The Horizontal Branch as a probe of stellar population history DISSERTATION Presented in Partial Ful¯llment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Grant Newsham ***** The Ohio State University 2007 Dissertation Committee: Approved by Professor Donald M. Terndrup, Adviser Professor Marc H. Pinsonneault Adviser Astronomy Graduate Program Professor David H. Weinberg ABSTRACT Color-magnitude diagrams of resolved stellar populations, especially when combined with metal abundance patterns from spectroscopy, are our principal means of determining the evolutionary history of the Galaxy, its stellar clusters, and dwarf satellites. In turn, they also guide our ideas about stellar structure and evolution. Improvements in the capability of evolutionary models allow better interpretations of the data, and better data lead to improvements in the theory. In this dissertation, I concentrate upon the horizontal branch (HB) of composite stellar populations as an aid to understanding the ages, helium content, and mass loss history of stellar systems. Hitherto, the HB has been studied primarily in single stellar populations, such as globular clusters, by comparing observations to theoretical stellar evolutionary models. The characterization of the HB has enabled constraints to be placed upon the ages, helium content, and red giant mass loss of the globular cluster populations. There is no reason in principle why such methods cannot be applied to composite population stellar systems to disentangle their complex history. ii In this dissertation, I utilize stellar evolutionary modeling and speci¯cally the morphology of the HB to investigate the hypothesized helium-rich stars in the globular cluster ! Centauri, and similar methodology to constrain the history of the metal-poor part of the stellar population in the Galactic bulge using the metallicity distribution of RR Lyrae variables. In response to proposed helium-rich stars as an explanation for the double main sequence observed in ! Centauri, I investigated the consequences of this hypothesis elsewhere on the color-magnitude diagram. I concentrated on the horizontal branch where the e®ects of high helium are expected to show themselves most clearly. In the process, I developed a new procedure for comparing the mass loss su®ered by di®ering stellar populations in a physically motivated manner. Stars that have high helium throughout their interiors are absent from the horizontal branch of ! Centauri in the numbers proposed unless their mass loss history is very di®erent from that of the majority population. It is possible to generate a double main sequence from existing ! Centauri stars via accretion of helium rich material consistent with the latest theoretical yields of AGB ejecta; such polluted stars are consistent with the observed HB morphology of ! Centauri. Polluted models are also consistent with the luminosity the main sequence merge as opposed to models of fully helium-rich stars. Using the (B R)=(B + V + R) statistic, I ¯nd that the high helium blue ¡ main sequence stars are required to be younger than the majority population by an iii amount not supported by observations of the main sequence turno®, whereas the pollution scenario stars have no such conict for ordinary mass losses. I then explored the metallicity distribution of RR Lyraes in the bulge to place constraints on the durantion of the early phases of star formation in the bulge. RR Lyrae variables o®er some real advantages as probes of stellar histories, especially if we know their metallicities because the metallicities that become RR Lyrae stars depend on age, mass loss and helium enrichment. I generated arti¯cal colour magnitude diagrams (CMD) of the Galactic bulge by sampling the known bulge metallicity distribution and assigning an age and mass loss for each model star. From these CMD's I created theoretical RR Lyrae metallicity distributions which I compared against the observed data. I used two scenarios to guide my selection of model parameters. The ¯rst is one in which star formation in the bulge starts at some time, t, and continues as a linear function of metallicity up to the highest metallicities, thus giving us an age spread ¢t. I call this a collapse scenario since it is motivated by monolithic collapse models of galaxy formation in which star formation in the bulge occurred all the way to the highest metallicities. The second is based on the hypothesis that the bulge formed from early accretion of small satellite systems already containing low metallicity stars ([Fe=H] 1:0) and gas. After some delay in time after the dwarfs formed, ¢t, · ¡ they merged to form the bulge accompanied by rapid star formation that created the higher metallicity stars seen in the bulge ([Fe=H] > 1:0). I chose the dividing ¡ iv line metallicity of [Fe=H] = 1:0 based upon the ¤CDM models of galaxy formation ¡ by satellite mergers. I call this a merger scenario. I found that for plausible giant branch mass loss the RR Lyrae stars need to be very old in both the collapse and merger scenarios. I also found that the age spread of the stars from the most metal-poor to the more metal-rich stars must be less than 1.5 Gyr. Bulge stars of solar metallicity or above would never appear as RR Lyrae variables without extreme mass loss on the giant branch. I also ¯nd that the stars of metallicities comprising the RR Lyrae metallicity distribution cannot be coeval either. Age spreads (collapse scenario), or time delay (merger scenario), of > 0:35 Gyr are implied in either of the two scenarios I investigated. The merger scenario is more restrictive on the start of star formation age and requires somewhat greater mass loss and a larger mass loss dispersion. In both scenarios the e®ect of helium enrichment is to reduce the initial age for the most metal poor stars by about 1 Gyr and to require a larger age spread for the star formation in the collapse scenario or time delay between the two bursts of star formation in the merger scenario. I also showed that the initial assumption that mass loss is independent of metallicity is satisfactory in the regimes of interest. A simple Reimers mass loss prescription is essentially independent of metallicity within our model parameter ranges. Mass losses for the more metal rich stars for a given age would actually be less than those for the lower metallicity stars, by a few hundredths of a solar mass, making them even less likely to appear as RR Lyrae variables. v Dedicated to those who care. vi ACKNOWLEDGMENTS I want to take this opportunity to thank many people without whom this last 5+ years would not have been possible. First I would like to thank my adviser Donald Terndrup whose knowledge, innate scienti¯c common sense (and why is it called common sense when it is seemingly so rare?) and even keel was much appreciated. To sum things up the entire experience of working with him was both interesting and fun. What more could one ask? I also have much appreciated the many conversations on all aspects of stellar physics with Marc Pinsonneault, whose knowledge of stars and physics intuition have always impressed me. I have walked into his o±ce more than a thousand times over the last few years, and the resulting knowledge learned has been a huge bonus in my studies. Another person I want to single out for thanks is Gerry Newsom. His positive demeanor and all around dedication in the teaching environment is truly an inspiration and a model to follow. Plus, how can you go wrong with a name so similar to mine! vii In my time in the Ohio State Astronomy Department I have interacted with numerous others whose help and discussions with have been most welcome. In no particular order and for varied and sundry reasons I shall mention the following: Zheng Zheng - a great o±ce mate, Nik Andronov - for truly strange movie tastes, James Pizagno - for pizza conversations, Julio Chaname - a man who should own a Donato's pizza franchise, Stefan Frank - despite German football, Matthias Dietrich - even more German football, Pavel Denissenkov - for discussions on stars, Rick Pogge - for knowing more than should be possible about Linux systems, David Will & Michael Savage - for their help with untold number of computer problems, David Weinberg - for astrophysical knowledge and obvious dedication and Pat Osmer for maintaining a department with a sure hand of professionalism and expertise. I would also like to thank all the people I was a teaching assistant for (some also mentioned above): Brad Peterson, Barbara Ryden, Smita Mathur, David Ennis, Kris Sellgren and Chris Kochanek. I enjoyed it all. I also am indebted to one inanimate part of the department, the Condor computer batch-job system. Without this I could not have attempted a large part of this dissertation. When you need 18 months plus of CPU time, as I did, then I should take time to acknowledge the University of Wisconsin team responsible for this tool. Thank you. viii Finally, on the personal side of life I want to thank my girlfriend Sue Wolfrum. This has been a di±cult last few years and her patience and understanding have meant the world to me despite the fact I may have been poor at showing how much this mattered to me. Believe me - it was appreciated, every single day. ix VITA June 9, 1965 . Born { Colne, Lancashire, England 1986 . B.Sc.(Honours) Physics, The Royal College of Science, Imperial College, University of London 1986 . Associateship of the Royal College of Science 2001 { 2007 . Graduate Teaching and Research Associate, The Ohio State University PUBLICATIONS Research Publications 1.