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Register a new userNonlinear Alfven Wave Model of Stellar Coronae and Winds from the Sun to M dwarfs
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M dwarfs atmosphere and wind is expected to be highly magnetized. The nonlinear propagation of Alfven wave could play a key role in both heating the stellar atmosphere and driving the stellar wind. Along this Alfven wave scenario, we carried out the one-dimensional compressive magnetohydrodynamic (MHD) simulation about the nonlinear propagation of Alfven wave from the M dwarfs photosphere, chromosphere to the corona and interplanetary space. Based on the simulation results, we develop the semi-empirical method describing the solar and M dwarfs coronal temperature, stellar wind velocity, and winds mass loss rate. We find that M dwarfs coronae tend to be cooler than solar corona, and that M dwarfs stellar winds would be characterized with faster velocity and much smaller mass loss rate compared to those of the solar wind.
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M dwarfs atmosphere is expected to be highly magnetized. The magnetic energy can be responsible for heating the stellar chromosphere and corona, and driving the stellar wind. The nonlinear propagation of Alfven wave is the promising mechanism for both heating stellar atmosphere and driving stellar wind. Based on this Alfven wave scenario, we carried out the one-dimensional compressive magnetohydrodynamic (MHD) simulation to reproduce the stellar atmospheres and winds of TRAPPIST-1, Proxima Centauri, YZ CMi, AD Leo, AX Mic, as well as the Sun. The nonlinear propagation of Alfven wave from the stellar photosphere to chromosphere, corona, and interplanetary space is directly resolved in our study. The simulation result particularly shows that the slow shock generated through the nonlinear mode coupling of Alfven wave is crucially involved in both dynamics of stellar chromosphere (stellar spicule) and stellar wind acceleration. Our parameter survey further revealed the following general trends of physical quantities of stellar atmosphere and wind. (1) The M dwarfs coronae tend to be cooler and denser than solar corona. (2) M dwarfs stellar winds can be characterized with relatively faster velocity and much smaller mass-loss rate compared to those of solar wind. The physical mechanisms behind these tendencies are clarified in this paper, where the stronger stratification of M dwarfs atmosphere and relatively smaller Alfven wave energy input from the M dwarfs photosphere are remarkable.
There have recently been detections of radio emission from low-mass stars, some of which are indicative of star-planet interactions. Motivated by these exciting new results, here we present stellar wind models for the active planet-hosting M dwarf AU Mic. Our models incorporate the large-scale photospheric magnetic field map of the star, reconstructed using the Zeeman-Doppler Imaging method. We use our models to assess if planet-induced radio emission could be generated in the corona of AU Mic, through a mechanism analogous to the sub-Alfvenic Jupiter-Io interaction. In the case that AU Mic has a mass-loss rate of 27 times that of the Sun, we find that both planets b and c in the system can induce radio emission from 10 MHz to 3 GHz in the corona of the host star for the majority of their orbits, with peak flux densities of 10 mJy. Our predicted emission bears a striking similarity to that recently reported from GJ 1151 by Vedantham et al. (2020), which is indicative of being induced by a planet. Detection of such radio emission would allow us to place an upper limit on the mass-loss rate of the star.
We investigated stellar winds from zero/low-metallicity low-mass stars by magnetohydrodynamical simulations for stellar winds driven by Alfven waves from stars with mass $M_{star}=(0.6-0.8)M_{odot}$ and metallicity $Z=(0-1)Z_{odot}$, where $M_{odot}$ and $Z_{odot}$ are the solar mass and metallicity, respectively. Alfvenic waves, which are excited by the surface convection, travel upward from the photosphere and heat up the corona by their dissipation. For lower $Z$, denser gas can be heated up to the coronal temperature because of the inefficient radiation cooling. The coronal density of Pop.II/III stars with $Zle 0.01Z_{odot}$ is 1-2 orders of magnitude larger than that of the solar-metallicity star with the same mass, and as a result, the mass loss rate, $dot{M}$, is $(4.5-20)$ times larger. This indicates that metal accretion on low-mass Pop.III stars is negligible. The soft X-ray flux of the Pop.II/III stars is also expected to be $approx (1-30)$ times larger than that of the solar-metallicity counterpart owing to the larger coronal density, even though the radiation cooling efficiency is smaller. A larger fraction of the input Alfvenic wave energy is transmitted to the corona in low $Z$ stars because they avoid severe reflection owing to the smaller density difference between the photosphere and the corona. Therefore, a larger fraction is converted to the thermal energy of the corona and the kinetic energy of the stellar wind. From this energetics argument, we finally derived a scaling of $dot{M}$ as $dot{M}propto L R_{star}^{11/9}M_{star}^{-10/9}T_{rm eff}^{11/2}left[max (Z/Z_{odot},0.01)right]^{-1/5}$, where $L$, $R_{star}$, and $T_{rm eff}$ are stellar luminosity, radius, and effective temperature, respectively.
There have recently been detections of radio emission from low-mass stars, some of which are indicative of star-planet interactions. Motivated by these exciting new results, in this paper we present Alfven wave-driven stellar wind models of the two active planet-hosting M dwarfs Prox Cen and AU Mic. Our models incorporate large-scale photospheric magnetic field maps reconstructed using the Zeeman-Doppler Imaging method. We obtain a mass-loss rate of $0.25~dot{M}_{odot}$ for the wind of Prox Cen. For the young dwarf AU Mic, we explore two cases: a low and high mass-loss rate. Depending on the properties of the Alfven waves which heat the corona in our wind models, we obtain mass-loss rates of $27$ and $590~dot{M}_{odot}$ for AU Mic. We use our stellar wind models to assess the generation of electron cyclotron maser instability emission in both systems, through a mechanism analogous to the sub-Alfvenic Jupiter-Io interaction. For Prox Cen we do not find any feasible scenario where the planet can induce radio emission in the stars corona, as the planet orbits too far from the star in the super-Alfvenic regime. However, in the case that AU Mic has a stellar wind mass-loss rate of $27~dot{M}_{odot}$, we find that both planets b and c in the system can induce radio emission from $sim10$ MHz to 3 GHz in the corona of the host star for the majority of their orbits, with peak flux densities of $sim10$ mJy. Detection of such radio emission would allow us to place an upper limit on the mass-loss rate of the star.
Beyond the main sequence solar type stars undergo extensive mass loss, providing an environment where planet and brown dwarf companions interact with the surrounding material. To examine the interaction of substellar mass objects embedded in the stellar wind of an asymptotic giant branch (AGB) star, three dimensional hydrodynamical simulations at high resolution have been calculated utilizing the FLASH adaptive mesh refinement code. Attention is focused on the perturbation of the substellar mass objects on the morphology of the outflowing circumstellar matter. In particular, we determine the properties of the resulting spiral density wake as a function of the mass, orbital distance, and velocity of the object as well as the wind velocity and its sound velocity. Our results suggest that future observations of the spiral pattern may place important constraints on the properties of the unseen low mass companion in the outflowing stellar wind.
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