The evolution of surface magnetic fields in young solar-type stars I: the first 250 Myr

TL;DR

This study investigates how the large-scale magnetic fields of young solar-type stars evolve from 20 to 250 million years, revealing a decline in magnetic strength with age and a correlation with rotation, driven by structural and rotational changes.

Contribution

It provides the first detailed observational analysis of magnetic field evolution in young solar-type stars during their first 250 million years.

Findings

Magnetic field strengths decrease with age from 14 to 140 G.

Strong correlation between magnetic strength and Rossby number.

Magnetic evolution driven by structural changes pre-main sequence and rotational evolution on the main sequence.

Abstract

The surface rotation rates of young solar-type stars vary rapidly with age from the end of the pre-main sequence through the early main sequence. Important changes in the dynamos operating in these stars may result from this evolution, which should be observable in their surface magnetic fields. Here we present a study aimed at observing the evolution of these magnetic fields through this critical time period. We observed stars in open clusters and stellar associations of known ages, and used Zeeman Doppler Imaging to characterize their complex magnetic large-scale fields. Presented here are results for 15 stars, from 5 associations, with ages from 20 to 250 Myr, masses from 0.7 to 1.2 solar masses, and rotation periods from 0.4 to 6 days. We find complex large-scale magnetic field geometries, with global average strengths from 14 to 140 G. There is a clear trend towards decreasing average large-scale magnetic field strength with age, and a tight correlation between magnetic field strength and Rossby number. Comparing the magnetic properties of our zero-age main sequence sample to those of both younger and older stars, it appears that the magnetic evolution of solar-type stars during the pre-main sequence is primarily driven by structural changes, while it closely follows the stars' rotational evolution on the main sequence.