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Red Giant Mass-Loss: Studying Evolved Stellar Winds with FUSE and HST/STIS

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Red Giant Mass-Loss: Studying Evolved Stellar Winds with FUSE and HST/STIS Red Giant Mass-Loss: Studying Evolved Stellar Winds with FUSE and HST/STIS A dissertation submitted to the University of Dublin for the degree of Doctor of Philosophy Cian Crowley Supervisor: Dr. Brian R. Espey Trinity College Dublin, July 2006 School of Physics University of Dublin Trinity College Dublin ii For Mam and Dad Declaration I hereby declare that this thesis has not been submitted as an exercise for a degree at this or any other University and that it is entirely my own work. I agree that the Library may lend or copy this thesis upon request. Signed, Cian Crowley July 25, 2006. Publications Crowley, C., Espey, B. R.,. & McCandliss, S. R., 2006, In prep., ‘FUSE and HST/STIS Observations of the Eclipsing Symbiotic Binary EG Andromedae’ Acknowledgments I wish to acknowledge and thank Brian Espey, Stephan McCandliss and Peter Hauschildt for their contributions to this work. Most especially I would like to express my gratitude to my supervisor Brian Espey for his enthusiastic supervision, patience and encourage- ment. His help, support and advice is very much appreciated. In addition, the helpful and insightful comments and advice from numerous people, inlcuding, Graham Harper, Philip Bennett, Alex Brown, Gary Ferland, Tom Ake, B-G Anderson and Dugan With- erick, were invaluable and again, very much appreciated. Also, a special word of thanks for their viva comments and feedback for Alex Brown and Peter Gallagher. This work was supported by Enterprise Ireland Basic Research grant SC/2002/370 from EU funded NDP. The FUSE data were obtained under the Guest Investigator Pro- gram and supported by NASA grants NAG5-8994 and NAG5-10403 to the Johns Hopkins University (JHU). The NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer is oper- ated for NASA by the JHU under NASA contract NAS5-32985. STIS data were obtained under the Guest Observer program STSI-GO0948701 provided by NASA through a grant to JHU from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Kait photometric data were obtained with the kind help of Weidong Li. Summary The changes that a star undergoes during the dying process are the most dramatic of its lifetime. These changes result in the most important interactions between a star and its environment, indeed it is in this area of research that some of the most challenging astrophysical problems still remain. For most stars, mass-loss becomes significant when they approach the end of their lives and enter the red giant evolutionary phase. This mass is lost in the form of a relatively dense and slow-moving wind which enriches the interstellar medium with material that has been processed inside the star and which is required for the formation of new stars and planets. However, despite the importance and ubiquity of these winds much remains unknown about the outflow conditions and charac- teristics and, furthermore, the physical processes which drive the mass-loss are unknown. Indeed, the mass-loss question remains arguably the most important outstanding problem of stellar astrophysics. For isolated giant stars, the diagnostics that can be obtained from observations are limited, with only disk-averaged information being directly observable. Spatially resolved observational constraints, in particular within the wind acceleration zone at the base of the outflow and close to the stellar photosphere, are required. This thesis presents a series of 20 FUSE (Far Ultraviolet Spectroscopic Explorer) and HST/STIS (Hubble Space Telescope/Space Telescope Imaging Spectrograph) observations of the bright symbiotic system EG Andromedae. The system consists of a low-luminosity white dwarf and a mass-losing, non-dusty M2.4 red giant star. The main motivation be- hind obtaining these ultraviolet spectra is to derive spatially resolved information on the giant wind in order to understand the mass-loss and wind heating mechanisms at work in evolved giants. The orbital elements of the binary are well understood and the dwarf star is known to undergo eclipse every 482 days. The ultraviolet observations follow the white dwarf continuum through periodic eclipses by the wind and chromosphere of the giant, providing a unique, spatially resolved diagnosis of the circumstellar gas in absorp- tion against the attenuated dwarf continuum. The atomic transitions in the ultraviolet wavelength region enable both the hot, ionised gas close to the dwarf and also the cooler material in the wind to be diagnosed. In addition, optical spectra and photometry are presented. Analysis of high and low-resolution optical spectra shows that the atmospheric struc- ture of the giant star is not severely perturbed, either radiatively or tidally, by the presence of the binary companion. It emerges that, although the photosphere is heated slightly and the atmosphere is elliptically distorted on the 7-8% level, these effects are minimal. The v photospheric spectrum remains similar to that of isolated stars and the atmosphere can be modelled as a normal giant. The photometry shows pulsational activity of the giant which is typical of similarly evolved stars, as well as relatively large periodic variations in the U-band. The ultraviolet spectral variations are dominated by the effects related to the eclipse of the dwarf by the giant atmosphere and wind. The uneclipsed spectra are dominated by the dwarf continuum with emission lines from an ionised portion of the giant wind superimposed on the continuum. The high ionisation features, such as the O VI resonance doublet (which is present as a variable, broad wind profile) diagnose the hot gas close to the dwarf component. This feature is variable on hourly timescales. During total eclipse of the hot component, the spectrum is dominated by emission lines originating from both the giant chromosphere and the extended photoionised section of the cool wind. Spectra observed at stages of partial eclipse display a host of low-ionisation, narrow absorption lines, with transitions observed from lower energy-levels up to 5 eV above ground. This ∼ absorption is due to chromospheric/wind material along the line of sight, with most lines being due to transitions of Si II, P II, N I, Fe II and Ni II. Photoionisation modelling shows that the white dwarf radiation does not dominate the wind acceleration region of the giant, and that any derived thermal and dynamic wind properties are most likely representative of isolated red giants. Analysis of the wind absorption features provides spatially resolved information through- out the base of the wind. The wind is found to be isothermal throughout the region that is probed, with a derived Fe II excitation temperature of 8, 000 K. The ionisation level ∼ along each line of sight is observed to be constant and symmetric around eclipse and hydrogen remains predominately neutral. The absorption spectra are modelled success- fully assuming collisional excitation and over 4,000 lines are identified and modelled. No molecular features are observed in the wind acceleration region despite the sensitivity of FUSE to molecular hydrogen. The terminal wind velocity is found to be 75 km s−1 and −8 −1 the mass-loss rate to be on the order of 10 M⊙yr . The damped wings of the hydrogen transitions define the ultraviolet continuum shape at absorbed phases and allow a mapping of the distribution of material around the giant. This information is used to derive a wind velocity profile that is incompatible with a low- order beta law and implies a delayed onset of acceleration. This result is confirmed by a photoionisation analysis of each sightline. The small-scale structure of the wind is found to be clumpy, with the flocculi having scale dimensions of 0.2 solar radii. Further analysis ∼ shows that the wind acceleration is unrelated to the presence of dust or molecular/atomic opacity, but is likely to be related to photospheric pulsations. Contents Contents vi List of Figures ix List of Tables xviii List of Acronyms xix 1 Red Giant Mass-loss and Symbiotic Stars 1 1.1 StellarEvolutionandMass-loss . .... 2 1.1.1 EvolutiontotheRedGiantBranch . 2 1.1.2 Stellar Photospheres, Chromospheres and Winds . ...... 4 1.1.3 Overview of Known Wind Driving Mechanisms . 10 1.1.4 Relevance to Other Fields of Study . 12 1.2 SymbioticStars ................................. 12 1.3 ThisThesis ................................... 14 1.3.1 Probing Giant Winds with Symbiotic Stars . ... 15 1.3.2 RelationshiptoIsolatedGiants . .. 16 1.3.3 OutlineofthisThesis. .. .. 17 2 Target Star, Observations and Data Processing 19 2.1 TheChoiceofBinarySystem . 19 2.2 EGAndromedae ................................ 21 2.3 The Ultraviolet Observing Program . .... 24 2.4 FUSE ...................................... 24 2.4.1 Data Processing and Instrumental Effects . ... 27 2.5 EG And FUSE Data .............................. 27 2.6 HST/STIS.................................... 29 vi CONTENTS vii 2.7 EGAndSTISData............................... 30 2.8 OpticalObservations . .. .. 32 2.8.1 LowResolutionData . .. .. 32 2.8.2 HighResolutionData. 33 2.8.3 PhotometricData............................ 33 3 The Giant Photosphere: Observations and Modelling 34 3.1 OpticalDataandAnalysis . 35 3.1.1 LowResolutionData . .. .. 35 3.1.2 HighResolutionData. 39 3.2 PhotosphericModelling. .. 42 3.2.1 ThePHOENIXModel ......................... 45 3.3 Photometry ................................... 45 3.4 MgIIhandkLines .............................. 47 3.5 Conclusions ..................................
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