DISCOVERY OF ROTATIONAL BRAKING IN THE MAGNETIC HELIUM-STRONG STAR SIGMA ORIONIS E

Abstract

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