >10). (ii) The ratio of chemically saturated to unsaturated species differs markedly between the hot core and the ridge cloud (specifically HC3N vs. CH3CH2CN, and CH2CHCN) with the latter region more closely resembling both the cloud TMC-1 and IRC+10216. The chemical selectivity in the ridge cloud is very pronounced, with CH3OH as abundant as H2CO while (CH3)2O and CH3OOCH are an order of magnitude less abundant. We have neither detected CH3CH2OH and CH3COOH (acetic acid)--isomers of the two latter species, respectively--nor HCOOH, CH3CHO, or CH2CHCHO. H2C2O (ketene) is two orders of magnitude less abundant than H2CO. From the isotopic species of CH3OH, OCS, and HC3N we find 12C/13C approximately 40, in agreement with independent estimates by others and a factor of two lower than the solar system isotope ratio. The observed intensities of the hydrogen recombination lines are consistent with optically thin LTE emission, indicating that our He alpha/H alpha intensity ratio is relatively model independent and thus is an accurate measure of the helium abundance. We estimate a helium abundance by mass of 28 +/- 2%. The excitation conditions and relative abundances in the IRC+10216 envelope are reviewed. It is emphasized that the low detection rate of molecular lines in this object does not necessarily imply a poorer chemistry compared with that in interstellar clouds, but is partly a reflection of low abundances and unsatisfied excitation requirements. However, the CH3CN data indicate less favorable conditions for species containing methyl groups in this environment relative to interstellar clouds. The chemistry appears to be dominated by linear, especially unsaturated carbon-chain, molecules (Table 9). Rotational temperatures for HC3N and HC5N are estimated to be on the order of 15 K. Guided by the selective chemistry in IRC+10216 we tentatively assign two close doublets at 76.2 and 98.0 GHz to the radical C3H. In addition to Orion KL and IRC+10216 a few other sources have been observed, however in a less systematic way. We present selected data for W 51 M. The W 51 M methanol data, including newly identified high energy lines, indicate a rotational temperature of about 100 K.