PAH processing in space
Micelotta, E.R.
Citation
Micelotta, E. R. (2009, November 12). PAH processing in space. Retrieved from https://hdl.handle.net/1887/14331
Version: Corrected Publisher’s Version
License: Licence agreement concerning inclusion of doctoral thesis in the Institutional Repository of the University of Leiden
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PAH Processing in Space
PAH Processing in Space
Proefschrift
ter verkrijging van
de graad van Doctor aan de Universiteit Leiden,
op gezag van de Rector Magnificus prof. mr. P.F. van der Heijden, volgens besluit van het College voor Promoties
te verdedigen op donderdag 12 november 2009 klokke 13.45 uur
door
Elisabetta Rita Micelotta
geboren te Torino, Itali¨e in 1974
Promotiecommissie
Promotores: Prof. dr. F. P. Israel
Prof. dr. A. G. G. M. Tielens
Co-promotor: Prof. dr. A. P. Jones (Institut d’Astrophysique Spatiale - Universit´e Paris-Sud XI, Orsay - France)
Overige leden: Dr. B. R. Brandl
Prof. dr. E. F. van Dishoeck
Dr. E. Dwek (NASA Goddard Space Flight Center, USA) Prof. dr. H. V. J. Linnartz
Dr. T. A. Schlath ¨olter (Rijksuniversiteit Groningen - Atoomfysica Kernfysisch Versneller Institute, KVI)
Alla mia Famiglia
Cover: Benzene molecule in the Leiden sky.
Photography of the sky by Elisabetta Micelotta.
Benzene molecule: pencil on paper by Gwena¨el Bou´e.
Design by Guilhem Lavaux & Elisabetta Micelotta.
Benzene (C6H6) is an aromatic hydrocarbon (not polycyclic since only one ring is present) and a fundamental unit of the Polycyclic Aromatic Hydro- carbons (PAHs) which are the subject of this thesis.
Table of contents vii
Table of contents
1 Introduction 1
1.1 The Interstellar Medium . . . 1
1.1.1 Components of the ISM . . . 1
1.1.2 The cosmic life cycle . . . 6
1.1.3 Supernova explosions . . . 7
1.2 Polycyclic Aromatic Hydrocarbons . . . 9
1.2.1 Definition, characteristics and structure . . . 9
1.2.2 Formation and evolution . . . 12
1.2.3 Excitation mechanisms and IR spectroscopy . . . 13
1.3 The importance of interstellar PAHs . . . 16
1.4 Dust processing in the ISM . . . 18
1.4.1 Processes affecting the dust-to-gas mass ratio . . . 18
1.4.2 Processes conserving the dust-to-gas mass ratio . . . 19
1.5 Why study collisional PAH processing in the ISM . . . 19
1.6 Thesis outline . . . 21
2 Spitzer observations of LMC-N157B 25 2.1 Introduction . . . 26
2.2 Observations and data processing . . . 27
2.3 Results and analysis . . . 29
2.3.1 IRAC and MIPS images . . . 29
2.3.2 IRS Spectroscopy of J05375027-6911071 . . . 31
2.3.3 IRS spectroscopy of the extended dust cloud . . . 33
2.4 Discussion . . . 34
2.4.1 The nature of the compact object J05375027-6911071 . . . 34
2.4.2 The northeast edge of the dust cloud . . . 38
2.4.3 Conditions in the extended dust cloud . . . 39
2.4.4 The SNR revisited . . . 41
2.5 Conclusions . . . 42
3 PAH processing in interstellar shocks 45 3.1 Introduction . . . 46
3.2 Ion interaction with solids . . . 47
3.2.1 Nuclear interaction . . . 47
3.2.2 Nuclear interaction above threshold . . . 51
3.3 PAHs in shocks . . . 55
viii Table of contents
3.3.1 Ion collisions: nuclear interaction . . . 57
3.3.2 Ion collisions: electronic interaction . . . 61
3.3.3 Electron collisions . . . 62
3.4 Results . . . 63
3.4.1 PAH destruction via nuclear interactions . . . 63
3.4.2 PAH destruction via electronic interaction by ion collisions . . . . 64
3.4.3 PAH destruction due to electron collisions . . . 64
3.4.4 Summary . . . 67
3.4.5 Uncertainties discussion . . . 68
3.5 Discussion . . . 70
3.5.1 PAH lifetime in shocks . . . 70
3.5.2 Astrophysical implications . . . 72
3.6 Conclusions . . . 75
3.7 Appendix A: Sn,σand<T > . . . 76
3.8 Appendix B: Low and high energy regime above threshold . . . 79
3.9 Appendix C: Orientation correction . . . 80
4 PAH processing in a hot gas 81 4.1 Introduction . . . 82
4.2 Ion interaction with PAHs . . . 83
4.2.1 Electronic interaction . . . 83
4.2.2 Nuclear interaction above threshold . . . 85
4.3 Electron collisions with PAHs . . . 86
4.4 PAH destruction . . . 88
4.4.1 Dissociation probability . . . 88
4.4.2 Collision rate . . . 93
4.5 Results . . . 94
4.5.1 PAH destruction in a hot gas . . . 94
4.5.2 PAH lifetime . . . 97
4.6 Discussion . . . 102
4.6.1 X-ray absorption . . . 102
4.6.2 PAHs as tracers of cold entrained gas . . . 104
4.6.3 Comparison with previous studies . . . 106
4.6.4 C2groups loss . . . 107
4.7 Conclusions . . . 107
5 PAH processing by Cosmic Rays 109 5.1 Introduction . . . 110
5.2 High energy ion interactions with solids . . . 112
5.2.1 Collisions with high energy ions . . . 112
5.2.2 Ion energy loss and dissociation probability . . . 115
5.3 Collisions with high energy electrons . . . 118
5.4 Cosmic ray spectrum . . . 121
5.5 Collision rate and C-atom ejection rate . . . 124
5.6 Results . . . 126
Table of contents ix
5.6.1 PAH lifetime . . . 126
5.6.2 Discussion of the uncertainties . . . 128
5.7 Discussion . . . 131
5.7.1 Lifetime of PAHs against cosmic ray processing . . . 131
5.7.2 PAHs in galactic halos . . . 133
5.7.3 PAHs in galactic winds . . . 136
5.7.4 PAHs in galaxy clusters . . . 139
5.8 Conclusions . . . 143
6 Conclusions and Perspectives 147 6.1 Key questions . . . 147
6.2 Results . . . 147
6.3 Answers to key questions . . . 152
6.4 Future perspectives . . . 153
Bibliography 157
Nederlandse samenvatting 167
Curriculum vitae 183
Acknowledgments 185
Was it all worth it, was it all worth it all these years.
Queen, “Was It All Worth It”, The Miracle
