Small-scale Dynamo in Cool Stars III. Changes in the photospheres of F3V to M0V stars

Abstract

Some of the quiet solar magnetic flux could be attributed to a small-scale dynamo (SSD) operating in the convection zone. An SSD operating in cool main-sequence stars is expected to affect the atmospheric structure, in particular the convection, and should have observational signatures. We aim to investigate the distribution of these fields as well as their effect on intensity characteristics, velocities and spatial distribution of kinetic (KE) and magnetic energy (ME) in the lower photosphere of spectral types F3V, G2V, K0V and M0V using 3D radiative-MHD simulations. PDFs of field strength at the $\tau =1$ surface are quite similar for all cases. The M0V star displays the strongest fields, but relative to the gas pressure, the fields on the F3V star reach the largest values. All stars display an excess of horizontal field relative to vertical field in the middle photosphere, with this excess becoming increasingly prominent towards later spectral types. These fields result in a decrease in upflow velocities, slightly smaller granules as well as the formation of bright points in intergranular lanes. The spatial distribution of KE and ME is also similar for all cases, implying a simple pressure scale height proportionality of important scales. SSD fields have rather similar effects on the photospheres of cool main-sequence stars, namely, a significant reduction in convective velocities as well as a slight reduction in granule size, and concentration of field to kG levels in intergranular lanes associated with the formation of bright points. The distribution of field strengths and energies is also rather similar.

arXiv comment11 pages, 14 figures, accepted for publication in A&A

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• The effect of local magnetic fields in quiet regions of stellar atmospheres simulated with MANCHA