Solar and Stellar Astrophysics (astro-ph.SR)

Abstract: The M dwarf star GJ 436 hosts a warm-Neptune that is losing substantial amount of atmosphere, which is then shaped by the interactions with the wind of the host star. The stellar wind is formed by particles and magnetic fields that shape the exo-space weather around the exoplanet GJ 436 b. Here, we use the recently published magnetic map of GJ 436 to model its 3D Alfv\'en-wave driven wind. By comparing our results with previous transmission spectroscopic models and measurements of non-thermal velocities at the transition region of GJ 436, our models indicate that the wind of GJ 436 is powered by a smaller flux of Alfv\'en waves than that powering the wind of the Sun. This suggests that the canonical flux of Alfv\'en waves assumed in solar wind models might not be applicable to the winds of old M dwarf stars. Compared to the solar wind, GJ 436's wind has a weaker acceleration and an extended sub-Alfv\'enic region. This is important because it places the orbit of GJ 436 b inside the region dominated by the stellar magnetic field (i.e., inside the Alfv\'en surface). Due to the sub-Alfv\'enic motion of the planet through the stellar wind, magnetohydrodynamic waves and particles released in reconnection events can travel along the magnetic field lines towards the star, which could power the anomalous ultraviolet flare distribution recently observed in the system. For an assumed planetary magnetic field of $B_p \simeq 2$ G, we derive the power released by stellar wind-planet interactions as $\mathcal{P} \sim 10^{22}$ -- $10^{23}$ erg s$^{-1}$, which is consistent with the upper limit of $10^{26}$ erg s$^{-1}$ derived from ultraviolet lines. We further highlight that, because star-planet interactions depend on stellar wind properties, observations that probe these interactions and the magnetic map used in 3D stellar wind simulations should be contemporaneous for deriving realistic results.

Abstract: WR21 and WR31 are two WR+O binaries with short periods, quite similar to the case of V444 Cyg. The XMM-Newton observatory has monitored these two objects and clearly revealed phase-locked variations as expected from colliding winds. The changes are maximum in the soft band (0.5--2.keV, variations by a factor 3--4) where they are intrinsically linked to absorption effects. The increase in absorption due to the dense WR wind is confirmed by the spectral analysis. The flux maximum is however not detected exactly at conjunction with the O star in front but slightly afterwards, suggesting Coriolis deflection of the collision zone as in V444 Cyg. In the hard band (2.--10. keV), the variations (by a factor of 1.5--2.0) are much more limited. Because of the lower orbital inclinations, eclipses as observed for V444 Cyg are not detected in these systems.

Abstract: The main goal of this work is to evaluate the correlation between Li abundance, age, and mass. Using high-quality ESO/HARPS data (R $\simeq$ 115 000; 270 $\leq$ SNR $\leq$ 1000), we measured Li abundances via spectral synthesis of the 6707.8 \r{A} $^7$Li line in 74 solar twins and analogs. Our joint analysis of 151 Sun-like stars (72 from our sample plus 79 solar twins from a previous study) confirms the strong Li abundance-age correlation reported by other works. Mass and convective envelope size also seem to be connected with Li abundance but with lower significance. We have found a link between the presence of planets and low Li abundances in a sample of 192 stars with a high significance. Our results agree qualitatively with non-standard models, and indicate that several extra transport mechanisms must be taken into account to explain the behaviour of Li abundance for stars with different masses and ages.