Parkes observations for project P997 semester 2018OCTS

Ever since its detection in 1937 by Swings & Rosenfeld, the methylidyne radical- CH, observed in diffuse clouds, has played an important role in studying the thermal, dynamical and chemical evolution these regions. In recent years the 532/536 GHz transitions of CH observed using HIFI on board Herschel has been established as an important tracer for H2 in diffuse and translucent clouds, which has led to a significant increase in its observations. The three ground state hyperfine structure (hfs) transitions of CH, observable in the radio range near 3.3 GHz although easily detected have weak intensities and non-LTE excitation and almost always appear in emission, even towards strong continuum sources. We would like to investigate the anomalous excitation of these lines utilizing the high sensitivity and spectral resolution provided by the new UWL receiver of the Parkes telescope, with the aim of (re-)establishing the 3.3 GHz lines of CH as a highly powerful probe of H2 in the Milky Way. In addition we would also like to observe the first rotational excited lines of CH at 0.7 GHz which due to Zeeman splitting will act as a useful potential magnetic probe. We would not only like to address the excitation conditions and enhanced emission at 3264 MHz but also constrain the physical conditions of the surrounding regions through non-LTE radiative transfer models utilizing collisional rate coefficients calculated by Paul Dagdigan, for which the Parkes observations will deliver strong constraints.