The Astron Astrophys Rev (2004) 12: 71–237 THE Digital Object Identifier (DOI) 10.1007/s00159-004-0023-2 ASTRONOMY AND ASTROPHYSICS REVIEW X-ray astronomy of stellar coronae Manuel Güdel Paul Scherrer Institut, Würenlingen and Villigen, 5232 Villigen PSI, Switzerland Received 27 February 2004 / Published online 25 August 2004 – © Springer-Verlag 2004 Abstract. X-ray emission from stars in the cool half of the Hertzsprung-Russell dia- gram is generally attributed to the presence of a magnetic corona that contains plasma at temperatures exceeding 1 million K. Coronae are ubiquitous among these stars, yet many fundamental mechanisms operating in their magnetic fields still elude an interpretation through a detailed physical description. Stellar X-ray astronomy is therefore contributing toward a deeper understanding of the generation of magnetic fields in magnetohydrody- namic dynamos, the release of energy in tenuous astrophysical plasmas through various plasma-physical processes, and the interactions of high-energy radiation with the stellar environment. Stellar X-ray emission also provides important diagnostics to study the structure and evolution of stellar magnetic fields from the first days of a protostellar life to the latest stages of stellar evolution among giants and supergiants. The disci- pline of stellar coronal X-ray astronomy has now reached a level of sophistication that makes tests of advanced theories in stellar physics possible. This development is based on the rapidly advancing instrumental possibilities that today allow us to obtain im- ages with sub-arcsecond resolution and spectra with resolving powers exceeding 1000. High-resolution X-ray spectroscopy has, in fact, opened new windows into astrophysical sources, and has played a fundamental role in coronal research. The present article reviews the development and current status of various topics in the X-ray astronomy of stellar coronae, focusing on observational results and on theoretical aspects relevant to our understanding of coronal magnetic structure and evolution. Key words: X-rays: stars – Stars: coronae – Stars: flare – Stars: late-type – Stars: magnetic fields For my part I know nothing with any certainty, but the sight of the stars makes me dream. Vincent van Gogh (1853–1890) This article is dedicated to the late Rolf Mewe, a prominent astrophysicist who contributed major work to the field of stellar X-ray astronomy and spectroscopy. He died on May 4, 2004. 72 M. Güdel Contents 1. Introduction ................................... 74 2. The study of stellar coronae ........................... 77 3. The early days of stellar coronal X-ray astronomy ............... 78 4. A walk through the X-ray Hertzsprung-Russell diagram ............ 81 4.1. Main-sequence stars .......................... 81 4.2. The coolest M dwarfs .......................... 84 4.3. Brown dwarfs (and planets?) ...................... 85 4.4. A-type stars ............................... 86 4.4.1. X-ray emission from normal A stars ............. 86 4.4.2. Chemically peculiar A stars ................. 87 4.4.3. Herbig Ae/Be stars ...................... 88 4.5. Giants and supergiants ......................... 88 4.6. Close binaries .............................. 89 4.7. Contact binary systems ......................... 89 5. X-ray activity and rotation ........................... 90 5.1. Rotation-activity laws .......................... 90 5.2. Activity and rotation in stars with shallow convection zones ..... 92 5.3. Rotation and saturation; supersaturation ................ 92 5.4. Physical causes for saturation and supersaturation ........... 93 5.5. Rotation and activity in pre-main sequence stars, giants and binaries . 94 6. Flux-flux relations ................................ 95 6.1. Chromosphere-transition region-corona ................ 95 6.2. Radio – X-ray correlations ....................... 96 7. Thermal structure of stellar coronae ...................... 97 7.1. Thermal coronal components ...................... 97 8. High-resolution X-ray spectroscopy ...................... 98 9. The differential emission measure distribution ................. 100 9.1. Theory ................................. 100 9.2. Interpretation .............................. 100 9.2.1. The DEM of a static loop ................... 100 9.2.2. The DEM of flaring structures ................ 102 9.3. Reconstruction methods and limitations ................ 103 9.4. Observational results .......................... 107 9.5. Coronal temperature-activity relations ................. 108 10. Electron densities in stellar coronae ...................... 110 10.1. Densities from Fe line ratios ...................... 110 10.2. Line ratios of He-like ions ....................... 112 10.3. Spectroscopic density measurements for inhomogeneous coronae . 115 11. The structure of stellar coronae ......................... 117 11.1. Loop models .............................. 117 11.2. Coronal structure from loop models .................. 119 11.2.1. Loop parameters ....................... 119 11.2.2. Loop-structure models .................... 119 11.2.3. Conclusions and limitations ................. 121 X-ray astronomy of stellar coronae 73 11.3. Coronal structure from densities and opacities ............. 122 11.4. Coronal constituents: Emission-measure interpretation ........ 123 11.5. Coronal imaging: Overview ...................... 124 11.6. Active-region modeling ......................... 125 11.7. Maximum-entropy image reconstruction ................ 126 11.8. Lucy/Withbroe image reconstruction .................. 127 11.9. Backprojection and Clean image reconstruction ............ 128 11.10. Coronal structure inferred from eclipses ................ 128 11.10.1. Extent of eclipsed features .................. 128 11.10.2. Structure and location of coronal features .......... 131 11.10.3. Thermal properties of coronal structures ........... 131 11.11. X-ray coronal structure in other eclipsing binaries ........... 132 11.12. Inferences from rotational modulation ................. 133 11.13. Rotationally modulated and eclipsed X-ray flares ........... 134 11.14. Inferences from Doppler measurements ................ 136 11.15. Inferences from surface magnetic fields ................ 137 11.16. Extended or compact coronae? ..................... 138 12. Stellar X-ray flares ............................... 139 12.1. General properties and classifications ................. 140 12.2. General flare scenario .......................... 141 12.3. Cooling physics ............................. 141 12.4. Interpretation of the decay time .................... 142 12.5. Quasi-static cooling loops ....................... 144 12.6. Cooling loops with continued heating ................. 145 12.7. Two-Ribbon flare models ........................ 147 12.8. Hydrodynamic models ......................... 149 12.9. Magnetohydrodynamic models ..................... 151 12.10. Summary of methods .......................... 152 12.11. Observations of stellar X-ray flares ................... 153 12.12. Flare temperatures ........................... 153 12.13. Flare densities .............................. 159 12.14. Correlation with UV and optical flares ................. 161 12.15. Correlation with radio flares ...................... 162 12.16. The “Neupert Effect” .......................... 162 12.17. Non-thermal hard X-rays? ....................... 164 13. The statistics of flares .............................. 165 13.1. Correlations between quiescent and flare emissions .......... 165 13.2. Short-term coronal X-ray variability .................. 166 13.3. Stochastic variability – what is “quiescent emission”? ......... 166 13.4. The solar analogy ............................ 168 13.5. The flare-energy distribution ...................... 170 13.6. Observables of stochastic flaring .................... 172 14. X-ray absorption features and prominences .................. 173 15. Resonance scattering and the optical depth of stellar coronae ......... 174 16. The elemental composition of stellar coronae ................. 175 16.1. Solar coronal abundances: A brief summary .............. 176 74 M. Güdel 16.2. Stellar coronal abundances: The pre-XMM-Newton/Chandra view . 177 16.3. Stellar coronal abundances: New developments with XMM-Newton and Chandra .............................. 178 16.4. Systematic uncertainties ........................ 182 16.5. Coronal and photospheric abundances ................. 183 16.6. Flare metal abundances ......................... 186 16.7. Theoretical models for abundance anomalies ............. 187 17. X-ray emission in the context of stellar evolution ............... 188 17.1. Main-sequence stars .......................... 188 17.2. Giants .................................. 190 17.3. Dividing lines .............................. 192 17.4. Hybrid stars ............................... 193 17.5. Evolution of X-ray emission in open stellar clusters .......... 194 17.5.1. Overview ........................... 194 17.5.2. Rotation-age-activity relations ................ 196 17.5.3. Binaries ............................ 199 17.5.4. Co-eval clusters ........................ 200 17.5.5. Toward older clusters ..................... 201 17.5.6. Toward younger clusters ................... 201 18. X-ray coronae and star formation ........................ 201 18.1. T Tauri stars ............................... 202 18.1.1. Overview ........................... 202 18.1.2. X-ray luminosity and age .................. 203 18.1.3. X-ray luminosity, saturation,