A Dissertation entitled Photoluminescence by Interstellar Dust by Uma Parvathy Vijh As partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics Advisor: Prof. Adolf N. Witt Graduate School The University of Toledo August 2005 An Abstract of Photoluminescence by Interstellar Dust Uma Parvathy Vijh Submitted in partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics The University of Toledo August 2005 In this dissertation, we report on our study of interstellar dust through the process of photoluminescence (PL). We present the discovery of a new band of dust PL, blue luminescence (BL) with λpeak ∼ 370 nm in the proto-planetary nebula known as the Red Rectangle (RR). We attribute this to fluorescence by small, 3-4-ringed polycyclic aromatic hydrocarbon (PAH) molecules. Further analysis reveals additional indepen- dent evidence for the presence of small PAHs in this nebula. Detection of BL using long-slit spectroscopic observations in other ordinary reflection nebulae suggests that the BL carrier is an ubiquitous component of the ISM and is not restricted to the particular environment of the RR. We present the spatial distribution of the BL in these nebulae and find that the BL is spatially correlated with IR emission structures attributed to aromatic emission features (AEFs), attributed to PAHs. The carrier of the dust-associated photoluminescence process causing the extended red emission (ERE), known now for over twenty five years, remains unidentified. We constrain the character of the ERE carrier by determining the wavelengths of the radiation that initiates the ERE – λ < 118 nm. We note that under interstellar ii conditions most PAH molecules are ionized to the di-cation stage by photons with E > 10.5 eV and that the electronic energy level structure of PAH di-cations is consistent with fluorescence in the wavelength band of the ERE. In the last few chapters of the dissertation we present first results from ongoing work: i) Using narrow-band imaging, we present the optical detection of the circum- binary disk of the RR in the light of the BL, and show that the morphology of the BL and ERE emissions in the RR nebula are almost mutually exclusive. It is very suggestive to attribute them to different ionization stages of the same family of carriers such as PAH molecules. ii) We also present a pure spectrum of the BL free of scattered light, resolved into seven molecular emission bands, superimposed upon a broad continuum. The relative intensity of the component bands varies with position within the nebula, suggesting an origin in a set of several related molecular species, most likely small PAHs. iii Acknowledgments First and foremost I express my heartfelt gratitude for my advisor Prof. Adolf Witt for his guidance and support. His knowledge, erudition and enthusiasm for astrophysical problems has encouraged me and will continue to motivate me. I must also thank my parents for their support, their belief and pride in me. I hope I live up to their pride. I also thank my husband and friend Aarohi who in spite of having to go through graduate school and dissertation-writing/defense at almost the same time as myself has never failed to support and encourage me. I hope I have been as good a friend to him as he has to me. We would like to acknowledge Karl Gordon for his efforts towards the observations at Steward Observatory and Karl was also the PI of the HST proposal, observations from which lead to the determination of the ERE excitation wavelength. We also acknowledge Paul Sell who made the measurements for the above study as a NSF-REU student at the University of Toledo. We acknowledge Donald York and the observing team at Apache Point Observatory for the spectroscopic mapping and narrow-band imaging effort, preliminary results from which are reported is this dissertation. We would also like to thank Dr. Louis Allamandola for his valuable comments on our first discovery manuscript. We also thank David Malin and Hans van Winckel for providing us with blue images of the Red Rectangle, Laurent Verstraete and W. W. Jochims for supplying us with data on ionization cross sections of PAH molecules, and Lewis Hobbs, Theodore Snow and Donald York for constructive discussions about the Red Rectangle nebula and its central source. I would like to thank my committee members for their support and helpful com- ments. This research was made possible through a generous allocation of observing time at CTIO, KPNO, Steward Observatory and through grants from the US Na- tional Science Foundation. Financial support for this study was provided through NSF Grant AST0307307 to The University of Toledo. I would also like to acknowl- edge a CTIO thesis student travel grant, for travel to Chile. This research has made use of NASA’s Astrophysics Data System (ADS) Biblio- graphic Services and the SIMBAD database, operated at CDS, Strasbourg, France, and also of the Aladin image server. Some of the data presented in this paper was obtained from the Multimission Archive at the Space Telescope Science Institute (MAST). STScI is operated by the Association of Universities for Research in As- tronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NAG5-7584 and by other grants and contracts. iv Contents Abstract ii Acknowledgments iv Contents v List of Figures x List of Tables xiv 1 Introduction 1 1.1 Dust Grains and Nanoparticles ..................... 2 1.2 Luminescence by Dust Grains: Extended Red Emission ........ 4 1.2.1 Observational Techniques for ERE Detection .......... 9 1.2.2 Observational Constraints .................... 13 1.2.3 Models for the ERE Carrier ................... 14 1.3 PAHs in the ISM ............................. 22 1.3.1 Unidentified Infrared Bands or the Aromatic Emission Features 23 1.3.2 Optical Fluorescence ....................... 25 v 1.3.3 Formation of PAHs in the ISM ................. 29 1.4 Outline ................................... 30 2 Discovery of Blue Luminescence 31 2.1 Introduction ................................ 32 2.2 Target and Observations ......................... 34 2.3 Results ................................... 38 2.3.1 Line-depth Technique ....................... 38 2.3.2 Identification ........................... 41 2.4 Discussion ................................. 44 3 Small PAHs in the Red Rectangle 45 3.1 Introduction ................................ 46 3.2 Observations ................................ 48 3.3 Analysis and Results ........................... 50 3.3.1 Blue Luminescence in the Red Rectangle ............ 50 3.3.2 Attenuation of HD 44179 ..................... 61 3.4 Discussion ................................. 75 3.4.1 Identification of the BL Carrier ................. 75 3.4.2 Spectral Variability of the BL .................. 77 3.4.3 Spatial Variation of the BL ................... 77 3.4.4 Attenuation of HD 44179 ..................... 79 3.5 Conclusions ................................ 81 vi 4 Detection of BL in Other Nebulae 84 4.1 Introduction ................................ 85 4.2 Observations ................................ 88 4.3 Results ................................... 89 4.3.1 Ced 201 .............................. 89 4.3.2 Ced 112 .............................. 93 4.3.3 NGC 5367 ............................. 100 4.3.4 NGC 2023 ............................. 105 4.4 Discussion ................................. 109 4.4.1 The BL Spectrum ......................... 109 4.4.2 Spatial Distribution of BL .................... 113 4.4.3 BL in Hydrogen Ionization Regions ............... 118 4.4.4 Survival vs. In-situ formation of BL Carriers .......... 119 4.5 Conclusions ................................ 122 5 The Excitation of Extended Red Emission 123 5.1 Introduction ................................ 125 5.2 Observations and Data Reduction .................... 131 5.3 Results ................................... 136 5.3.1 The ERE, H2, and z-Band Morphology of NW Filaments ... 136 5.3.2 Determination of RV in NW Filaments ............. 142 5.3.3 Width of the ERE Filaments .................. 145 5.3.4 Wavelength of ERE Initiation .................. 146 vii 5.4 Discussion ................................. 149 5.4.1 Impact on Existing ERE Models ................ 150 5.4.2 Possible New ERE Carriers ................... 153 5.4.3 Consistency Check: The Red Rectangle ............. 158 5.4.4 Consistency Check: The High-|b| Galactic Cirrus ....... 162 5.5 Conclusions ................................ 164 6 Optical Emission Band Morphologies of the Red Rectangle 167 6.1 Introduction ................................ 167 6.2 Observations and Reductions ...................... 168 6.3 Results and Discussion .......................... 170 6.3.1 Blue Luminescence in the Red Rectangle ............ 170 6.3.2 Sharp Emission Feature at 5800 A˚ ................ 174 6.3.3 ERE ................................ 177 6.4 Conclusions ................................ 181 7 Spectral Characteristics of BL in the Red Rectangle 183 7.1 Introduction ................................ 184 7.2 Observations and Reductions ...................... 186 7.3 Results ................................... 189 7.3.1 The Band Structure of the BL .................. 189 7.3.2 Variation of BL Spectra with Position in the RR ........ 191 7.3.3 Variation of BL Intensities with Position in the RR ...... 193 7.3.4 Morphology of BL in the RR .................. 194 viii 7.4 Discussion ................................. 197