The impact of disk-locking on convective turnover times of low-mass pre-main sequence and main sequence stars

¹ Universidade Federal de Viçosa, Campus UFV Florestal, CEP 35690-000 Florestal, MG, Brazil
² Depto. de Física, Universidade Federal de Minas Gerais, C.P.702, 31270-901 Belo Horizonte, MG, Brazil
³ Depto. de Engenharia Eletrônica, Universidade Federal de Minas Gerais, C.P.702, 31270-901 Belo Horizonte, MG, Brazil

^⋆ Corresponding author; nlandin@ufv.br

Received: 10 March 2024
Accepted: 5 November 2024

Abstract

Aims. The impact of disk-locking on the stellar properties related to magnetic activity from the theoretical point of view is investigated.

Methods. We use the ATON stellar evolution code to calculate theoretical values of convective turnover times (τ_c) and Rossby numbers (Ro, the ratio between rotation periods and τ_c) for pre-main sequence (pre-MS) and main sequence (MS) stars. We investigate how τ_c varies with the initial rotation period and with the disk lifetime, using angular momentum conserving models and models simulating the disk-locking mechanism. In the latter case, the angular velocity is kept constant during a given locking time to mimic the magnetic locking effects of a circumstellar disk.

Results. The local convective turnover times generated with disk-locking models are shorter than those obtained with angular momentum conserving models. The differences are smaller in the early pre-MS, increase with stellar age, and become more accentuated for stars with M ≥ 1 M_⊙ and ages greater than 100 Myr. Our new values of τ_c are used to estimate Ro for a sample of stars selected from the literature in order to investigate the rotation-activity relationship. We fit the data with a two-part power-law function and find the best fitting parameters of this relation.

Conclusions. The differences found between both sets of models suggest that the star’s disk-locking phase properties affect its Rossby number and its position in the rotation-activity diagram. Our results indicate that the dynamo efficiency is lower for stars that had undergone longer disk-locking phases.