ALBERT

Description: The relation of activity to rotation in M dwarfs is of high astrophysical interest because it provides observational evidence of the stellar dynamo, which is poorly understood for low-mass stars, especially in the fully convective regime. Previous studies have shown that the relation of X-ray activity to rotation consists of two different regimes: the saturated regime for fast-rotating stars and the unsaturated regime for slowly rotating stars. The transition between the two regimes lies at a rotation period of ∼10 d. We present here a sample of 14 M dwarf stars observed with XMM-Newton and Chandra, for which we also computed rotational periods from Kepler Two-Wheel (K2) Mission light curves. We compiled X-ray and rotation data from the literature and homogenized all data sets to provide the largest uniform sample of M dwarfs (302 stars) for X-ray activity and rotation studies to date. We then fit the relation between Lx − Prot using three different mass bins to separate partially and fully convective stars. We found a steeper slope in the unsaturated regime for fully convective stars and a nonconstant Lx level in the saturated regime for all masses. In the Lx/Lbol − RO space we discovered a remarkable double gap that might be related to a discontinuous period evolution. Then we combined the evolution of Prot predicted by angular momentum evolution models with our new results on the empirical Lx − Prot relation to provide an estimate for the age decay of X-ray luminosity. We compare predictions of this relationship with the actual X-ray luminosities of M stars with known ages from 100 Myr to a few billion years. We find remarkably good agreement between the predicted Lx and the observed values for partially convective stars. However, for fully convective stars at ages of a few billion years, the constructed Lx-age relation overpredicts the X-ray luminosity because the angular momentum evolution model underpredicts the rotation period of these stars. Finally, we examine the effect of different parameterizations for the Rossby number (RO) on the shape of the activity-rotation relation in Lx/Lbol − RO space, and we find that the slope in the unsaturated regime and the location of the break point of the dual power-law depend sensitively on the choice of RO.

Description: Aims. In this paper we present a deep X-ray observation of the nearby M dwarf GJ 357 and use it to put constraints on the atmospheric evolution of its planet, GJ 357 b. We also analyse the systematic errors in the stellar parameters of GJ 357 in order to see how they affect the perceived planetary properties. Methods. By comparing the observed X-ray luminosity of its host star, we estimate the age of GJ 357 b as derived from a recent XMM-Newton observation (log Lx [erg s−1] = 25.73), with Lx− age relations for M dwarfs. We find that GJ 357 presents one of the lowest X-ray activity levels ever measured for an M dwarf, and we put a lower limit on its age of 5 Gyr. Using this age limit, we performed a backwards reconstruction of the original primordial atmospheric reservoir. Furthermore, by considering the systematic errors in the stellar parameters, we find a range of possible planetary masses, radii, and densities. Results. From the backwards reconstruction of the irradiation history of GJ 357 b’s we find that the upper limit of its initial primordial atmospheric mass is ~38 M⊕. An initial atmospheric reservoir significantly larger than this may have survived through the X-ray and ultraviolet irradiation history, which would not be consistent with current observations that suggest a telluric composition. However, given the relatively small mass of GJ 357 b, even accreting a primordial envelope ≳10 M⊕ would have been improbable as an unusually low protoplanetary disc opacity, large-scale migration, and a weak interior luminosity would have been required. For this reason, we discard the possibility that GJ 357 b was born as a Neptunian- or Jovian-sized body. In spite of the unlikelihood of a currently existing primordial envelope, volcanism and outgassing may have contributed to a secondary atmosphere. Under this assumption, we present three different synthetic IR spectra for GJ 357 b that one might expect, consisting of 100% CO2, 100% SO2, and 75% N2, 24% CO2 and 1% H2O, respectively. Future observations with space-based IR spectroscopy missions will be able to test these models. Finally, we show that the uncertainties in the stellar and planetary quantities do not have a significant effect on the estimated mass or radius of GJ 357 b.