Earnshaw Medal
Earnshaw 2008 Nominees: Brendan O'Dwywe
Observing Cyanide in Space
Stars form from vast cold and dark clouds composed of molecules and dust grains. These molecular clouds are completely opaque at optical wavelengths so astronomers must tune to the millimetre portion of the electromagnetic spectrum to detect them. The dust particles glow weakly by thermal emission while the molecules emit spectral lines as they undergo rotational transitions. In practice, there are only a very few molecules that can be used investigate the interiors of these clouds. The molecule has to be abundant enough that it can be detected yet not so abundant that molecules at the edge of the cloud re-absorb the radiation emitted by the same molecules deeper inside, where the star formation is taking place.
HCN appears at first to be an ideal molecule with which to probe the interior of molecular clouds. The rotational line spectrum has a hyperfine structure to each transition which in principle can be used to measure opacity and temperature of the gas. However, HCN exhibits anomolies in its hyperfine spectrum that are not clearly understood.
These anomolies were first discovered in nearby quiescent low-mass star forming clouds. In this project, the student carefully analysed a huge dataset of HCN emission from a distant, highly turbulent high- mass star forming cloud complex. He discovered, for the first time, that hyperfine anomolies in HCN are also present in clouds with very different physical conditions than in low-mass star forming environments. In addition, he was able to show that the character of the anomolies is quite different in the two cases. This work is extremely important because it shows that until these anomolies are ubiquitous and until they are understood, HCN must be regarded as an untrustworthy probe of molecular clouds and star formation.
Supervisor: Dr. Matt Redman
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