The model of dynamo with small number of modes and magnetic activity of T Tauri stars

TL;DR

This paper presents a dynamo model for fully convective stars using a finite set of modes, predicts differences in magnetic activity cycles compared to stars with radiative cores, and analyzes the long-term activity of V410 Tau.

Contribution

It introduces a dynamo model based on superposition of modes for fully convective stars and explores its implications for magnetic activity cycles and spot distribution.

Findings

Dynamo cycles differ qualitatively between fully convective stars and those with thin convective envelopes.

V410 Tau exhibits quasi-cyclic activity with a characteristic period of about 4 years.

Long-term brightness variations are due to redistribution of cool spots, not strict periodicity.

Abstract

The model that describes operation of dynamo in fully convective stars is presented. It is based on representation of stellar magnetic field as a superposition of finite number of poloidal and toroidal free damping modes. In the frame of adopted low of stellar differential rotation we estimated minimal value of dynamo number D, starting from which generation of cyclic magnetic field in stars without radiative core is possible. We also derived expression for period of the cycle. It was found that dynamo cycles of fully convective stars and stars with thin convective envelopes differ in a qualitative way: 1) distribution of spots over latitude during the cycle is different in these stars; 2) the model predicts that spot formation in fully convective stars should be strongly suppressed at some phases of the cycle. We have analyzed historical lightcurve of WTTS star V410 Tau and found that long term activity of the star is not periodic process. Rather one can speak about quasi cyclic activity with characteristic time of $\sim 4$ yr and chaotic component over imposed. We concluded also that redistribution of cool spots over longitude is the reason of long term variations of V410 Tau brightness. It means that one can not compare directly results of photometric observations with predictions of our axially symmetric (for simplicity) model which allows to investigate time evolution of spot's distribution over latitude. We then discuss what kind of observations and in which way could be used to check predictions of the dynamo theory.