The corona of the dMe flare star AD Leo

Abstract

We have studied the X-ray emission (both the quiescent component and the flares) of the dM3e star AD Leo, analyzing the Einstein IPC, ROSAT PSPC and ASCA SIS observations. Using a consistent method which explicitly considers sustained heating we have analyzed six flares with sufficient statistics, deriving constraints on the physical parameters of the flaring regions. In all cases the flaring loops are likely compact (L approx 0.3 R*), and confined to a rather narrow range of sizes, incompatible with the large (L >= R*) tenuous loops claimed by previous analyses of flares on AD Leo and other similar stars. The flaring loops appear to have a larger cross section (beta = r/L approx. 0.3) than customarily assumed (e.g. beta 0.1). All flares show evidence of significant heating during the decay phase. Although the derived peak pressures are high (up to P approx. 10^4 dyne/cm^2) with a peak temperature of approx. 50 MK, the magnetic fields required to confine such loops and to produce the observed flare luminosity are relatively modest (B approx. 1 to 2 kG) and fully compatible with the photospheric magnetic fields measured in several flare stars. If the narrow range of loop sizes obtained is extrapolated to the quiescent structures responsible for the active corona, the latter can be naturally scaled up from the solar case through a modest (a factor of 10) increase in pressure in otherwise solar-like active structures with a small surface filling factor (approx 5%). The quiescent component of the corona shows no evidence for abundance peculiarities with respect to the photosphere, and the quiescent coronal luminosity is remarkably constant (with variations of less than a factor of 2) across the almost 20 yr span of the observations discussed here.