Coronal Loops Heating in the Atmosphere of the Ad Leo Red Dwarf

Аннотация

We study the possible origin of long-lasting soft X-ray flares on the AD Leo star, which were observed onboard the Extreme Ultraviolet Explorer (EUVE) spacecraft for the period of 1993-2000 [1]. These flares have relatively long rise and decay times of the radiation intensity (τ_R ≈ 10⁴ s and τ_d ≈ 5 · 10³, respectively), as well as a relatively large emission measure EM ≈ 10⁵¹cm⁻³, which exceeds by 1–3 orders of magnitude the emission measure of soft X-ray flares on the Sun. Assuming that the radiation appears in magnetic loops and basing on the observed values of the emission measure and radiation decay time, the authors of [1] determined the typical length \( \overline{l}\approx 1.5\cdot {10}^{10} cm \), electron number density \( \overline{n}\approx 3\cdot {10}^{11}c{m}^{-3} \), and plasma temperature \( \overline{T}\approx 2.5\cdot 107 \) K of the loops. This paper considers plasma heating due to dissipation of the electric currents in the coronal magnetic loops of the star induced by the photospheric convection. The large inductance of the loop as an equivalent electric circuit determines the long time of the current rise in the source and explains the observed time of plasma heating and the rise time of the X-ray radiation intensity. It is shown that the parameters of the X-ray sources in the AD Leo atmosphere agree with the parameters calculated under the assumption of simultaneous emission of a great number of loops (about 50) with electric currents greater than 10¹³ A, which exceeds the electric currents in the solar coronal magnetic loops by 1–3 orders of magnitude. Such an exceeding can be related to the higher photospheric convection velocities on the late-type stars compared with the Sun.