Evolution of Stellar Activity in Early Post-Main-Sequence Phases

TL;DRAbstract

We present preliminary observational evidences on the variation of the activity level in late type stars, during the evolutionary phases on the main sequence and beyond. We have selected a sample of 51 stars (Fig. 1), lying mostly along evolutionary tracks between 1.3 and 1.7 solar masses, which have been observed in soft X-rays with the Einstein Observatory, and in UV with IUE (Maggio et al. 1990; Haisch et al .1990). Two ROSAT targets, and four new IUE observations are also included. We find that the radiative emission from the outer atmospheres of stars with M > 1.6 M ⊙ seems to behave differently than for stars with lower mass. On the main sequence, the X-ray luminosity of most stars with B-V < 0.42 (spectral type F3) is relatively low, at L x ~ 3 × 10 28 erg s -1 (Fig. 2). In the early evolutionary phases beyond the main sequence, the X-ray luminosity of the higher mass stars tend to increase sistematically up to ~ 10 30 erg s -1 , while the lower mass stars show an initial

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