Chandra Observations of the Pleiades Open Cluster: X-ray Emission from Some Late-B to Early-F stars

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  • 8 April 2002
Abstract
We present the analysis of a 38.4 ks and a 23.6 ks observation of the core of the Pleiades open cluster. The Advanced CCD Imaging Spectrometer (ACIS-I) on board the Chandra X-ray Observatory has revealed 99 X-ray sources in a 17'X17' region, including eighteen (of 23) Pleiades members. Fifty-seven sources have no optical or near-infrared counterparts to limiting magnitudes V=22.5 and K=13.5. The unidentified {\it Chandra} sources are probably background AGN and not stars. The Chandra field of view contains seven intermediate mass (B6-F4) cluster members. Four of the intermediate mass members, HII 980 (B6 IV), HII 956 (A7 V), HII 1338 (F3 V), and HII 1122 (F4 V), are X-ray luminous, have soft X-ray spectra, and show no obvious signs of flaring. K- and M-type cluster members with comparable X-ray luminosities have harder X-ray spectra and exhibit moderate flares. For non-flaring K- and M-type stars, L_X is 1-2 orders of magnitude lower than the B- and A-type stars. HII 1284 (A9 V), has X-ray properties comparable to non-flaring K-type stars, and HII 1362 (A7 V) and HII 1375 (A0 V) are not detected. Despite the low number statistics, some late-B to early-F stars in our Pleiades sample (HII 980, HII 956, HII 1122, and HII 1338) appear to be intrinsic X-ray sources. Some late-B to early-F stars, like HII 1284, may have late-type companions, and some, like HII 1362 and HII 1375, may be single, non-X-ray emitting stars. X-ray spectra and light curves of a larger sample of stars are needed to confirm this trend. The X-ray properties of five candidate Pleiades members confirm their cluster membership.

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