The CH radical at radio wavelengths: Revisiting emission in the 3.3GHz
  ground state lines
  The intensities of the three widely observed radio-wavelength hyperfine
structure (HFS) lines between the {\Lambda}-doublet components of the
rotational ground state of CH are inconsistent with LTE and indicate ubiquitous
population inversion. While this can be qualitatively understood assuming a
pumping cycle that involves collisional excitation processes, the relative
intensities of the lines and in particular the dominance of the lowest
frequency satellite line has not been well understood. This has limited the use
of CH radio emission as a tracer of the molecular interstellar medium. We
present the first interferometric observations, with the Karl G. Jansky Very
Large Array, of the CH 9 cm ground state HFS transitions at 3.264 GHz, 3.335
GHz, and 3.349 GHz toward four high mass star-forming regions (SFRs) Sgr B2
(M), G34.26+0.15, W49 (N), and W51. We investigate the nature of the
(generally) weak CH ground state masers by employing synergies between the
ground state HFS transitions themselves and with the far-infrared lines, near
149 {\mu}m (2 THz), that connect these levels to an also HFS split rotationally
excited level. Employing recently calculated collisional rate coefficients, we
perform statistical equilibrium calculations with the non-LTE radiative
transfer code MOLPOP-CEP in order to model the excitation conditions traced by
the ground state HFS lines of CH and to infer the physical conditions in the
emitting regions while also accounting for the effects of far-infrared line
overlap.
