X-rays from the open cluster NGC 6633

NGC 6633 is a young, open cluster with a similar age to the Hyades and Praesepe, but probably a lower metallicity. We present the results of ROSAT High Resolution Imager observations of an optically selected catalogue of likely members of NGC 6633. 8 out of 51 NGC 6633 members have been detected, with main-sequence spectral types A to G, above a threshold X-ray luminosity of ≈6−12×1028 erg s−1. We find that NGC 6633 does not contain cool stars that are as X-ray luminous as the most active objects in the Hyades and that the median X-ray luminosity of F-G stars in NGC 6633 is less than that in the Hyades, but probably greater than in Praesepe. However, when X-ray activity is expressed as the X-ray to bolometric flux ratio we find that NGC 6633 and the Hyades are very similar and display similar peak levels of coronal activity. We attribute this discrepancy to a number of possible wide binary systems with higher X-ray (and bolometric) luminosities in the Hyades sample and either a low metallicity in NGC 6633, which makes its cool stars both X-ray and bolometrically less luminous at the same colour, or a distance to NGC 6633 that has been underestimated, which would decrease stellar X-ray luminosities without changing X-ray to bolometric flux ratios

The EUV mini-survey with the ROSAT wide field camera

Following a successful launch on 1990 June 1, the ROSAT spacecraft and its payload, consisting of an X-ray and an EUV telescope, underwent two months of in-orbit calibration and detailed performance checks. A preliminary observation of a small section of the sky, carried out over July 11–16 (the ‘mini-survey’) showed all ROSAT systems to be functioning well and has allowed predictions to be made on the ultimate productivity of both ROSAT all-sky surveys. An analysis of the mini-survey data from the UK Wide Field Camera, has revealed 35 EUV sources, including several white dwarf stars, a variety of active cool stars and several other objects. In all, 23 sources have probable optical counterparts. Consideration of the effective exposure and sky coverage in the mini-survey allows the prediction that the recently completed ROSAT all-sky survey will yield in excess of a thousand new EUV sources