Stellar coronal astronomy

Fabio Favata ([email protected]) Astrophysics Division of ESA’s Research and Science Support Department – P.O. Box 299, 2200 AG Noordwijk, The Netherlands Giuseppina Micela ([email protected]) INAF – Osservatorio Astronomico G. S. Vaiana – Piazza Parlamento 1, 90134 Palermo, Italy

Abstract. Coronal astronomy is by now a fairly mature discipline, with a quarter century having gone by since the detection of the first stellar X-ray coronal source (Capella), and having benefitted from a series of major orbiting observing facilities. Several observational charac- teristics of coronal X-ray and EUV emission have been solidly established through extensive observations, and are by now common, almost text-book, knowledge. At the same time the implications of coronal astronomy for broader astrophysical questions (e.g. Galactic structure, stellar formation, stellar structure, etc.) have become appreciated. The interpretation of stellar coronal properties is however still often open to debate, and will need qualitatively new ob- servational data to book further progress. In the present review we try to recapitulate our view on the status of the field at the beginning of a new era, in which the high sensitivity and the high spectral resolution provided by Chandra and XMM-Newton will address new questions which were not accessible before.

Keywords: Stars; X-ray; EUV; Coronae Full paper available at ftp://astro.esa.int/pub/ffavata/Papers/ssr-preprint.pdf

1 Foreword 3 2 Historical overview 4 3 Relevance of coronal astronomy 10 4 Characteristics of stellar coronal X-ray emission 13 4.1 X-ray emission from solar-type stars 13 4.2 X-ray emission from stars at the convective boundary 15 4.3 X-ray emission at the low mass end 18 4.4 X-ray emission from active binaries 24 4.5 Correlation between X-ray activity and rotation 26 5 Open problems in coronal physics 29 5.1 Dynamo mechanisms 29 5.2 Activity cycles 30 6 The structure of stellar coronae 34 6.1 Structuring of the solar corona 34 6.2 Tools for studying the coronal structures 37 7Flares 47 7.1 Typical stellar events 48 7.2 Modeling approaches 55 7.3 Notable results from flare analyses 58 7.4 The Neupert effect 62 8 Activity in the pre-main sequence phase 63 9 Evolution of activity in the main-sequence phase 64

© 2003 Space Science Reviews, 2003 in press (Kluwer Academic Publishers). Printed in the Netherlands.

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9.1 Results from open cluster studies: solar mass stars 67 9.2 Results from open cluster studies: low mass stars 69 9.3 Coronal spectral evolution 70 9.4 Is the age evolution of activity unique? 74 9.5 Chemical abundance effects on coronal emission 75 9.6 Evidence for the existence of old active field stars 77 10 Coronal abundances 79 10.1 The solar case 79 10.2 Initial stellar evidence 79 10.3 Photospheric abundances 85 10.4 Present-day evidence 86 11 Other results from high-resolution spectroscopy 95 11.1 Density 96 11.2 Thermal structuring 102 12 X-ray surveys as a tool for the study of Galactic structure 107 12.1 Constraints on stellar birthrate 109 12.2 Identification of a large scale structure: the Gould Belt 110 13 Conclusions 112 Index of individual objects 131

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