Mapping, interstellar clouds

Molecular hydrogen is the dominant molecule the second most abundant molecule, CO, is four orders of magnitude less abundant. But H2 has no strong transitions in the microwave regions, CO is mainly used to map interstellar clouds in our galaxy and others, and also in quasars. The observation of... 

The fractionation of deuterium in interstellar molecules continues to excite considerable interest. Cosmologists identify the cosmic D/H ratio as a parameter critical to the assessment of cosmological models. Astrophysicists can use the isotopic ratio of species found in interstellar clouds as a probe of the conditions in those clouds. Isotopic abundances can help ion chemists to map synthetic pathways for forming interstellar molecules. Rnally to chemical kineticists, interested in the formation of interstellar molecules at temperatures approaching absolute zero, isotope effects offer a unique challenge — what is a minor perturbation at 300 K must exercise a profound influence at 10 K. Thus the equilibrium constant for the reaction... 

The excellent imaging capabilities of the Herschel space telescope have revealed, with unprecedented detail, the structure of molecular clouds and the regions where dense cores are formed. It is now clear that dense cores form within the ubiquitous filamentary structure of interstellar clouds, as localized density and column density maxima. There is a threshold in gas colunm density for dense core formation, estimated to be at extinctions of seven magnitudes from the analysis of either deep extinction images [15] or dust sub-millimetre emission maps [16], With such a threshold, the dense cores will be commOTily well shielded from the far ultra violet (FUV) radiation, with some exceptions in massive star forming regions or in photodissociation regions (PDRs) where dense molecular gas becomes directly exposed to FUV radiation. 

As we mentioned in the introduction to this section, it was known forty years ago from optical spectroscopy that CH is a component of interstellar gas clouds and the search was on for a spectroscopic detection of the radical at higher resolution so that the /l-doubling in the lowest rotational level (J = 1/2) could be measured or predicted accurately. This would enable the detection of CH by radio-astronomers and so allow the distribution of CH in these remote sources to be mapped out. The race was won by Evenson, Radford and Moran [48] using the then new technique of far-infrared LMR in the Boulder laboratories of the NBS (now known as NIST). They realised that there was a good near-coincidence between the water discharge laser line at 118.6 qm (84.249 cm ) and the N = 3 <- 2, J = 7/2 5/2 transition of CH in the / ) spin... 

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