Surveying the Hyades with ROSAT

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Session 110 -- Open Clusters
Display presentation, Saturday, January 15, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[110.01] Surveying the Hyades with ROSAT

R.A. Stern (LPARL), J.H.M.M. Schmitt, P.T. Kahabka (MPE/Garching)

During the course of the ROSAT All Sky Survey (RASS), the ROSAT PSPC surveyed the entire Hyades cluster region, over 30$^\circ \times$30$^\circ$ of the sky. Analysis of the RASS deta reveals over 180 probable or possible cluster members which are detected as X-ray sources with an X-ray luminosity L$_x$ $\ga$ 1--2$\times$10$^{28}$ erg s$^{-1}$. The detection rate for the F8-G8 (solar-type) stars is over 90\%. All four Hyades giants are also detected in X-rays, two of which are among the X-ray brightest Hyads, and two are among the faintest. Many objects that are anomalously X-ray bright for their spectral type turn out to be close binaries, including some BY Dra systems. A comparison of 56 Hyades members detected in both the present study and in the Einstein Observatory surveys over a decade ago indicates little evidence for long term variability beyond counting rate statistics, with the giant stars being notable exceptions. The main sequence variation in the X-ray luminosity function and its increasing dispersion for later spectral types will be discussed in the context of angular momentum evolution in open stellar clusters.

R.A.S. was supported in part by NASA Contract NAS5-32070 and the Lockheed Independent Research Program.

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