X-Ray Observations of Stellar Coronae.

The Einstein X-Ray Observatory (HEAO-2) has been used to search for X-ray emission from a wide variety of stars in our galaxy. A significant fraction of the data was obtained via the 8.5 magnitude stellar survey, which is reported here. This survey searches for X-ray emission from all stars brighter than visual magnitude 8.5 that serendipitously fell into the Imaging Proportional Counter field-of-view of the Einstein Observatory. The survey includes 227 separate fields that contain a total of 276 stars with V < 8.5. A wide range of spectral types and luminosity classes are represented. X-Ray emission in excess of 3-sigma above local background was discovered from 33 stars; 3-sigma upper limits have been determined for the remaining 243 stars. Comparison of X-ray source detection statistics with the expected frequency of stars brighter than V = 8.5 as a function of spectral type and luminosity class shows that the present visual magnitude limited survey can define the X-ray luminosity function for dwarf F stars and provides constraints for the high luminosity tails of the X-ray luminosity functions for other types of stars constructed from other Einstein observations. A second survey reported here is called the Gliese survey. This survey consists of all nearby stars listed in either of 2 catalogues of the nearest stars that accidentally fell into any CFA (Center for Astrophysics) Einstein observation. A total of 29 nearby stars are included in the Gliese survey. Ten were detected, including 4 dwarf M stars and 3 dwarf K stars. Late dwarf K and dwarf M stars are not effectively sampled by the 8.5 survey because most are fainter than 8.5 magnitude. The results of the 8.5 and Gliese survey have been integrated with those of four other Einstein surveys. In total, X-ray emission has been detected from 156 stars, with most of these representing initial discoveries. A correlation of X-ray luminosity with spectral type (soft X-ray H-R Diagram) is presented. Most of the observations can be explained by the presence of hot coronae around stars of nearly every spectral type. However, the data is at variance in many respects to existing theoretical coronal models based on acoustic flux dissipation. The consequences of these observations in terms of the presence of stellar outer convection zones, surface magnetic fields, and rotation rates are discussed.