XMM - Newton and the Pleiades – I : Bright coronal sources and the X - ray emission from intermediate - type stars

We present results of X-ray spectral and timing analyses of solar-like (spectral types F5–K8) and intermediate-type (B4–F4) Pleiads observed in a 40-ks XMM-Newton EPIC exposure, probing X-ray luminosities (L X) up to a factor 10 fainter than previous studies using the ROSAT PSPC. All 8 solar-like members have " quasi-steady " L X > ∼ 10 29 erg s −1 consistent with the known rotation–activity relation, and 4 exhibit flares. Using a hydrodynamic modelling technique we derive loop half-lengths L < ∼ 0.5 R ⋆ for the two strongest flares, on HII 1032 and HII 1100. Near the beginning of its flare, HII 1100's lightcurve shows a feature with a profile suggestive of a total occultation of the flaring loop. Eclipse by a substellar companion in a close orbit is possible but would seem an extraordinarily fortuitous event; absorption by a fast-moving cloud of cool material requires N H at least two orders of magnitude greater than any solar or stellar prominence. An occultation may have been mimicked by the coincidence of two flares, though the first, its decay time shorter than its rise time and suggestive of L ∼ 0.02 R ⋆ , would be unusual. Spectral modelling of the quasi-steady emission shows a rising trend in coronal temperature from F and slowly-rotating G stars through K stars to fast-rotating G stars, and a preference for low coronal metallicity. These features are consistent with those of nearby solar-like stars, although none of the three stars showing " saturated " emission bears the significant component at 2 keV seen in the saturated coronae of AB Dor and 47 Cas. Of 5 intermediate-type stars, 2 are undetected (L X < 4 × 10 27 erg s −1) and 3 show X-ray emission with a spectrum and L X consistent with origin from an active solar-like companion. Key words: X-rays: stars – stars: activity – stars: coronae – stars: early-type – stars: late-type – open clusters and associations: individual: the Pleiades