2010A&A...516A..38R

We investigate a Chandra observation of the remarkable planet-bearing A5V star HR 8799, more precisely classified as a kA5hF0mA5 star. We search for intrinsic X-ray emission, a diagnostic for studying the possible activity of intermediate-mass stars. In the regime of mid/late A-type stars a strong decline in magnetic activity occurs towards hotter stars because of the vanishing of the outer convection zone. We clearly detect HR 8799 at soft X-ray energies with the ACIS-S detector in a 10 ks exposure; minor X-ray brightness variability is present during the observation. The coronal plasma is described well by a model with a temperature of around 3MK and an X-ray luminosity of about L_X=1.3x10²⁸erg/s in the 0.2-2.0keV band, corresponding to an activity level of log L_X/L_bol≃-6.2. Altogether, these findings point to a rather weakly active and given a RASS detection, long-term stable X-ray emitting star. The X-ray emission from HR 8799 resembles those of late A/early F-type stars, in agreement with its classification on the basis of hydrogen lines and its effective temperature determination and thus resolving the apparent discrepancy with the standard picture of magnetic activity that predicts mid A-type stars to be virtually X-ray dark.