XMM-Newton and the Pleiades - I. Bright coronal sources and the X-ray emission from intermediate-type stars.
journal contributionposted on 08.12.2009, 16:24 by K. R. Briggs, J. P. Pye
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 (LX) up to a factor 10 fainter than previous studies using the ROSAT PSPC. All eight solar-like members have ‘quasi-steady’LX≳ 1029erg s−1, consistent with the known rotation–activity relation and four exhibit flares. Using a hydrodynamic modelling technique, we derive loop half-lengths Graphic for the two strongest flares, on H II 1032 and H II 1100. Near the beginning of its flare, the light curve of H II 1100 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 NH 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, with its decay time being shorter than its rise time and suggestive of Graphic, would be unusual. Spectral modelling of the quasi-steady emission shows a rising trend in coronal temperature from F and slowly rotating G stars to 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 five intermediate-type stars, two are undetected (LX < 4 × 1027erg s−1) and three show X-ray emission with a spectrum and LX consistent with origin from an active solar-like companion.