Detection of Magnetic Massive Stars in the Open Cluster NGC 3766

TL;DR

This study surveys magnetic fields in B-type stars within the open cluster NGC 3766, discovering a few magnetic stars and analyzing their properties to understand the origin of stellar magnetism.

Contribution

It provides the first spectropolarimetric survey of B-type stars in NGC 3766 and challenges the fossil field hypothesis for their magnetic origins.

Findings

Detected two magnetic B stars, including one with a 2 kG field.

No correlation between magnetic field strength and stellar rotation.

Models do not support a purely fossil origin for the magnetic fields.

Abstract

A growing number of observations indicate that magnetic fields are present among a small fraction of massive O- and B-type stars, yet the origin of these fields remains unclear. Here we present the results of a VLT/FORS1 spectropolarimetric survey of 15 B-type members of the open cluster NGC 3766. We have detected two magnetic B stars in the cluster, including one with a large field of nearly 2 kG, and we find marginal detections of two additional stars. There is no correlation between the observed longitudinal field strengths and the projected rotational velocity, suggesting that a dynamo origin for the fields is unlikely. We also use the Oblique Dipole Rotator model to simulate populations of magnetic stars with uniform or slightly varying magnetic flux on the ZAMS. None of the models successfully reproduces our observed range in B_l and the expected number of field detections, and we rule out a purely fossil origin for the observed fields.