X-ray emission of late-type stars

A theoretical estimate of the dependence of the X-ray coronal flux from late-type stars on stellar parameters is given and compared with observations. The theoretical approach is based on the assumptions that magnetic energy in coronal loops is converted into thermal energy and that the amplitude of the emerging magnetic field generated in the deepest part of the convection zone by an aw-dynamo mechanism is limited by buoyancy. Using simple physics, an approximate relationship is derived linking the X-ray surface flux to the mass, the angular velocity of rotation, and the depth of the convection zone of the star. When this relationship is calibrated with the Sun, the theory predicts a trend in agreement with observations, except for K stars, within the uncertainties of the measured rotationaI velocities. Rotation, convection, and magnetic field appear, therefore, to be the basic ingredients of the X-ray coronal emission from late-type stars.