Detection of interstellar rotationally excited CH

Astronomy and Astrophysics – Astronomy

Scientific paper

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Abundance, Interstellar Matter, Molecular Spectra, Radio Sources (Astronomy), Far Infrared Radiation, Hyperfine Structure, Interstellar Chemistry, Optical Thickness

Scientific paper

Rotationally excited CH has been detected for the first time in the ISM. The first excited level (F(1), N = 1, J = 3/2) has been observed via its Lambda-doubling transitions at 700 MHz toward W51 - the lowest frequency molecular lines detected to date. The two main hyperfine components are seen in absorption. The spectra qualitatively resemble the line profiles of the ground state at the same position, suggesting that both transitions originate in the same gas. The column density derived for the excited state is at least 1.2-1.4 x 10 to the 14th/sq cm, comparable to that of the ground state in W51, but could be as high as 9.8 x 10 to the 17th/sq cm. The large excited-state population cannot apparently be explained by far-infrared radiative excitation from dust; collisions with gas densities n(H2) greater than or equal to 1,000,000/cu cm are probably the dominant source of excitation. CH is therefore present with considerable abundance at high densities in W51, contrary to predictions of chemical models.

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