Are Coronae of Late-Type Stars Made of Solar-like Structures? The X-Ray Surface Flux versus Hardness Ratio Diagram and the Pressure-Temperature Correlation

This work is dedicated to the solar-stellar connection, i.e., the close similarity of the Sun and late-type stars; in particular, this work shows that stellar coronae can be composed of X-ray-emitting structures similar to those present in the solar corona. To this end we use a large set of ROSAT PSPC observations of late-type stars of all spectral types and activity levels and a large set of solar X-ray data collected with Yohkoh SXT. Solar data have been analyzed and formatted to study the Sun as an X-ray star; they include observations of the solar corona at various phases of the solar cycle and data on various kinds of X-ray coronal structures, from flares to the background corona, i.e., the most quiet regions. We use the X-ray surface flux (FX) versus spectral hardness ratio (HR) diagram as a fundamental tool for our study. We find that FX is strongly correlated to HR in stellar coronae, in the solar corona at all phases of the solar cycle, and in the individual solar coronal structures; all the above follow the same law. Schmitt found the same correlation in stellar coronae. We therefore claim that coronae of late-type stars are formed with X-ray structures very similar to those in the Sun, since their behavior is identical to that of the solar coronal structures and of the whole solar corona. The spatial location of the X-ray structures on the star, however, could be very different from those on the Sun. In this scenario, the fraction of the stellar surface covered with active regions and their bright cores increases with activity; the most active stars are brighter and hotter than if they were entirely covered with active regions, so they can be explained only with the additional presence of several flares (or flarelike structures) at any time. On the basis of the FX versus HR correlation, corresponding to FX~T6, we then derive a set of new laws relating the temperature, pressure, volumetric heating, and characteristic loop length of the coronal plasma on all the late-type stars. In addition, individual solar coronal structures and the whole solar corona follow the same laws. These laws also agree with recent findings of higher plasma density at higher temperatures in stellar coronae. We claim that the strong correlation between surface flux and temperature and the laws mentioned above are just the effect of more fundamental physical mechanisms driving the coronal structures of all the late-type stars from the emergence of new magnetic structures to their dispersal and dissipation.