The Magnetic Early B-type Stars III: A main sequence magnetic, rotational, and magnetospheric biography

TL;DR

This study investigates the magnetic, rotational, and magnetospheric evolution of 56 early B-type stars, revealing how magnetic flux and rotation change with age and mass, and identifying conditions for detectable Hα emission.

Contribution

It provides the first detailed analysis of magnetic and rotational evolution in early B-type stars, including derived parameters and magnetospheric models for a significant sample.

Findings

Magnetic flux increases with stellar mass but decreases with age.

All stars with Hα emission are rapid rotators, strongly magnetic, and young.

Magnetic flux decay rate appears to increase with stellar mass.

Abstract

Magnetic confinement of stellar winds leads to the formation of magnetospheres, which can be sculpted into Centrifugal Magnetospheres (CMs) by rotational support of the corotating plasma. The conditions required for the CMs of magnetic early B-type stars to yield detectable emission in H$\alpha$ -- the principal diagnostic of these structures -- are poorly constrained. A key reason is that no detailed study of the magnetic and rotational evolution of this population has yet been performed. Using newly determined rotational periods, modern magnetic measurements, and atmospheric parameters determined via spectroscopic modelling, we have derived fundamental parameters, dipolar oblique rotator models, and magnetospheric parameters for 56 early B-type stars. Comparison to magnetic A- and O-type stars shows that the range of surface magnetic field strength is essentially constant with stellar mass, but that the unsigned surface magnetic flux increases with mass. Both the surface magnetic dipole strength and the total magnetic flux decrease with stellar age, with the rate of flux decay apparently increasing with stellar mass. We find tentative evidence that multipolar magnetic fields may decay more rapidly than dipoles. Rotational periods increase with stellar age, as expected for a magnetic braking scenario. Without exception, all stars with H$\alpha$ emission originating in a CM are 1) rapid rotators, 2) strongly magnetic, and 3) young, with the latter property consistent with the observation that magnetic fields and rotation both decrease over time.