M Dwarf Coronae in the Hyades: ROSAT AO-1/2 Results
Abstract
X-ray observations of red dwarfs in the Hyades cluster provide one of the best opportunities to study magnetic activity in a coeval stellar population. A program of long duration ( ~ 20 - 40 ks), pointed observations with the ROSAT PSPC is in progress; so far, 7 fields have been completed. A preliminary analysis indicates that 51 Hyads out of 79 catalogued have been detected, including 24 out of 38 Hyades dM-stars from catalogued Hyades to V ~ 16-17. We detect 4 out of 5 dM's with V > 16. The long pointed PSPC exposures yield a sensitivity improvement over the ROSAT all-sky survey of more than a factor 5, thus enabling us to probe the X-ray luminosity function at the faint end of the Hyades main sequence to L_X ~ 6 times 10(27) \ erg\ s(-1) (for VA 203, V=16.7). The mean and median values of L_X for the M dwarfs, constructed using detections and upper limits, are both ~ 1.6 times 10(28) \ erg\ s(-1) . The coolest stars detected in X-rays (VA 203 and VA 260, at R-I ~ 1.4) are probably fully convective according to standard stellar models. Comparisons of the ROSAT X-ray data with earlier Hα observations obtained by Stauffer et al. (1991) indicate a clear correlation of X-ray surface flux and excess Hα surface flux. This work was supported in part by NASA Contract NAS5-31828 and the Lockheed Independent Research Program and in the UK by the SERC.
- Publication:
-
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1992
- Bibcode:
- 1992AAS...181.7204S