From radio to X-ray: The quiescent atmosphere of the dMe Flare Star EV Lac

Abstract

We report on multi-wavelength observations spanning radio to X-ray wavelengths of the M dwarf flare star, EV Lacertae, probing the characteristics of the outer atmospheric plasma from the upper chromosphere to the corona. We detect the star at a wavelength of 2 cm (15 GHz) for the first time. UV and FUV line profiles show evidence of nonthermal broadening, and the velocity width appear to peak at lower temperatures than in the Sun; this trend is confirmedin another active M dwarf flare star. Electron density measurements indicate nearly constant electron pressures between log T =5.2 and 6.4. At higher coronal temperatures, there is a sharp increase of two orders of magnitude in density (ne ~1013 cm?3 at log T =6.9). X-ray, EUV, FUV and NUV spectra constrain the DEM from the upper chromosphere through the corona. The coronal pressures are inconsistent with the assumption of hydrostatic equilibrium, either through EM modeling or application of scaling laws, and imply large conductive loss rates and a large energy input at the highest temperatures. The timescales for radiative and conductive losses in EV Lac?s upper atmosphere imply that significant continued heating must occur for the corona to maintain its quiescent properties. The high frequency radio detection requires the high temperature X-ray-emitting coronal plasma to be spatially distinct from the radio emission source. Length scales in the low-temperature corona are markedly larger than those in the hightemperature corona, further suggestions of an inhomogeneous mixture of thermal and nonthermal coronal plasma.