DISAPPEARANCE OF CORONAL X-RAY-EMISSION IN STARS WITH COOL DENSE WINDS

THE Einstein observatory survey of cosmic X-ray sources a decade ago showed that coronae were common among diverse types of star, and were in many cases more energetic than the Sun's corona. Such coronae seemed, however, to disappear abruptly across a 'dividing line' in the Hertzsprung-Russell (H-R) diagram describing the evolution of intermediate-mass stars towards the red giant phase1-5. Here we use results from the Rosat all-sky survey, which increases by an order of magnitude the number of X-ray stars, to show that the dividing line is not an artefact of poor sampling. Optical and ultraviolet observations show that the dividing line in the H-R diagram coincides approximately with the onset of cool, massive stellar winds6-15, but we show that these winds are not sufficiently dense for simple X-ray absorption to be the cause of the disappearance of coronal emission. We conclude, therefore, that the dividing line represents a true evolutionary transition in these stars, at which the hot coronae are replaced by cool winds.