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Paper: Statistical Equilibrium Calculation of OH: Interpretation of the 1612 MHz Absorption Line in HCL2
Volume: 476, New Trends in Radio Astronomy in the ALMA Era
Page: 349
Authors: Inokuma, H.; Sakai, N.; Maezawa, H.; Menten, K.; Yamamoto, S.
Abstract: We have recently observed four hyperfine components of the ground state Λ-doublet-type transition of OH toward several positions in the Taurus Molecular Cloud with the Effelsberg 100 m radio telescope. We have found that the 1612 MHz line appears in absorption toward the eastern position of HCL2, where the [CI](3P1-3P0) emission shows a local peak. Furthermore, the spectrum observed toward the other positions in HCL2 shows two velocity components. The higher velocity component is seen in absorption, whereas the lower velocity component is seen in emission. In order to understand the above results, we have made statistical equilibrium calculations of the OH molecule. It is found that the 1612 MHz line appears in absorption only when the gas kinetic temperature is higher than 40 K. Thus, intensities of the four hyperfine components of OH can be a good thermometer. The combination of emission and absorption observed in HCL2 would reflect a cloud structure like cold dense cores surrounded by a warm less-dense envelope.
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