CORRELATING CORONAL TEMPERATURE AND GRAVITATIONAL POTENTIAL: A TEST OF THE NONTHERMAL BOUNDARY HYPOTHESIS

摘要

It has been suggested that a nonthermal boundary condition can account for the existence of a coronal temperature inversion in stars. The nonthermal hypothesis makes falsifiable predictions of the maximum coronal temperature obtainable. This maximum temperature corresponds to the depth of the potential well at the boundary, which is effectively R_*, and so varies as M_*/R_*. Here, we compare this prediction with one-temperature fits to single stars in Einstein data. We find that the coronal temperatures in dwarfs are consistent with a nonthermal boundary condition, but the nonthermal hypothesis cannot readily account for the coronal temperatures of class III giants. However, interpreting the emission from the noncompliant giants in alternate terms suggests that a dwarf companion may be the source of the X-rays. More generally, gravitational potential is found to be a useful quantity to ordering coronal observations: (1) the "dividing line" near K4, beyond which giant star coronae are not observed, corresponds to a rather precipitous factor of 10 drop in the strength of the stellar gravitational potential; and (2) M dwarfs and A stars of class III-V, whose coronae have been somewhat of a puzzle, all have relatively deep gravitational potentials.