Chandra observations of the Pleiades open cluster:: X-ray emission from late B- to early F-type binaries

We present the analysis of a 38.4 and 23.6 ks observation of the core of the Pleiades open cluster. The Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory detected 99 X-ray sources in a 17' x 17' region, including 18 of 23 Pleiades members. Five candidate Pleiades members have also been detected, confirming their cluster membership. Fifty-seven sources have no optical or near-infrared counterparts to limiting magnitudes V = 22.5 and J = 14.5. The unidentified X-ray sources are probably background active galactic nuclei and not stars. The Chandra field of view contains seven intermediate-mass cluster members. Five of these, HII 980 ( B6+G), HII 956 (A7+F6), HII 1284 (A9+K), HII 1338 (F3+F6), and HII 1122 (F4+K), are detected in this study. All but HII 1284 have high X-ray luminosity and soft X-ray spectra. HII 1284 has X-ray properties comparable to non flaring K-type stars. Since all five stars are visual or spectroscopic binaries with X-ray properties similar to F-G stars, the late-type binary companions are probably producing the observed coronal X-ray emission. Strengthening this conclusion is the nondetection by Chandra of two A stars, HII 1362 (A7, no known companion) and HII 1375 (A0+A SB) with X-ray luminosity upper limits 27-54 times smaller than HII 980 and HII 956, the B6-A7 stars with cooler companions. Despite the low number statistics, the Chandra data appear to confirm the expectation that late B and A stars are not strong intrinsic X-ray sources. The ACIS spectra and hardness ratios suggest a gradual increase in coronal temperature with decreasing mass from F4 to K. M stars appear to have somewhat cooler coronae than active K stars.