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Astron. Astrophys. 361, 614-628 (2000)

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Evolution of X-ray activity of 1-3 [FORMULA] late-type stars in early post-main-sequence phases

N. Pizzolato 1, A. Maggio 2 and S. Sciortino 2

1 Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, Sezione di Astronomia, Piazza del Parlamento 1, 90134 Palermo, Italy (nicola@oapa.astropa.unipa.it)
2 Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy (maggio@oapa.astropa.unipa.it)

Received 14 March 2000 / Accepted 7 July 2000


We have investigated the variation of coronal X-ray emission during early post-main-sequence phases for a sample of 120 late-type stars within 100 pc, and with estimated masses in the range 1-3 [FORMULA], based on Hipparcos parallaxes and recent evolutionary models. These stars were observed with the ROSAT/PSPC, and the data processed with the Palermo-CfA pipeline, including detection and evaluation of X-ray fluxes (or upper limits) by means of a wavelet transform algorithm. We have studied the evolutionary history of X-ray luminosity and surface flux for stars in selected mass ranges, including stars with inactive A-type progenitors on the main sequence and lower mass solar-type stars. Our stellar sample suggests a trend of increasing X-ray emission level with age for stars with masses [FORMULA], and a decline for lower-mass stars. A similar behavior holds for the average coronal temperature, which follows a power-law correlation with the X-ray luminosity, independently of their mass and evolutionary state. We have also studied the relationship between X-ray luminosity and surface rotation rate for stars in the same mass ranges, and how this relationships departs from the [FORMULA] law followed by main-sequence stars. Our results are interpreted in terms of a magnetic dynamo whose efficiency depends on the stellar evolutionary state through the mass-dependent changes of the stellar internal structure, including the properties of envelope convection and the internal rotation profile.

Key words: stars: coronae – stars: late-type – X-rays: stars – stars: activity – stars: evolution

Send offprint requests to: N. Pizzolato

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Online publication: October 2, 2000