A SURVEY STRATEGY FOR LIGHT ECHOES FROM HISTORICAL SUPERNOVAE IN THE MILKY WAY A SURVEY STRATEGY FOR LIGHT ECHOES FROM HISTORICAL SUPERNOVAE IN THE MILKY WAY By LINDSAY E. OASTER, B.Sc. A Thesis Submitted to the School of Graduate Studies in Partial Fulfilment of the Requirements for the Degree Master of Science August 2008 McMaster University © 2008 Lindsay Oaster I grant McMaster University the non-exclusive right to use this work for the University's own purposes and to make single copies of the work available to the public on a not-for-profit basis if copies are not otherwise available. Lindsay Oaster The thesis of Lindsay Oaster was reviewed and approved* by the following: Dr. Douglas Welch Professor, McMaster University Thesis Advisor, Chair of Committee Dr. Christine Wilson Professor, McMaster University Committee member Dr. Laura Parker Professor, McMaster University Committee member *Signatures are on file in the Graduate School. MASTER OF SCIENCE (2008) McMaster University (Department of Physics and Astronomy) Hamilton, Ontario TITLE: A Survey Strategy for Light Echoes from Historical Supernovae in the Milky Way AUTHOR: Lindsay Oaster, B.Sc. SUPERVISOR: Douglas Welch NUMBER OF PAGES: viii, 77 ii L..--_____ABSTRAC~ Hundreds of years after exploding, the original light from a supernova can still be observed in the form of light echoes. This light scatters off interstellar dust and is re-directed back toward Earth; due to the extra travel time, we observe the echo after the initial outburst. At some time t after observing the outburst, the surface of equal travel paths defines an ellipsoid with Earth and the supernova at the foci. If dust intersects this ellipsoid it is possible to scatter the light and produce an echo. In this thesis, I develop a relative probability model for the detection of supernova light echoes based on the physical characteristics of interstellar dust and absorption near the Galactic plane. This model includes a dust scat­ tering function, distribution (scale height) of dust in the Galaxy, the dilution of echo flux with distance, and absorption along the supernova-dust-Earth travel paths. I have tested the model's predictions against observations and compared it with a prior survey strategy based on IRIS (re-processed IRAS) maps. Currently the IRIS-based strategy is more effective at selecting good paintings but its detection rate is only around 5%, highlighting the elusiveness of echo appearances. This work considers six historical supernovae in the Milky Way, all of which exploded in the pre-telescopic era (with the possible exception of Cas A) and were recorded as "guest stars" in astronomy records from Asia, Europe, and the Middle East. Their light echoes could give us information on these historically significant events and an opportunity to simultaneously study a supernova in outburst and several hundred years later. Early investigations suggest that the distribution of CO in the Galaxy may anti-correlate with the best paintings for light echoes; if a CO-echo link can be established, this would be useful in future light echo surveys. iii ~----ACKNOWLEDGMENT~ During my experiences here, I have been supported by more people than I could possibly name within these pages. I would like to give my sincere thanks to the following people. To Doug Welch: I got to come to McMaster and study astronomy with a fantastic supervisor - everyone should be so lucky! Thank you for sharing your wisdom and experience (and many stories!), and for all of your guidance during my time here. To Christine Wilson and Laura Parker: thank you for being on my thesis com­ mittee and for your valued input on my research. To the incredible staff in the department office: Mara Esposto, Cheryl John­ ston, Daphne Kilgour, Rosemary McNeice, Liz Penney, and Tina Stewart. You are amazing! Thank you for helping me get organized and for answering all my questions, big and small. To the many dear friends I've found here, especially Rob Cockcroft and Pam Klaassen: I have been so blessed to know you, and I couldn't have asked for better people to journey with here in Hamilton. Thanks for sharing your in­ herent awesomeness with me. To my amazing husband Zach Oaster, my mom and stepdad Marilynn and Gil Springman, my sister Niaomi and her husband Chad Curtis, and my husband's parents Nilene and John Oaster: You have been a source of balance when I needed it most. Thank you for your ever-present support. IV &....--__TABLE OF CONTENTS! Acknowledgments iv List of Figures vii List of Tables viii Introduction 1 Chapter 1 A supernova review 2 1.1 Type Ia supernovae . 2 1.2 Core-collapse supernovae . 9 1.3 Light echoes . 11 1.3.1 Nova Persei 1901 . 11 1.3.2 SN 1987 A . 12 1.3.3 Outside the galaxy 13 1.3.4 SuperMACHO and the LMC discoveries 14 1.3.5 The historical supernovae . 15 1.3.6 Echo detectability and models 19 1.3.7 Surveying for light echoes 20 Chapter 2 Methods 21 2.1 The probability model 21 2.2 Absorption .. 24 2.3 Observations . 26 v Chapter 3 Results 28 3.1 Model results . 28 3.2 Observational results 29 3.3 Absorption . 29 3.4 Efficacy of the survey strategies 31 Chapter 4 Discussion & Conclusions 45 4.1 Extinction and the probability . 45 4.2 CO investigations 48 4.3 Conclusions . 49 Appendix A Code 55 A.1 Derror.m ....... 55 A.2 Cas_prob_commands.m 56 References 70 vi &...--____LIST OF FIGURE~ 1.1 Supernova classification . 3 1.2 Combined supernova, CMB, galaxy cluster results 7 1.3 The layered structure of CC supernovae . 9 1.4 Kepler ellipsoid . 12 1.5 Couderc ellipsoid explanation . 13 1.6 Apparent superluminal expansion 14 2.1 Kepler and Crab ellipsoids ... 22 2.2 Extrapolated absorption .... 25 3.1 Probability maps for the six remnants. 34 3.2 Tycho and Cas A echoes. 37 3.3 Tycho and Cas A field probabilities 38 3.4 Tycho and Cas A maps, fields 39 3.5 Earth-dust absorption: Cas A . 40 3.6 Earth-dust absorption: Tycho 41 3.7 The effect of absorption: Cas A 42 3.7 The effect of absorption: Tycho 43 3.7 The effect of absorption: Cas A and Tycho . 44 4.1 Tycho and Cas A echo labels . 46 4.2 CO line profile for echo #2729 . 52 4.3 Sample CO emission: Cas A 53 4.4 Sample CO emission: Tycho (1) 53 4.5 Sample CO emission: Tycho (2) 54 4.6 Sample CO emission: Tycho (3) 54 vii L...--____LIST OF TABLE~ 1.1 Galactic supernovae and remnants . 16 3.1 The twelve scattered light echoes . 30 3.2 Strategy success rates . 31 3.3 Random-pointing success rates: Tycho 32 3.4 Random-pointing success rates: Cas A 32 3.5 Average probability near Tycho 33 3.6 Average probability near Cas A . 33 viii L. Gaster • M.Sc. Thesis Dept. of Physics & Astronomy, McMaster University _______INTRODUCTIOJ Much of our current knowledge of the end stages of stars, the cosmological distance scale, and acceleration of the universe has been established through supernova observations. The outburst of SN 1987A in the Large Magellanic Cloud provided the opportunity to thoroughly observe a local supernova and trace the development of its remnant and light echoes from the earliest stages. Scattered-light echoes from supernovae have the potential to reveal informa­ tion about the explosion asymmetries and interstellar dust structure (Rest et al. 2005, 2008). Identifying echoes in significant numbers has been a chal­ lenge, however, and the optimum search strategy isn't obvious at present. In this work, I will introduce a new survey strategy that considers the dust distri­ bution, scattering function, flux-distance relation, and interstellar absorption and attempts to determine where light echoes are most likely to be found. The efficacy of this model will be evaluated based on observations performed in 2006 and 2007. Chapter 1 is a review of supernovae, the physics of their explosions, and their use in cosmology. I will also introduce the light echo phenomenon and its history along with the six historical, galactic supernova remnants that are considered in this thesis. Chapter 2 explains the development of the prediction model and the details of our observations, and in Chapter 3 the observational results are presented and analyzed. Chapter 4 contains the rates of success for each detection method and a discussion of future plans for similar work. 1 L. Oaster • M.Sc. Thesis Dept. of Physics & Astronomy, McMaster University CHAPTERl ________________________~ IL..---__A SUPERNOVA REVIEW Supernova observations are used to set the cosmological distance scale and determine the acceleration of the Universe (Wilson 1939; Perlmutter et al. 1998; Schmidt et al. 1998; Leibundgut 2008). The explosion mechanisms for the low- and high-mass regimes are summarized below, along with how each type has contributed to our knowledge of cosmology. Minkowski (1941) was the first to designate sub-groups for supernovae based on the absence ( "Type I") or presence ( "Type II") of hydrogen lines in the SN spectrum. These types were further subdivided based on the presence of helium and silicon lines (da Silva 1993); the current classification scheme is shown below in Figure 1.1 and will be referenced when discussing supernova types. 1.1 Type Ia supernovae Explosion details When a white dwarf (WD) in a binary system accretes matter from its compan­ ion and exceeds 1.44 M 8 , electron degeneracy pressure is no longer sufficient to support the star against collapse (Nomoto & Sugimoto 1977; Hoeflich & Khokhlov 1996). A thermonuclear reaction begins at the center and spreads outward; if it propagates by a supersonic shock wave it is called a detona­ tion, whereas subsonic flame propagation is called deflagration (Hillebrandt & Niemeyer 2000).