A VOLUME-LIMITED ROSAT SURVEY OF EXTREME-ULTRAVIOLET EMISSION FROM ALL NONDEGENERATE STARS WITHIN 10 PARSECS

We report the results of a volume-limited ROSAT Wide Field Camera (WFC ) survey of all nondegenerate stars within 10 pc. Of the 220 known sta r systems within 10 pc, we find that 41 are positive detections in at least one of the two WFC filter bandpasses (S1 and S2), while we consi der another 14 to be marginal detections. We compute X-ray luminositie s for the WFC detections using Einstein Imaging Proportional Counter ( IPC) data, and these IPC luminosities are discussed along with the WFC luminosities throughout the paper for purposes of comparison. Extreme ultraviolet (EUV) luminosity functions are computed for single stars of different spectral types using both S1 and S2 luminosities, and the se luminosity functions are compared with X-ray luminosity functions d erived by previous authors using IPC data. We also analyze the S1 and S2 luminosity functions of the binary stars within 10 pc. We find that most stars in binary systems do not emit EUV radiation at levels diff erent from those of single stars, but there may be a few EUV-luminous multiple-star systems which emit excess EUV radiation due to some effe ct of binarity. In general, the ratio of X-ray luminosity to EUV lumin osity increases with increasing coronal emission, suggesting that coro nally active stars have higher coronal temperatures. We find that our S1, S2, and IPC luminosities are well correlated with rotational veloc ity, and we compare activity-rotation relations determined using these different luminosities. Late M stars are found to be significantly le ss luminous in the EUV than other late-type stars. The most natural ex planation for this result is the concept of coronal saturation-the ide a that late-type stars can emit only a limited fraction of their total luminosity in X-ray and EUV radiation, which means stars with very lo w bolometric luminosities must have relatively low X-ray and EUV lumin osities as well. The maximum level of coronal emission from stars with earlier spectral types is studied also. To understand the saturation levels for these stars, we have compiled a large number of IPC luminos ities for stars with a wide variety of spectral types and luminosity c lasses. We show quantitatively that if the Sun were completely covered with X-ray-emitting coronal loops, it would be near the saturation li mit implied by this compilation, supporting the idea that stars near u pper limits in coronal activity are completely covered with active reg ions.