Small-scale dynamo in cool stars

Abstract

Context. All cool main sequence stars including our Sun are thought to have magnetic fields. Observations of the Sun revealed that small-scale turbulent magnetic fields are present even in quiet regions. Simulations further showed that such magnetic fields affect the subsurface and photospheric structure, and thus the radiative transfer and emergent flux. Since small-scale turbulent magnetic fields on other stars cannot be directly observed, it is imperative to numerically study their effects on the near surface layers. Aims. Until recently comprehensive three-dimensional simulations capturing the effect of small-scale turbulent magnetic fields only exist for the solar case. A series of investigations extending small-scale dynamo simulations for other stars has been started. Here we aim to examine small-scale turbulent magnetic fields in stars of solar effective temperature but different metallicity. Methods. We investigate the properties of three-dimensional simulations of the magneto-convection in boxes covering the upper convection zone and photosphere carried out with the MURaM code for metallicity values of M/H = {–1.0,0.0,0.5} with and without a small-scale dynamo. Results. We find that small-scale turbulent magnetic fields enhanced by a small-scale turbulent dynamo noticeably affect the subsurface dynamics and significantly change the flow velocities in the photosphere. Moreover, significantly stronger magnetic field strengths are present in the convection zone for low metallicity. Instead, at the optical surface the averaged vertical magnetic field ranges from 64G for M/H = 0.5 to 85G for M/H = –1.0.