EINSTEIN-OBSERVATORY CORONAL TEMPERATURES OF LATE-TYPE STARS

We present the results of an extensive survey of coronal temperatures of late-type stars using the Einstein Observatory Imaging Proportional Counter and discuss various biases that may arise during the process of determining X-ray temperatures from low-resolution, broad-band data. From the Bright Star Catalog and its Supplement and the Woolley Catalog, we derive a sample consisting of 130 late-type stars which were observed with sufficiently high signal-to-noise ratio and whose B - V colors are redder than 0.0. Our spectral analysis shows that the frequently found one- and two-temperature descriptions are largely influenced by the signal-to-noise ratio of the data, and that models employing continuous emission measure distributions can provide equally adequate and physically more meaningful and more plausible descriptions. For four groups of stars (main-sequence F and G stars, main-sequence M stars, yellow giants, and RS CVn systems), we find intrinsic differences in their differential emission measure distributions. In general, M dwarfs - whenever observed with sufficient signal-to-noise ratio - show evidence for high-temperature gas (T > 107 K) in conjunction with lower temperature (T ∼ 3 × 106 K) material, while main-sequence stars of spectral type F and G have the high-temperature component either absent or very weak. For the majority of the giant stars we studied (all of which have X-ray luminosities above the median for this class), there is evidence of very hot coronae (T > 107 K), but without the lower temperature component which appears in the dwarf stars. As a group, RS CVn systems also show evidence for extremely hot coronae (T > 107.5 K), sometimes with no accompanying lower temperature material; in this sense, they resemble the behavior of the giants, but with significantly higher coronal temperatures.