Price, Richard John; (2002) Ultra-high-resolution spectroscopy of the ISM towards Orion. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Firstly, we report ultra-high-resolution observations(R = 880,000) of Na 1 D1, Ca II K, K 1, CH and CH+ for interstellar sightlines towards twelve bright stars in Orion, including four stars in the M42 region. Secondly, we report high-resolution observations (R = 110,000) of Na 1 D1 and D2 and Ca II H and K towards twelve stars with various locations in and around the [lambda] Orionis association. Model fits have been constructed for the absorption-line profiles, providing estimates for the column density, velocity dispersion, and central velocity for each constituent velocity component. These data enable the detection of many more absorption components than previously recognised, providing a more accurate perspective on the absorbing medium. This is especially so for the line-of-sight to the Orion Nebula, a region not previously studied at very-high resolution. A comparison between the absorption occurring in sightlines with small angular separations (namely the M42 and [lambda] Orionis association) has been used, along with comparisons with other studies, to estimate the line-of-sight velocity structures. Na /Ca+ abundance ratios are derived for individual clouds, providing an indication of their physical state. Where absorption from both Na and K is observed for a particular cloud, a comparison of the velocity dispersions measured for each of these species provides rigorous limits on both the kinetic temperature and turbulent velocity prevailing in each cloud. Our results indicate the turbulent motions to be subsonic in each case. The comparison of observations made at different epochs has revealed temporal variability in the interstellar absorption-line profiles of two stars, [delta] Ori and [zeta] Ori. In the case of [delta] Ori, our observations reveal the transient cloud to be cool (Tk 280 K), with a heliocentric velocity of +21.3 km s-1. The component is detected in Na1 D1 where clear hyperfine splitting is seen, and Ca 11 K. Following a discussion of the possible origins of this component it is concluded that an interstellar, rather than circumstellar, origin is most likely. This is one of very few detections of variable interstellar absorption reported in the literature, and we suggest an origin within filamentary material associated with the expanding H 1 shell surrounding the Orion-Eridanus superbubble. These detections highlight the presence of small-scale spatial structure in the interstellar medium down to scales on the order of a few astronomical units. Furthermore, the comparison of observations made towards the two closely spaced stars [delta] Ori A and [delta] Ori C, highlights the presence of variations in absorption between the two sight-lines. Such differences are indicative of small-scale spatial structure in the interstellar medium in this direction over distances of less than 15,000 AU (the projected separation of the two stars).

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