Rotationally excited interstellar CH - detection of satellite lines and analysis of abundance and excitation

The two satellite lines of the lowest rotationally excited state (F1, N = 1, J = 3/2) have been detected at 42 cm wavelength toward W51M. The four hyperfine lines of this state exhibit intensity ratios in absorption which are consistent with a large optical depth. However, it is shown that the effects of trapping of the interlocking and overlapping far-IR hyperfine lines at 560 microns which connect the ground and first excited rotational states can also explain the observed hyperfine intensity ratios at 42 cm. Collisions dominate the excitation of the first excited rotation state. A column density of 6(15)/sq cm is derived for the CH toward W51, more than 10 times higher than previous estimates, yielding a fractional abundance of about 2(-8) over a region of mean density about 2(4)/cu cm. Shock chemistry is implied for CH in such regions, of which several are now known. 36 references