The impact of disk-locking on convective turnover times of low-mass pre-main sequence and main sequence stars
Creators: name: Landin, Natália Rezende
name: Mendes, Luiz Themystokliz Sanctos
name: Vaz, Luiz Paulo Ribeiro
name: Alencar, Silvia Helena Paixão
Publisher: name: Zenodo
Year: 2021
Subjects: stellar rotation
stellar magnetic activity
pre-main sequence
disk-locking
evolutionary codes

description: We present theoretical studies about the magnetic activity of low-mass (0.1-2.0\({\rm M_{\odot}}\)) pre-main sequence and main sequence stars through the use of stellar structure and evolution models. The ATON stellar evolution code was used to obtain new values of theoretical convective turnover times (\(\tau_{\rm c}\)) for low-mass pre-main sequence and main sequence stars by using both models conserving angular momentum and models simulating the disk-locking mechanism. In the later set of models, the angular velocity is kept constant during a given locking time, in order to mimic the magnetic locking effect of a circumstellar disk. We investigate how convective turnover times vary with the initial rotation period and with the disk lifetimes. The local convective turnover times generated with models simulating the disk-locking mechanism are smaller than those obtained with models conserving angular momentum. The differences are smaller in the early pre-main sequence (\(\sim\)1%) and increase with stellar age, reaching 8-10% for ages greater than 100 Myr. The most accentuated differences are found in stars with larger masses (\(M\geq 1 {\rm M_{\odot}}\)). Our new values of convective turnover times were used to estimate the Rossby number (defined as the ratio between rotation period and \(\tau_{\rm c}\)) of a sample of stars selected from the literature, in order to investigate the relationship between rotation and magnetic activity. By using \({\rm Ro_{sat} =0.1}\) (where \({\rm Ro_{sat}}\) is the Rossby number at which X-ray saturation occurs), we fit the data with a two-part power-law function and the best fitting slope for the unsaturated part of the rotation-activity relashionship was \(-1.76\pm 0.18\) and the saturation level of the fractional X-ray luminosity was \(-3.27\pm 0.05\). The differences we found between disk-locking models and models conserving angular momentum suggest that properties of the disk phase of the star affect its Rossby number and its position in the rotation-activity diagram. Finally, our fit to the data is consistent to those usually found in the literature.
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