Numerical quantification of the wind properties of cool main sequence stars

As a cool star evolves, it loses mass and angular momentum due to magnetized stellar winds which affect its rotational evolution. This change has consequences that range from the alteration of its activity to influences over the atmosphere of any orbiting planet. Despite their importance, observations constraining the properties of stellar winds in cool stars are extremely limited. Therefore, numerical simulations provide a valuable way to understand the structure and properties of these winds. In this work, we simulate the magnetized winds of 21 cool main-sequence stars (F-type to M-dwarfs), using a state-of-the-art 3D MHD code driven by observed large-scale magnetic field distributions. We perform a qualitative and quantitative characterization of our solutions, analyzing the dependencies between the driving conditions (e.g., spectral type, rotation, magnetic field strength) and the resulting stellar wind parameters (e.g., Alfv\'en surface size, mass loss rate, angular momentum loss rate, stellar wind speeds). We compare our models with the current observational knowledge on stellar winds in cool stars and explore the behaviour of the mass loss rate as a function of the Rossby number. Furthermore, our 3D models encompass the entire classical Habitable Zones (HZ) of all the stars in our sample. This allows us to provide the stellar wind dynamic pressure at both edges of the HZ and analyze the variations of this parameter across spectral type and orbital inclination. The results here presented could serve to inform future studies of stellar wind-magnetosphere interactions and stellar wind erosion of planetary atmospheres via ion escape processe.Comment: 21 pages, 10 figure

Mercury's Na exosphere as seen with very high spectral resolution from the ground, and from space with MESSENGER

International audienceThe MErcury Surface, Space ENvironment, GEochemistry, and Ranging NASA’s spacecraft, known as MESSENGER, flew by Mercury on September 29, 2009. It was the spacecraft’s third and final flyby, before it went into orbit around the planet. The flyby presented a unique trajectory approach and perspective on the planet’s exosphere, not available when in orbit. We present very high spectral resolution ground-based data obtained at the University of Texas McDonald 2.7-m telescope. These data were acquired within hours of the data taken with the Ultraviolet and Visible Spectrometer (UVVS) onboard MESSENGER. Both datasets targeted similar spatial regions, in the polar altitudes of Mercury. We compare the sodium emissions from both measurements in the exosphere. We find that close to the surface, both intensity measurements match, but the intensities fall off differently with altitude, with the MESSENGER data showing an exponential drop off, sharper than that of the ground-based data; an effect that we attribute to atmospheric seeing. In addition, our ground-based data provided Full Width Half Maximum (fwhm) speeds and Doppler shift speeds; our results suggest energetic processes took place in the polar regions on the dusk side of the planet, but arguably on the dawn side as well. We confirm previous conclusions of Leblanc et al. (2008, 2009) where signatures of energetic processes seem to be coupled with high fwhm speeds and intensity peaks. We compare our Doppler shift velocities with previous works, and find agreement within the uncertainties with Potter et al., (2013) on their transit velocity measurements. Although our peak emissions along the terminator vary in structure and in brightness, they do not exhibit distinctive signatures in the intensity profiles at altitudes above the poles, when compared with convolved MESSENGER space data

Simulating the Space Weather in the AU Mic System : Stellar Winds and Extreme Coronal Mass Ejections

Two close-in planets have been recently found around the M-dwarf flare star AU Microscopii (AU Mic). These Neptune-sized planets (AU Mic b and c) seem to be located very close to the so-called "evaporation valley" in the exoplanet population, making this system an important target for studying atmospheric loss on exoplanets. This process, while mainly driven by high-energy stellar radiation, will be strongly mediated by the space environment surrounding the planets. Here we present an investigation of this last area, performing 3D numerical modeling of the quiescent stellar wind from AU Mic, as well as time-dependent simulations describing the evolution of a highly energetic coronal mass ejection (CME) event in this system. Observational constraints on the stellar magnetic field and properties of the eruption are incorporated in our models. We carry out qualitative and quantitative characterizations of the stellar wind, the emerging CMEs, as well as the expected steady and transient conditions along the orbit of both exoplanets. Our results predict extreme space weather for AU Mic and its planets. This includes sub-Alfvenic regions for the large majority of the exoplanet orbits, very high dynamic and magnetic pressure values in quiescence (varying within 10(2)-10(5) times the dynamic pressure experienced by Earth), and an even harsher environment during the passage of any escaping CME associated with the frequent flaring observed in AU Mic. These space weather conditions alone pose an immense challenge for the survival of exoplanetary atmospheres (if any) in this system