Small-scale vortical motions in cool stellar atmospheres

¹ Istituto ricerche solari Aldo e Cele Daccò (IRSOL), Faculty of Informatics, Università della Svizzera italiana, 6605 Locarno, Switzerland
² Euler Institute, Università della Svizzera italiana (USI), 6900 Lugano, Switzerland
³ Kiepenheuer-Institut für Sonnenphysik (KIS), Georges-Köhler-Allee 401a, 79110 Freiburg i.Br., Germany

Received: 30 January 2024
Accepted: 4 July 2024

Abstract

Context. Small-scale vortices in the solar atmosphere have received considerable attention in recent years. These events are considered potential conduits for the exchange of energy and mass between the solar atmospheric layers from the convective surface to the corona. Similar events may occur in the atmospheres of other stars and play a role in energy transfer within their atmospheres.

Aims. Our aim is to study the presence and properties of small-scale swirls in numerical simulations of the atmospheres of cool main-sequence stars. Our particular focus is on understanding the variations in these properties for different stellar types and their sensitivity to the surface magnetic field. Furthermore, we aim to investigate the role of these events in the energy transport within the simulated atmospheres.

Methods. We analyzed three-dimensional, radiative-magnetohydrodynamic, box-in-a-star, numerical simulations of four main-sequence stars of spectral types K8V, K2V, G2V, and F5V. These simulations include a surface small-scale dynamo responsible for amplifying an initially weak magnetic field. Thus, we can study models characterized by very weak, or, in near equipartition magnetic fields. To identify small-scale vortices in horizontal layers of the simulations, we employed the automated algorithm SWIRL.

Results. Small-scale swirls are abundant in the simulated atmospheres of all the investigated cool stars. The characteristics of these events appear to be influenced by the main properties of the stellar models and by the strength of the surface magnetic field. In addition, we identify signatures of torsional Alfvénic pulses associated with these swirls, which are responsible for a significant vertical Poynting flux in the middle photospheres of the simulated stellar models. Notably, this flux is particularly significant in the K8V model, suggesting that if ~70% of it is dissipated in the low chromosphere, small-scale vortical motions may play a role in the enhanced basal CaII H and K fluxes observed in the range of B − V color index 1.1 ≤ B − V ≤ 1.4. Finally, we present a simple analytical model, along with an accompanying scaling relation, to explain the peculiar result of the statistical analysis that the rotational period of surface vortices increases with the effective temperature of the stellar model.

Conclusions. Our study shows that small-scale vortical motions are not unique to the solar atmosphere and that their interplay with the stellar surface magnetic field may effect the observable chromospheric activity of main-sequence cool dwarf stars.