Saturation of StellarWinds from Young Suns

Abstract

We investigated mass losses via stellar winds from Sun-like main-sequence stars with a wide range of activity levels. We performed forward-type magnetohydrodynamical numerical experiments for Alfvén wave-driven stellar winds with a wide range of input Poynting flux from the photosphere. Increasing the magnetic field strength and the turbulent velocity at the stellar photosphere from the current solar level, the mass-loss rate rapidly at first increases, owing to suppression of the reflection of the Alfvén waves. The surface materials are lifted up by the magnetic pressure associated with the Alfvén waves, and the cool dense chromosphere is intermittently extended to 10%–20% of the stellar radius. The dense atmospheres enhance the radiative losses, and eventually most of the input Poynting energy from the stellar surface escapes by radiation. As a result, there is no more sufficient energy remaining for the kinetic energy of the wind; the stellar wind saturates in very active stars, as observed in Wood et al. (2002, ApJ, 574, 412; 2005, ApJ, 628, L143). The saturation level is positively correlated with Br,0f0, where Br,0 and f0 are the magnetic field strength and the filling factor of open flux tubes at the photosphere. If Br,0f0 is relatively large ≳5G, the mass-loss rate could be as high as 1000 times. If such a strong mass loss lasts for ∼1 billion years, the stellar mass itself would be affected, which could be a solution to the faint young Sun paradox. We derived a Reimerstype scaling relation that estimates the mass-loss rate from an energetics consideration of our simulations. Finally, we derived the evolution of the mass-loss rates, Ṁ∝t-1.23, of our simulations, combining with an observed time evolution of X-ray flux from Sun-like stars, which are shallower than Ṁ∝t-2.33±0.55 in Wood et al. (2005).

Atmospheric Escape by Magnetically Driven Wind From Gaseous Planets

“…The simulation code is generally applicable to stars with a surface-convective layer. So far, we have applied it to red giant winds (Suzuki 2007) and young active solar-type stars (Suzuki et al 2013). Hot Jupiters generally possess a surfaceconvective layer.…”

“…In the simulations for the solar and stellar winds (Suzuki & Inutsuka 2005, 2006Suzuki 2007;Suzuki et al 2013), we set the inner boundary at the photosphere. For the planetary winds in this paper, we set the inner boundary at the position that gives p 0 = 10 5 dyn cm −2 (= 0.1 bar).…”

“…In the simulations for the solar and stellar winds (Suzuki & Inutsuka 2005, 2006Suzuki 2007;Suzuki et al 2013), we have taken the optically thin radiative cooling for the coronal plasma (Landini & Monsignori-Fossi 1990;Sutherland & Dopita 1993) and empirical radiative cooling for the chromospheric gas that takes into account optically thick effects based on observations of the solar chromosphere (Anderson & Athay 1989a). For the simulations of hot Jupiters, we need to prescribe the radiative cooling for gas with lower temperature down to ∼1000 K. In this paper, we adopt a simple treatment by extending the empirical cooling rate (Anderson & Athay 1989a), which is proportional to density, 4.5 × 10 9 ρ (erg cm −3 s −1 ).…”

Response of Solar Wind on Extreme Solar Activity

“…Although we have three parameters on the magnetic field, we here focus on B r,0 f 0 , where B r,0 is the radial magnetic field strength and f 0 is the areal filling factor of open magnetic field regions measured at the photosphere; B r,0 f 0 denotes the average magnetic field strength contributed from open flux regions. We performed 163 simulation runs for stellar winds from a star with the solar mass and radius by changing these parameters (see (23) for detail). Figure 2 shows the relation between the input energy by the Alfvénic wave in units of luminosity, erg s −1 , from the photosphere (horizontal axis) and the kinetic energy luminosity of the stellar wind (vertical axis).…”

“…Therefore, the radiation loss rapidly increases for an increasing input energy, and eventually the sufficient energy is not remained for the kinetic energy of the wind. In order to compare our simulation results to the observation of solar-type stars (3; 4; 5), we show the wind kinetic energy on the radiative flux in Figure 4 (see (23) for detail). The observed maximum mass loss is ≈ 100 times of the current mass loss rate from the Sun, our result shows that some cases give even larger mass losses up to ≈ 1000 times of the present solar mass loss for strong magnetic field, B r,0 f 0 = 10 G. Most of the simulation runs with a large energy input by the Alfvénic waves from the photosphere show an extended chromosphere because of the support from the magnetic pressure of the Alfvénic waves (Figure 3).…”