Activity of M-dwarf L 98-59 and Atmospheric Escape of its Exoplanets

Abstract

A study of the activity of a bright M-dwarf L 98-59, located at a distance of 10.6 pc from Earth is reported. The star has a multiplanetary system, where one of the planets, L 98-59 b, is a rocky planet with a mass not exceeding the mass of Venus. It was found that the level of chromospheric activity of L 98-59 does not exceed the average value characteristic of other cold dwarfs with similar color index values $(B-V)$. This level is below solar, the mean $\log R{{}^{\prime}}_{\textrm{HK}}$ parameter for L 98-59 is equal to $-$5.297 dex. The data about the parent star and its activity was used to estimate the magnitude of the atmospheric escape of five planets (including the planet candidate 05). To calculate the loss of matter from the planetary atmospheres (without a detailed modeling of the processes in the ‘‘star–planet’’ system), we applied an approximation formula corresponding to the energy-limited atmospheric loss model. Estimates of the XUV photon flux—extreme ultraviolet (EUV) and X-ray photons for the medium and high activity levels, as well as the estimates of the tidal parameter $K_{\textrm{tide}}(\xi)$ were found. Variations of the $\dot{M}$ parameter have been specified for five planets of the L 98-59 system, taking into account the possible activity level variations of the star. The $\dot{M}$ parameter variations for planets c–e range from $7.6\times 10^{6}$ g s${}^{-1}$ to $3.8\times 10^{7}$ g s${}^{-1}$ (average activity level) and $1.2$–$5.7\times 10^{7}$ g s${}^{-1}$ (high activity level). For the planet L 98-59 b, the an analogue of Venus, we got the following estimates: $1.3\times 10^{7}$ and $2.0\times 10^{7}$ g s${}^{-1}$ (for the intermediate and high activity levels, respectively). It can be assumed that the relatively modest outflow of matter from their atmospheres $\dot{M}$, despite the proximity of the planets to their star, is due to a small flux of XUV photons from the star due to the low level of its activity.