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Radio observations of several interstellar molecules
Abstract
We present observations of rotational transitions for several interstellar molecules whose formation may be related to grain processes and discuss their implications for interstellar chemistry. Interstellar hydrogen sulfide has been observed at fractional abundances f(H$\sb2$S) $\sim$ 10$\sp{-9}$ relative to H$\sb2$ towards cold, dark clouds, while its abundance is enhanced by a factor of 1000 in the Orion hot core and the plateau. H$\sb2$S may be evaporating from the grain mantles in the hot core, and even in the cold, dark clouds, grain surface reactions may be responsible for the gas-phase H$\sb2$S abundances. We also derive an upper limit for the HDS abundance (HDS) / (H$\sb2$S) $\leq$ 6 $\times$ 10$\sp{-4}$ in the Orion hot core. H$\sb2$CS ortho-to-para ratios have been observed to be $\sim$1.8 towards TMC-1, which may suggest that H$\sb2$CS is in equilibrium with the expected grain temperature (10 K) and gas-grain exchanges are taking place effectively in cold, dark clouds. We derive a ratio of $\sim$3, the statistical value, for Orion(3N1E) and NGC7538, and $\sim$2 for Orion(KL). We derive upper limits of the ethyl cyanide column densities of $\sim$3 $\times$ 10$\sp{12}$ cm$\sp{-2}$ towards TMC-1 and L134N. Together with the detection for vinyl cyanide, there may be no necessity of invoking grain surface synthesis for these highly saturated species in cold clouds, but the desorption processes seem to be quite inefficient for these heavy molecules. Finally, we have surveyed HOCO$\sp+$ as a tracer of interstellar CO$\sb2$ towards many galactic sources, and derive f(HOCO$\sp+$) $\sim$ 10$\sp{-8} - 10\sp{-9}$ in the Galactic center and $\leq$10$\sp{-10}$ for cold dark clouds. The observed abundance of HOCO$\sp+$ in the Galactic center is $\sim$3 and $\sim$1 orders of magnitude larger than that predicted by ion-molecule chemistry and shock chemistry, respectively. UV photolysis of grain mantles may produce CO$\sb2$ efficiently, resulting in a large abundance of HOCO$\sp+$ in the Galactic center. Interstellar grains are thought to play crucial roles in the chemistry of interstellar molecular clouds, and our results give some constraints on the highly uncertain grain processes, as well as on the gas phase processes.
Subject Area
Astrophysics
Recommended Citation
Minh, Young Chol, "Radio observations of several interstellar molecules" (1990). Doctoral Dissertations Available from Proquest. AAI9022722.
https://scholarworks.umass.edu/dissertations/AAI9022722