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

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 M_sun, 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 M > 1.5 M_sun, 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 L_x ~ v_rot² 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.