X-RAY ACTIVITY IN THE OPEN CLUSTER IC-4665

We present the results of a joint ROSAT High Resolution Imager (HRI) a nd optical investigation of the open cluster IC 4665. The ROSAT data c ontains detections for 28 stellar sources in the held, including 22 cl uster members and candidate members spanning the color range -0.18 les s than or equal to (B-V)(0) less than or equal to 1.63 (similar to B3- M3). Upper limits are given for the remaining members (or candidate me mbers) in the HRI field. Keck HIRES spectra have been obtained that yi eld radial and rotational velocity measures, respectively, for faint, low-mass candidate members located within the held of the ROSAT HRI ob servation. In addition, photometry of possible optical counterparts to previously uncataloged X-ray sources in the HRI held is presented. Th e trends in X-ray properties With B-V color in IC 4665 are found to be quite similar to that for other, more nearby young clusters such as t he Pleiades and alpha Persei. In particular a maximum in normalized X- ray luminosity of log (L-X/L-bol) similar or equal to -3 is observed, beginning in the color range of(B-V)(0) = 0.7-0.8. This is similar to the corresponding color range among Pleiades members, in agreement wit h the earlier estimate by Presser & Giampapa that the age of IC 4665 i s similar to the age of the Pleiades. The correlation of rotation and X-ray emission levels is consistent with that in other young clusters. Among the high-mass stars in IC 4665, five B stars are detected as X- ray sources. Of these, one is a spectroscopic binary while the remaini ng objects are apparently single stars. The level of intrinsic X-ray e mission observed in the rapidly rotating (v sin i > 200 km s(-1)), sin gle B stars is consistent with an origin due to shock heating of the a mbient medium by radiatively driven, rotationally enhanced winds. On t he basis of these observations and the results for other clusters, we argue that observed levels of X-ray emission in high-mass stars of log (L-X/L-bol) > - 6.0 are likely due to intrinsic emission associated w ith the B star itself rather than emission from a late-type secondary. Finally, our results suggest that the initial angular momentum distri butions and the subsequent evolution of rotation, L-X, and (L-X/L-bol) occur in practically identical fashions in clusters with ages less th an or similar to 100 Myr.