The evolution of surface magnetic fields in young solar-type stars II: The early main sequence (250-650 Myr)

TL;DR

This study investigates how large-scale surface magnetic fields in young solar-type stars evolve between 120 and 650 million years, revealing a decline in magnetic strength with age and potential saturation at low Rossby numbers.

Contribution

It provides new observational data on magnetic field strength and geometry in stars aged 120-650 Myr, highlighting the evolution and possible saturation of magnetic fields during early main sequence.

Findings

Magnetic field strength decreases with age.

Large-scale magnetic fields correlate with Rossby number.

Possible saturation of magnetic fields at low Rossby numbers.

Abstract

There is a large change in surface rotation rates of sun-like stars on the pre-main sequence and early main sequence. Since these stars have dynamo driven magnetic fields, this implies a strong evolution of their magnetic properties over this time period. The spin-down of these stars is controlled by interactions between stellar winds and magnetic fields, thus magnetic evolution in turn plays an important role in rotational evolution. We present here the second part of a study investigating the evolution of large-scale surface magnetic fields in this critical time period. We observed stars in open clusters and stellar associations with known ages between 120 and 650 Myr, and used spectropolarimetry and Zeeman Doppler Imaging to characterize their large-scale magnetic field strength and geometry. We report 15 stars with magnetic detections here. These stars have masses from 0.8 to 0.95 Msun, rotation periods from 0.326 to 10.6 days, and we find large-scale magnetic field strengths from 8.5 to 195 G with a wide range of geometries. We find a clear trend towards decreasing magnetic field strength with age, and a power-law decrease in magnetic field strength with Rossby number. There is some tentative evidence for saturation of the large-scale magnetic field strength at Rossby numbers below 0.1, although the saturation point is not yet well defined. Comparing to younger classical T Tauri stars, we support the hypothesis that differences in internal structure produce large differences in observed magnetic fields, however for weak lined T Tauri stars this is less clear.