NGC 6611 601: A hot pre-main sequence spectroscopic binary containing a centrifugal magnetosphere host star

Abstract

W 601 (NGC 6611 601) is one of the handful of known magnetic Herbig Ae/Be stars. We report the analysis of a large dataset of high-resolution spectropolarimetry. The star is a previously unreported spectroscopic binary, consisting of 2 B2 stars with a mass ratio of 1.8, masses of 12 M${}_{\odot }$ and 6.2 $M_{\odot }$, in an eccentric 110-day orbit. The magnetic field belongs to the secondary, W 601 B. The H$\alpha$ emission is consistent with an origin in W 601 B's centrifugal magnetosphere; the star is therefore not a classical Herbig Be star in the sense that its emission is not formed in an accretion disk. However, the low value of $\log g=3.8$ determined via spectroscopic analysis, and the star's membership in the young NGC 6611 cluster, are most consistent with it being on the pre-main sequence. The rotational period inferred from the variability of the H$\alpha$ line and the longitudinal magnetic field $⟨B_z⟩$ is 1.13 d. Modelling of Stokes $V$ and $⟨B_z⟩$ indicates a surface dipolar magnetic field $B_{\mathrm{d}}$ between 6 and $11$ kG. With its strong emission, rapid rotation, and strong surface magnetic field, W 601 B is likely a precursor to H$\alpha$-bright magnetic B-type stars such as $\sigma$ Ori E. By contrast, the primary is an apparently non-magnetic ($B_{\mathrm{d}}<300$ G) pre-main sequence early B-type star. In accordance with expectations from magnetic braking, the non-magnetic primary is apparently more rapidly rotating than the magnetic star.

arXiv comment20 pages, 13 figures, accepted for publication in MNRAS

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