On the diffuse interstellar bands and the local interstellar medium
This thesis presents results from four different studies on the diffuse interstellar bands and the local interstellar medium.
Firstly, a detailed investigation into the profile of the λ6614 diffuse interstellar band (DIB) is presented, which was aimed at ascertaining the origin of the observed fine structure and overall band shape, and why both of these properties change between a number of different lines-of-sight. A new method for normalising DIBs is described, and revealed new information about how the λ6614 band evolves between lines-of-sight. Observations and modelling of the DIB profile towards HD 147889 suggests that this line-of-sight has the highest internal excitation of the carrier, which accounts for its greater width and additional structure.
As a continuation of this project, a further fifteen DIB profiles were examined towards HD 179406 and HD 147889, some of which are thought to correlate with the column density of the C2 molecule. It was shown that HD 179406 and HD 147889 demonstrated almost identical band profiles for the ‘C2’ DIBs, which differed considerably to the ‘non-C2’ DIBs investigated in this study. Analysis presented in this chapter strongly supports the idea that the ‘C2’ DIBs form a distinct class, and the normalisation method used provides a new criterion for validating whether a DIB may be classed as a ‘C2’ DIB.
A study of small-scale structure in the ultraviolet (UV) region towards three stars within the ρ Ophiuchus system was undertaken using observations from the Hubble Space Telescope, to deduce column densities of atomic and molecular species, and to obtain physical parameters such as density and kinetic temperature through chemical modelling. The biggest absorption differences were found within the C2 molecule profile, and it was found that the medium in which C2 molecules reside is denser towards ρ Oph A and ρ Oph B than ρ Oph D. Modelling additional species, such as Fe I and Fe II which are also thought to be present in the data, may help to further characterise the ISM towards these three stars.
Optical and near-infrared (IR) observations of Herschel 36 were undertaken using the Southern African Large Telescope (SALT) and Gemini North. The aim of this project was to investigate the full DIB spectrum along this unusual sight-line, and determine how many DIBs were present and how many demonstrate the behaviour previously observed. A number of issues arose with the optical observations and the data reduction processes, meaning that the overall aim was not fully realised. However, observations from Gemini North showed more promise but did not detect either near-IR DIB, although higher S/N observations are recommended to verify their absence.