Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...501..624g&link_type=abstract
Astrophysical Journal v.501, p.624
Astronomy and Astrophysics
Astronomy
16
Stars: Evolution, Galaxy: Open Clusters And Associations: Individual Alphanumeric: Ic 4665, Stars: Rotation, X-Rays: Stars
Scientific paper
We present the results of a joint ROSAT High Resolution Imager (HRI) and optical investigation of the open cluster IC 4665. The ROSAT data contains detections for 28 stellar sources in the field, including 22 cluster members and candidate members spanning the color range -0.18 <= (B-V)0 <= 1.63 (~B3-M3). Upper limits are given for the remaining members (or candidate members) in the HRI field. Keck HIRES spectra have been obtained that yield radial and rotational velocity measures, respectively, for faint, low-mass candidate members located within the field of the ROSAT HRI observation. In addition, photometry of possible optical counterparts to previously uncataloged X-ray sources in the HRI field is presented. The 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 the Pleiades and alpha Persei. In particular a maximum in normalized X-ray luminosity of log (LX/Lbol) ~= -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 with the earlier estimate by Prosser & Giampapa that the age of IC 4665 is 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 remaining objects are apparently single stars. The level of intrinsic X-ray emission observed in the rapidly rotating (v sin i > 200 km s-1), single B stars is consistent with an origin due to shock heating of the ambient medium by radiatively driven, rotationally enhanced winds. On the 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 (LX/Lbol) > -6.0 are likely due to intrinsic emission associated with the B star itself rather than emission from a late-type secondary. Finally, our results suggest that the initial angular momentum distributions and the subsequent evolution of rotation, LX, and (LX/Lbol) occur in practically identical fashions in clusters with ages <~100 Myr.
Fleming Thomas Anthony
Giampapa Mark S.
Prosser Charles F.
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