Rossby numbers of fully and partially convective stars

TL;DR

This study models stellar magnetic activity by calculating convective turnover times and Rossby numbers for stars, revealing how these parameters relate to X-ray emissions across different stellar types and ages.

Contribution

It introduces a new method for determining the convective turnover time in fully convective stars, improving the accuracy of Rossby number calculations for magnetic activity analysis.

Findings

Stars with Rossby number below saturation level show constant X-ray emission.

X-ray emission decays with Rossby number following a power-law with exponent -2.4.

The new radial position for calculating $ au_c$ improves the rotation-activity relation fit.

Abstract

We investigate stellar magnetic activity from the theoretical point of view, by using stellar evolution models to calculate theoretical convective turnover times ($\tau_{\rm c}$) and Rossby numbers (${\rm Ro}$) for pre-main-sequence and main-sequence stars. The problem is that the canonical place where $\tau_{\rm c}$ is usually determined (half a mixing length above the base of the convective zone) fails for fully convective stars and there is no agreement on this in the literature. Our calculations were performed with the ATON stellar evolution code. We concentrated our analysis on fully and partially convective stars motivated by recent observations of slowly rotating fully convective stars, whose X-ray emissions correlate with their Rossby numbers in the same way as in solar-like stars, suggesting that the presence of a tachocline is not required for magnetic field generation. We investigate the behaviour of $\tau_{\rm c}$ over the stellar radius for stars of different masses and ages. As ${\rm Ro}$ depends on $\tau_{\rm c}$, which varies strongly with the stellar radius, we use our theoretical results to determine a better radial position at which to calculate it for fully convective stars. Using our alternative locations, we fit a sample of 847 stars in the rotation-activity diagram ($L_{\rm X}/L_{\rm bol}$ versus ${\rm Ro}$) with a two-part power-law function. Our fit parameters are consistent with previous work, showing that stars with ${\rm Ro}$$\leq$${\rm Ro_{sat}}$ are distributed around a saturation level in $L_{\rm X}/L_{\rm bol}$ and, for stars with ${\rm Ro}$$>$${\rm Ro_{sat}}$, $L_{\rm X}/L_{\rm bol}$ clearly decays with ${\rm Ro}$ with an exponent of $-2.4\!\pm\!0.1$.