Discovery of Rotational Braking in the Magnetic Helium-Strong Star Sigma Orionis E

TL;DR

This study reports the detection of rotational braking in the magnetic helium-strong star Sigma Ori E, showing its rotation period is increasing over time due to magnetized wind-driven angular momentum loss, aligning with theoretical models.

Contribution

It provides observational evidence of rotational braking in Sigma Ori E, confirming theoretical predictions of angular momentum loss in magnetic massive stars.

Findings

Rotation period increasing at 77 ms/year

Spin-down time estimated at 1.34 million years

Consistent with magnetohydrodynamical simulations

Abstract

We present new U-band photometry of the magnetic Helium-strong star Sigma Ori E, obtained over 2004-2009 using the SMARTS 0.9-m telescope at Cerro Tololo Inter-American Observatory. When combined with historical measurements, these data constrain the evolution of the star's 1.19 d rotation period over the past three decades. We are able to rule out a constant period at the p_null = 0.05% level, and instead find that the data are well described (p_null = 99.3%) by a period increasing linearly at a rate of 77 ms per year. This corresponds to a characteristic spin-down time of 1.34 Myr, in good agreement with theoretical predictions based on magnetohydrodynamical simulations of angular momentum loss from magnetic massive stars. We therefore conclude that the observations are consistent with Sigma Ori E undergoing rotational braking due to its magnetized line-driven wind.