Other
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt.........1m&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF MASSACHUSETTS, 1995.Source: Dissertation Abstracts International, Volume: 56-11, Section: B, page:
Other
3
Dark Clouds
Scientific paper
We have carried out radio observations for the interstellar molecules NO, NS, and HCCN in order to investigate the role of nitrogen in the chemistry of the interstellar medium (ISM). Abundances of these species and implications for chemistry models are discussed. In addition, we have conducted a spectral line survey towards the star forming region Orion(KL) over the frequency range 160-170 GHz, which revealed a large number of spectral features arising from such nitrogen-bearing molecules as NS, HNCO, HCCCN, CHCN, CHCHCN, and CHCHCN. The first detection of interstellar nitric oxide (NO) in the cold dark cloud L134N is reported, and we also confirm the subsequent detection towards TMC-1. The inferred NO fractional abundance relative to molecular hydrogen for L134N is f ~ 5 W 10 towards the position of peak SO emission in that cloud. The inferred fractional abundance for TMC-1 is f ~ 2 W 10 towards the position of peak NH emission. These fractional abundances are in good agreement with predictions of quiescent cloud ion-molecule chemistry. We estimate f(N)/f(NO) ~ 140, which suggests that the bulk of the gas-phase nitrogen in quiescent clouds is in the form of N , as the ion-molecule chemistry models suggest. The first detection of interstellar nitrogen sulfide (NS) in cold dark clouds is reported. Several components of the , J = 3/2 --> 1/2 and J = 5/2 --> 3/2 rotational transitions were observed towards TMC-1 and L134N. The inferred column density for TMC-1 is f ~ 8 W 10 towards the NH peak in that cloud, and in L134N is f ~ 6 W 10 towards the position of peak NH emission. We have found that NS emission is extended in both clouds. The inferred NS fractional abundances are significantly higher than those predicted by some recent gas-phase ion-molecule models. Our astronomical observations of NS have led to new laboratory spectroscopy which has resulted in more accurate NS transition frequencies, and a corresponding determination of the NS molecular constants to a much higher precision. We also report the detection of SO towards L134N by its , J = 3/2 --> 1/2 parity-e transition. SO was not seen towards TMC-1. We have carried out the first comprehensive survey of NS towards regions of massive star formation. The inferred fractional abundance of NS relative to molecular hydrogen for sources with extended, optically thin emission ranges from 0.5 - 2 W 10, which is in good agreement with some recent gas-phase ion-molecule chemistry models developed for cold dark cloud conditions. The largest NS fractional abundance, 14 W 10, was inferred for the Galactic center cloud, Sgr B2(M). We have also mapped the distribution of NS towards OMC-1, where the NS integrated line intensity was found to be strongly peaked towards the (KL) position, but to extend all along the Orion ridge. The [J = 5/2 --> 3/2]/[J = 3/2 --> 1/2] ratio observed for Orion(KL), Orion(1.5S) and W3(OH) suggests that an emission source in these clouds may be spatially compact. In addition, we have identified a spectral feature seen towards DR21(OH), Orion(KL), Orion(1.5S) and W51(MS) as being the ortho N = 4 --> 2, J = 3 --> 2, fine structure component of CH, and another spectral feature seen towards W51(MS) as being the , J = 3/2 --> 1/2, parity-e transition of SO. The first astronomical detection of HCCN, a member of a possible new interstellar carbon chain series (HCN), has been reported at millimeter wavelengths towards the carbon star IRC+10216 by Guilin and Cernicharo (1992). We have conducted a deep search for HCCN towards TMC-1 and several GMC's. HCCN was not detected in any of the sources. The inferred fractional abundance upper limit, with respect to molecular hydrogen, for TMC-1 is f 2 W 1 0. We find the ratio [HCN:HCCN:HCCCN] in TMC-1 to be [1 : < 0.01 : 0.3]. We estimate the HCCN total column density towards Sgr B2(NW), G34.3+0.2, and W51(HO) to be N 4 - 10 W 10 cm s. Finally, a spectral survey of Orion-KL has been carried out in the 2 mm atmospheric window covering the frequency range 159.6 - 170.4 GHz, using the FCRAO 14 m radio telescope. Over 235 spectral lines were detected, including ~ 35 unidentified features. Sixteen known interstellar species were conclusively identified in Orion from this data, with the largest numbers of lines arising from ethyl cyanide, methyl formate, methanol, and dimethyl ether. Several other molecular species have been tentatively identified as producing emission lines in this band, including formamide, isothiocyanic acid, and possibly ethanol. This survey has been instrumental in the detection of interstellar SO. We also report on the tentative detection of a new interstellar molecular-ion, HCOH.
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