OH 18 cm TRANSITION AS A THERMOMETER FOR MOLECULAR CLOUDS

作者：Ebisawa Yuji^*; Inokuma Hiroshi; Sakai Nami; Menten Karl M; Maezawa Hiroyuki; Yamamoto Satoshi

来源：Astrophysical Journal, 2015, 815(1): 13.

DOI：10.1088/0004-637X/815/1/13

• ISM: individual objects (HCL2, L134N, rho-Ophiuchi)
• ISM: molecules

We have observed the four hyperfine components of the 18 cm OH transition toward the translucent cloud eastward of Heiles Cloud 2 (HCL2E), the cold dark cloud L134N, and the photodissociation region of the rho-Ophiuchi molecular cloud with the Effelsberg 100 m telescope. We have found intensity anomalies among the hyperfine components in all three regions. In particular, an absorption feature of the 1612 MHz satellite line against the cosmic microwave background has been detected toward HCL2E and two positions of the rho-Ophiuchi molecular cloud. On the basis of statistical equilibrium calculations, we find that the hyperfine anomalies originate from the non-LTE population of the hyperfine levels, and can be used to determine the kinetic temperature of the gas over a wide range of H-2 densities (10(2)-10(7) cm(-3)). Toward the center of HCL2E, the gas kinetic temperature is determined to be 53 +/- 1 K, and it increases toward the cloud peripheries (similar to 60 K). The ortho-to-para ratio of H-2 is determined to be 3.5 +/- 0.9 from the averaged spectrum for the eight positions. In L134N, a similar increase of the temperature is also seen toward the periphery. In the rho-Ophiuchi molecular cloud, the gas kinetic temperature decreases as a function of the distance from the exciting star HD 147889. These results demonstrate a new aspect of the OH 18 cm line that can be used as a good thermometer of molecular cloud envelopes. The OH 18 cm line can be used to trace a new class of warm molecular gas surrounding a molecular cloud, which is not well traced by the emission of CO and its isotopologues.

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  RIKEN