Variable magnetic field and adaptive mixing-length: reproducing Li abundances and constraining rotational evolution of solar-type stars in clusters

TL;DR

This study develops a dynamic model of solar-type stars incorporating variable magnetic fields and mixing-length parameters, improving Li abundance predictions and rotational evolution understanding, though it still faces challenges in matching solar rotation and magnetic flux.

Contribution

It introduces a novel approach with time-dependent magnetic field strength and mixing-length parameter variations, reducing free parameters and better capturing physical variability in stellar models.

Findings

Models reproduce observed Li abundance in Sun-like stars.

Incorporating variable B and alpha-MLT improves Li and rotation predictions.

Discrepancies remain in matching solar rotation and magnetic flux.

Abstract

Investigating the apparent anomalies in lithium (Li) surface abundance observed in the Sun and young stellar globular clusters holds significant promise for advancing our understanding of the mechanisms influencing Li depletion. This study delves into the intricate interplay between rotational mixing and rotational hydrostatic effects in pre-main-sequence (PMS) and main-sequence (MS) solar-type stars by employing grids of rotating models. We implement a novel approach in which both the magnetic field strength (B) and the mixing-length parameter (alpha-MLT) vary dynamically with stellar parameters. This avoids fixed values and aims to reduce free parameters while capturing key physical variability. Our models reproduce the observed Li abundance of Sun-like stars (A(Li) = 1.12 dex) consistent with the present-day solar value (1.1 +- 0.1 dex) and yield qualitatively consistent rotational spin-down trends across PMS and MS phases. However, at the solar age (4.57 Gyr), the same models over-predict the equatorial rotation rate (v = 4.72 kms-1 vs. 2.0 kms-1) and the mean surface magnetic field (B = 36.9 G vs. 1 G). These discrepancies reflect the omission of additional angular momentum loss mechanisms and possible oversimplifications in magnetic saturation physics. While the adaptive alpha-MLT converges to the solar-calibrated value ([1.76, 1.78]) at the present age, its variability during earlier phases significantly influences Li depletion. We compare predictions with observational data from 64 open clusters. The results demonstrate that incorporating time-dependent B and alpha-MLT improves Li predictions and captures rotational evolution trends, but cannot yet reproduce the present-day solar rotation and magnetic flux without additional physics. We discuss these limitations and outline future work for a more complete model of solar-type stars.