X-ray activity and evolutionary status of late-type giants

We study the evolution of stellar activity in a volumelimited sample of single giants within 35 pc distance from the Sun as measured by the amount of soft X-ray emission. This sample of 36 stars is assumed to be complete for absolute magnitude MV <∼ 3.0 and for X-ray luminosities Lx >∼ 1.5 × 1028 erg s−1. We use ROSAT data to determine stellar activity, Hipparcos parallaxes to place stars into the HRD, and the empirically well tested evolutionary code by P. Eggleton (see Pols et al. 1998) together with Kurucz colour tables to derive individual masses and ages. Based on more X-ray data and much improved HR diagram positions, we confirm the suggestion by Hünsch & Schröder (1996), that stellar activity evolution is strongly coupled to stellar mass and that it is a very common feature among giants with M >∼ 1.3M . Most pointed ROSAT observations on the giant branch (GB) and also in the “K giant clump” (with masses betweeen about 1.3 and 2.3 M ) resulted in detections at typically solar levels. This indicates that magnetic activity mostly (for M >∼ 1.3M ) even survives the He-flash and, possibly, also persists on the asymptotic giant branch. The more massive stars (>∼ 3M ) show even a larger amount of activity in their advanced evolutionary stages (blue loop giants).