University of Groningen Optical spectroscopy of interstellar and circumstellar molecules Wehres, Nadine IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2011 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Wehres, N. (2011). Optical spectroscopy of interstellar and circumstellar molecules: a combined laboratory and observational study. Rijksuniversiteit Groningen. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). 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Download date: 29-09-2021 RIJKSUNIVERSITEIT GRONINGEN Optical Spectroscopy of Interstellar and Circumstellar Molecules A combined laboratory and observational study Proefschrift ter verkrijging van het doctoraat in de Wiskunde en Natuurwetenschappen aan de Rijksuniversiteit Groningen op gezag van de Rector Magnificus, dr. E. Sterken, in het openbaar te verdedigen op vrijdag 18 maart 2011 om 16.15 uur door Nadine Wehres geboren op 30 april 1979 te Viersen, Duitsland Promotores: Prof. dr. A. G. G. M. Tielens Prof. dr. H. V. J. Linnartz Beoordelingscommissie: Prof. dr. P. Sarre Prof. dr. J. M. van der Hulst Prof. dr. E. F. van Dishoeck ISBN: 978-90-367-4804-9 ISBN: 978-90-367-4805-6 (electronic version) Für meine Eltern. Front Cover: The Red Rectangle Proto-Planetary Nebula. Credits: Nasa – This image was taken with Hubble’s Wide Field Planetary Camera 2. “The secret of getting ahead is getting started. The secret of getting started is breaking your complex overwhelming tasks into small manageable tasks, and then starting on the first one.” – Mark Twain Table of Contents 1 Introduction 1 1.1 Introduction . 2 1.2 This Thesis . 2 1.3 Observational Spectroscopy of Molecules in the ISM . 2 1.3.1 The Molecular Inventory . 3 1.3.2 The Radio Regime of the ISM . 3 1.3.3 The Infrared Spectrum of the ISM . 6 1.3.4 The Optical Spectrum of the ISM . 8 1.4 Post-Asymptotic Giant Branch (AGB) Objects . 11 1.4.1 The Red Rectangle Proto-Planetary Nebula . 11 1.5 Thesis Outline . 12 2 Laboratory Astrophysics 17 2.1 Absorption Spectroscopy . 18 2.2 Production of Transient Species . 20 2.2.1 Pinhole Nozzle . 21 2.2.2 Slit Nozzle . 21 2.3 The Experimental Set-Up - CRDS . 23 2.4 The Experimental Set-Up - LIF . 25 2.5 Rotational Contour Simulations . 28 2.6 Optical Spectroscopy using the New Technology Telescope . 31 2.7 Optical Spectroscopy using the Mercator Telescope . 33 3 A Coincidence between a Hydrocarbon Plasma Absorption Spectrum and the λ5450 DIB 35 3.1 Introduction . 36 3.2 Cavity Ring-Down Spectroscopy . 38 3.3 Optical Observations . 39 3.3.1 HERMES @ Mercator Telescope . 39 3.3.2 McKellar @ DAO Telescope . 40 3.4 Results . 40 3.5 Discussion . 41 3Σ− 3Σ− 4 Rotationally Resolved A u –X g Spectrum of HC7H 45 4.1 Introduction . 46 4.2 Experimental . 47 4.3 Results and Discussion . 47 vii Table of Contents 5 Electronic Spectra and Molecular Geometry of the non-linear Carbon Chain C9H3 53 5.1 Introduction . 54 5.2 Experimental . 57 5.3 Results and Discussion . 59 5.3.1 Experimental Spectra and Analysis . 59 5.3.2 Consideration of the Molecular Geometry . 60 6 C2 Emission Features in the Red Rectangle – a combined observational/laboratory study 67 6.1 Introduction . 68 6.2 Astronomical Observations and Data Reduction . 70 6.2.1 Astronomical Results . 71 6.2.2 Laboratory Experiment . 72 6.2.3 Experimental Results . 73 6.2.4 The C2 Rotational Contour: Excitation Temperature and Velocity Shifts . 75 6.3 Fluorescent Emission in the Red Rectangle . 77 6.3.1 Model . 77 6.3.2 Results . 80 6.3.3 The Abundance of C2 ........................ 86 6.4 Implications . 87 6.5 Conclusions . 88 7 The Spatial Distribution of the Optical Emission Features in the Red Rectan- gle Proto-planetary Nebula 91 7.1 Introduction . 92 7.2 Observations . 94 7.3 Results . 96 7.3.1 An Offset Dependent Catalogue . 96 7.4 Discussion . 106 7.4.1 Spatial Behaviour of the Emission Bands . 106 7.4.2 Constraints on the Carriers of the Emission Bands . 109 7.4.3 The Red Rectangle Emission Bands and the DIBs . 109 7.4.4 Summary . 111 7.5 Conclusions . 112 Nederlandse Samenvatting 115 Zusammenfassung der Arbeit 121 Bibliography 127 Publications 133 viii Table of Contents Curriculum Vitae 135 Acknowledgements 137 ix Introduction1 1 Chapter 1 Introduction 1.1 Introduction Modern astronomy is still mainly based on the observations of the material that is sur- rounding the Earth. Most conclusions and theories about the main building blocks, the species, and even the formation and ongoing development of the Universe are based on the observations of light that reaches the Earth’s atmosphere in each moment. Light in this respect is a universal carrier of vast information about our past as well as our present. To understand the information that is encoded in form of electromagnetic wave packages is the task of spectroscopists and is the task I was facing when starting my thesis. Detecting light of different energies that reaches us from places far away from the Earth can give information about the ongoing chemistry, molecule formation, reaction channels and even about the physical conditions that occur in environments far away from Earth. Last but not least the information that we obtain from the spectroscopic investigation of light gives also hint about formation and creation of life. The origin of life provides a strong motivation to investigate the chemistry that is going on thousands of lightyears away and that took place millions of years ago. The challenge that modern astrochemistry is facing is the limited number of building blocks that we know about, it is the harsh conditions in circumstellar shells, in proto- planetary disks, in photon-dominated regions (PDRs) or in general in the interstellar medium (ISM). These harsh conditions put constraints on possible reaction pathways. The restrictions and different conditions push researchers to a re-think of the rules of chemistry and physics prejudiced by the experiences we make on Earth. The field of observational astrophysics combined with experimental astrophysics or astrochemistry is a complex field and will be limited here mainly to reflect the ongoing search of the molecules in the ISM. The search for the “Molecular Universe”, its main building blocks and its understanding and the challenges that it provides will be described in the following chapters. 1.2 This Thesis In this thesis the importance of the correlation between laboratory spectroscopy and ob- servational astronomy is discussed. It will be shown how the two different approaches correlate to each other and that mainly due to the combination of both techniques, the laboratory ∼ and the observational study, conclusions can be drawn on the molecular in- ventory of the interstellar and circumstellar medium. The outcome of the study on the molecular inventory can in turn put constraints on the chemistry and also on the physical conditions in specific environments. 1.3 Observational Spectroscopy of Molecules in the ISM Information of the species that are abundant in space is important. The information assist our understanding of the mechanisms that drive the evolution of the universe. Atoms or 2 1.3 Observational Spectroscopy of Molecules in the ISM molecules are not only abundant around stars, but they are also an important component in the medium in between the stars. As stars evolve, stellar winds can blow-off material from the stars that subsequently enriches the ISM. That way atoms like H, He, but also C, O and N become part of the ISM. Supernovae can also form heavier elements that also enrich the ISM. The molecular constituents of the ISM are known to be important for the heating and cooling mechanisms. At low densities for example, CO is an important coolant and dominates the process because of the high abundance of CO compared to other species. At higher densities, other species take over and dominate the cooling in molecular clouds, for example H2O and O2. In neutral regions, HI regions, heating can be traced mainly by observations of other ionized trace species, mainly larger molecules, but also dust grains. Dust grains can also be important in regions with a strong UV field. Here they absorb the photons and become excited. De-excitation can be through emission of photons in the IR, which lead to the so-called PAH (polycyclic aromatic hydrocarbons) emission bands at specific frequencies. The emitted photons can in turn excite other molecular species ro- vibrationally. In this way molecules have a strong influence of the mechanisms that drive the ISM, they influence the thermal balance and provide coupling between key processes that also drive star and planet formation.