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Studies of Dust and Gas in the Interstellar Medium of the Milky Way

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Studies of Dust and Gas in the Interstellar Medium of the Milky Way Studies of Dust and Gas in the Interstellar Medium of the Milky Way Francisco Javier Salgado Cambiazo ISBN: 978-94-6259-803-4 Studies of Dust and Gas in the Interstellar Medium of the Milky Way, Thesis, Leiden University 170 pages; illustrated, with bibliographic references and summary in Dutch Printed by Ipskamp Drukkers, Enschede Cover design Francisco Salgado Studies of Dust and Gas in the Interstellar Medium of the Milky Way Proefschrift ter verkrijging van de graad van Doctor aan de Universiteit Leiden, op gezag van de Rector Magnificus prof. mr. C. J. J. M. Stolker, volgens besluit van het College voor Promoties te verdedigen op woensdag 2 september 2015 klokke 10:00 uur door Francisco Javier Salgado Cambiazo Geboren te Santiago, Chili in 1983 Promotiecommissie Promotor Prof. Dr. A. G. G. M. Tielens Co-promotores Dr. J. B. R. Oonk Overige Leden Prof. Dr. H. J. A. Röttgering Prof. Dr. B. R. Brandl Prof. Dr. C. M. Walmsley Prof. Dr. A. Abergel Prof. Dr. G. J. White ii Contents List of Figures viii List of Tables ix 1 Introduction 11 1.1 The Interstellar Medium of the Milky Way . 11 1.2 Low Frequency Carbon Radio Recombination Lines . 12 1.2.1 CRRL Observations . 14 1.2.2 LOw Frequency ARray (LOFAR) . 15 1.2.3 Carbon Radio Recombination Line models . 16 1.3 HII Regions and Photodissociation Regions . 17 1.4 PAH and Dust Grains . 19 1.5 SOFIA FORCAST . 22 1.6 This Thesis . 23 1.7 Future Outlook . 26 2 First Results from SOFIA/FORCAST: The Mid-Infrared view of the Compact HII Region W3(A) 27 2.1 Introduction . 28 2.2 Observations and data . 29 2.3 Results . 31 2.3.1 Observational analysis of W3A . 31 2.3.2 Dust Spectral Energy Distribution . 34 2.3.3 Lyman alpha dust heating . 34 2.3.4 Dust-to-gas ratio . 35 3 The Orion HII region and the Orion Bar in the Mid-Infrared 37 3.1 Introduction . 38 3.2 Observations and Data . 40 3.3 Analysis . 42 3.3.1 An Overall View of the Orion Nebula . 42 3.3.2 The Orion Bar . 43 iii 3.3.3 Spectral Energy Distribution Fitting . 47 3.4 Results . 53 3.4.1 Geometry of the Orion Bar . 53 3.4.2 The Swept-up Shell . 55 3.4.3 Dust extinction . 55 3.5 Discussion . 57 3.5.1 The Dust Temperature and Lyα Heating . 57 3.5.2 Properties of the Dust . 58 3.5.3 Dust in HII Regions . 60 3.5.4 Cooling Lines and Photoelectric Heating Efficiency . 62 3.6 Summary and Conclusions . 65 4 Low Frequency Carbon Radio Recombination Lines I: Calcu- lations of Departure Coefficients 67 4.1 Introduction . 68 4.2 Theory . 70 4.2.1 Level Population of Carbon Atoms under Non-LTE Con- ditions . 71 4.2.2 Numerical Method . 74 4.2.3 Rates Used in this Work . 75 4.3 Results . 81 4.3.1 Departure Coefficient for Carbon Atoms . 81 4.3.2 Comparison with Previous Models . 86 4.4 Summary and Conclusions . 92 A List of Symbols . 95 B Level population . 97 B.1 Hydrogenic atoms . 98 C Radial Matrices and Einstein A coefficients . 100 D Radiative recombination cross-section . 101 E Energy changing collision rates . 103 F Collisional ionization . 103 5 Low Frequency Carbon Radio Recombination Lines II: The Diffuse Interstellar Medium 105 5.1 Introduction . 106 5.2 Theory . 108 5.2.1 Radiative transfer of carbon radio recombination lines . 108 5.2.2 The far-infrared fine structure line of C+ . 110 5.2.3 Line profile of recombination lines . 111 5.3 Method . 113 5.4 Results . 114 5.4.1 Line widths . 114 iv 5.4.2 Integrated line to continuum ratio . 117 5.4.3 CRRLs as diagnostic tools for the physical conditions of the ISM . 120 5.5 On the Observed Behavior of CRRLs . 124 5.5.1 General considerations . 124 5.5.2 Illustrative examples . 126 5.5.3 Discussion . 129 5.6 Summary and Conclusions . 130 A List of Symbols . 132 B Radiative Transfer . 134 B.1 Doppler and Lorentzian broadening . 137 B.2 Collisional/Stark broadening . 138 C Radiation broadening . 138 C.1 The FIR fine structure line of C+ . 141 6 Samenvatting 143 Samenvatting 143 6.1 Het belang van het interstellaire medium . 143 6.2 Koolstof Radio Recombinatie Lijnen op lage frequenties . 146 6.3 LOw Frequency ARray (LOFAR) . 146 6.4 Het belang van PAKs en interstellair stof . 147 6.5 SOFIA FORCAST . 148 6.6 Dit Proefschrift . 149 6.7 De toekomst . 151 Curriculum Vitae 153 Acknowledgements 155 Bibliography 168 v vi List of Figures 1.1 The phases of the ISM . 13 1.2 Comparison of LOFAR LBA with WSRT observation . 18 1.3 Observations and models of CRRLs towards Cas A . 19 1.4 Example of using CRRL models and observations to constrain the physical parameters of a cloud . 20 1.5 A schematic of a PDR . 21 1.6 Examples of dust emission . 23 1.7 Atmospheric transmission . 24 2.1 Color image of W3 main region . 30 2.2 SEDs for W3(A) . 31 2.3 Cross-cuts for the W3(A) region . 33 3.1 False color image for the Orion HII region . 44 3.2 FORCAST images . 45 3.3 A comparison between the 19 µm PDR-subtracted emission (green) and Hα ............................... 46 3.4 Cross-cuts for the Orion Bar . 47 3.5 Comparison between dust emission and cooling lines . 48 3.6 Examples of the SEDs at four positions. 52 3.7 Comparison between 13CO and PACS emission . 52 3.8 Temperature and optical depth maps of the two dust components revealed by the SED modeling. 53 3.9 Total infrared luminosity from the blackbody fits . 54 3.10 Temperature as a function of distance for different dust grain sizes 59 3.11 Photoelectric heating efficiency map . 63 3.12 Photoelectric heating efficiency plots . 64 4.1 R value as a function of electron temperature . 74 4.2 Comparison of energy changing collision rates . 77 4 2 4.3 bn and bnβn values (right) for hydrogen at Te = 10 and 10 K. 83 4.4 Example of hydrogenic bnl values . 84 2 4.5 Departure coefficients for the P3=2 parent ions as a function of n 86 vii final 4.6 Final departure coefficients for carbon atoms (bn ) as a func- tion of n level . 87 3=2 4.7 bn values for carbon atoms . 88 4.8 bnβn values for carbon atoms at different temperatures and den- sities . 89 4.9 Level where the bnβn values go to zero . 90 4.10 Difference between emissivities for low nl lines at low densities 90 4.11 A comparison of the effect of different collision rates on the final bn values . 91 4.12 Comparison between our bn values and the results from Hummer and Storey (1992) . 92 4.13 Departure coefficients from Ponomarev and Sorochenko (1992) 93 5.1 tn factor as a function of electron density . 113 5.2 A comparison between the continuum radiation fields. 115 5.3 A comparison between different line broadening mechanisms . 116 5.4 α and β line width transitions for diffuse regions as a function of frequency for different power law radiation fields . 118 5.5 Line-to-continuum ratio of CRRL as a function of principal quan- tum number (n) . 119 5.6 Ratio of the excitation to background temperature (TX =Tbg) . 119 5.7 Integrated line to continuum ratio normalized to the value at the level n = 500 ............................ 121 5.8 Example ratio diagnostic plots for different electron tempera- tures and densities. 122 5.9 Level where lines transition from emission to absorption (nt) as a function of electron density (Ne) . 123 5.10 Comparison between the integrated line to continuum I(α)=I(β) ratio as a function of frequency for different densities . 124 5.11 Ratio of the line to continuum ratio, for n = 700, to the carbon FIR line as a function of density. 125 5.12 Summary of the constraints derived from CRRL observations and models for Cas A . 128 5.13 Summary of the constraints derived from CRRL observations and models for Galactic plane regions . 129 6.1 Fases van het ISM . 144 6.2 Voorbeelden van emissie door stof . 150 6.3 Atmosferische transmissie . 151 viii List of Tables 1.1 Main characteristics of the phases of the ISM (Ferrière, 2001). 13 1.2 Physical characteristics of HII regions (Kurtz, 2005). 20 1.3 Filters characteristics and resolution of FORCAST for the filters used in this work. 24 3.1 FORCAST and PACS angular resolution . ..
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