CORONAL X-RAY SOURCES IN THE HYADES - A 40-KILOSECOND ROSAT POINTING

We present results of a 40 ks ROSAT pointed observation of the Hyades cluster. The limiting L(x) is almost-equal-to 2 x 10(27) ergs s-1 at field center, increasing to almost-equal-to 2 x 10(28) ergs s-1 at 40' off-axis. This represents the most sensitive X-ray observation to date in the Hyades region. More than 30 sources have been detected in the PSPC standard processing, of which 15 are Hyades members, five are cluster candidates that are likely nonmembers, four are foreground or background stars, and the remainder are unidentified. One Hyad, VB 173 (= VA 276), lies in a confused region, but is detected as a distinct source in the soft band only (E less than or similar to 0.5 keV). We report upper limits for four other Hyades members in our field, all M dwarfs. Of the 16 Hyades detections, two represent the optically faintest members seen to date in X-rays: VA 260 (V = 16.68) and VA 368 (V = 16.25). These are both M dwarfs of mass almost-equal-to 0.15-0.2 M. and are fully convective stars according to current theory. Analysis of X-ray light curves using 1 ks bins indicates some variability in the strongest sources and a possible flare in VA 383. Two Hyades stars, VB 141 and VB 71, were also detected with the co-aligned WFC EUV instrument. VB 141, the second brightest X-ray source in the Hyades, remains an enigma: a rapidly rotating F0 star with a fainter, long-period companion, this object has an X-ray spectrum indicative of strong coronal activity. X-ray pulse-height analysis demonstrates that coronal models with at least two temperatures are required for most of the stronger X-ray sources. The ROSAT X-ray spectra generally require higher temperatures for the hotter component in the M dwarfs compared to the F-G dwarfs.