Ultra-High-Resolution Spectroscopy of the ISM Towards Orion Richard John Price Thesis submitted for the Degree of Doctor of Philosophy of the University of London UCL Department of Physics & Astronomy UNIVERSITY COLLEGE LONDON March 2002 ProQuest Number: U643002 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest U643002 Published by ProQuest LLC(2015). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 To my parents “If we knew what it was we were doing, it would not be called research, would it?” Albert Einstein. ’ * • A b s t r a c t Firstly, we report ultra-high-resolution observations {R % 880,000) of Na I Di, Ca II if, K I, 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 py 110, 000) of Na I Di Sz. Ü2 and Ca, u H k. K towards twelve stars with various locations in and around the A 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 veloc­ ity 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 stud­ ied at very-high resolution. A comparison between the absorption occurring in sightlines with small angular separations (namely the M42 and A 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 hais revealed temporal vari­ ability in the interstellar absorption-line profiles of two stars, S Ori and Ori. In the case of 5 Ori, our observations reveal the transient cloud to be cool (Tk^280 K), with a heliocentric velocity of -|-21.3 km s“^. The component is detected in Na I j^i, where clear hyperfine splitting is seen, and Ca II K. Following a discussion of the possible origins of 6 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 I 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 5 Ori A and S 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). C o n t e n t s Frontispiece 4 A bstract 5 Table of Contents 7 List of Figures 10 List of Tables 13 1 Introduction 15 1.1 The Interstellar Medium ........................................................................................... 15 1 .1 .1 The G a s ............................................................................................................. 17 1.1.2 The D u s t .......................................................................................................... 20 1 .2 Absorption Line Spectroscopy .................................................................................. 2 2 1.2.1 Absorption Line P rofiles ................................................................................ 22 1.2.2 Equivalent Width .......................................................................................... 25 1.2.3 Fine and Hyperfine Atomic Structure ...................................................... 26 1.3 Spectrographs ............................................................................................................... 27 1.4 Aims and O b jectives .................................................................................................. 31 2 Observations and Data Reduction 32 2.1 The O bservations ........................................................................................................ 32 2.1.1 The U H R F ....................................................................................................... 34 2.1.2 The Mount Stromlo Echelle Spectrograph ................................................. 37 2.2 D ata R eduction ........................................................................................................... 38 7 Contents 8 2.2.1 E xtraction ............................................................................................................ 38 2.2.2 Background Correction .................................................................................. 38 2.2.3 F latfield in g ......................................................................................................... 39 2.2.4 Wavelength Calibration .................................................................................. 39 2.2.5 Rectification ......................................................................................................40 2.2.6 Atmospheric Correction .................................................................................. 41 2.2.7 Zero-Level Errors ............................................................................................... 42 2.3 Line Profile Analysis ..................................................................................................... 44 2.3.1 V a p i d ...................................................................................................................44 2.3.2 Atomic D a t a ......................................................................................................47 3 The Orion Region 48 3.1 Introduction ......................................................................................................................48 3.2 The Observations ............................................................................................................ 52 3.3 Na°/Ca+ Abundance Ratios .........................................................................................79 3.3.1 The Orion-Eridanus Shell ............................................................................... 82 3.4 Discussion ......................................................................................................................... 83 3.4.1 The M42 R e g io n ................................................................................................83 3.4.2 The A Ori Association ......................................................................................93 3.4.3 The Belt Region ................................................................................................93 3.4.4 (3 O rio n is ........................................................................................................ 95 3.4.5 K O r io n is ......................................................................................................... 96 3.5 C onclusions .................................................................................................................. 97 4 T he A Ori Association 99 4.1 Introduction ......................................................................................................................99 4.2 The Observations .......................................................................................................... 100 4.2.1 Stellar Line R em oval ....................................................................................... 102 4.3 Discussion ....................................................................................................................... 128 4.4 Conclusions .................................................................................................................... 133 5 Small-Scale Structure in the Interstellar Medium 135 5.1 Introduction ....................................................................................................................135 5.2 Search For Temporal Variability .............................................................................137 Contents 9 5.2.1 Introduction ......................................................................................................137 5.2.2 6 Orionis ...........................................................................................................