HD 164492C: a rapidly-rotating, H$\alpha$-bright, magnetic early B star associated with a 12.5d spectroscopic binary

TL;DR

This study characterizes the physical and magnetic properties of the multiple star system HD 164492C, revealing a magnetic early B star with a rapid rotation and a centrifugal magnetosphere, and clarifies its binary nature and distance.

Contribution

First detailed spectropolarimetric analysis of HD 164492C, identifying magnetic properties and binary components, and constraining the system's physical parameters and distance.

Findings

Detection of a strong magnetic field in the broad-lined B star component.

Identification of a 12.5351-day eccentric binary orbit for two components.

Confirmation of a centrifugal magnetosphere around the magnetic star.

Abstract

We employ high resolution spectroscopy and spectropolarimetry to derive the physical properties and magnetic characteristics of the multiple system HD 164492C, located in the young open cluster M20. The spectrum reveals evidence of 3 components: a broad-lined early B star (HD 164492C1), a narrow-lined early B star (HD 164492C2), and a late B star (HD 164492C3). Components C2 and C3 exhibit significant ($>100$ km/s) bulk radial velocity variations with a period of $12.5351(7)$ d that we attribute to eccentric binary motion around a common centre-of-mass. Component C1 exhibits no detectable radial velocity variations. Using constraints derived from modeling the orbit of the C2+C3 binary and from synthesis of the combined spectrum, we determine the approximate physical characteristics of the components. We conclude that a coherent evolutionary solution consistent with the published age of M20 implies a distance to the system of $0.9\pm 0.2$ kpc, corresponding to the smallest published values. We confirm the detection of a strong magnetic field in the combined spectrum. The field is clearly associated with the broad-lined C1 component of the system. Repeated measurement of the longitudinal magnetic field allows the derivation of the rotation period of the magnetic star, $P_{\rm rot}=1.36986(6)$ d. We derive the star's magnetic geometry, finding $i=63\pm 6$ deg, $\beta=33\pm 6$ deg and a dipole polar strength $B_{\rm d}=7.9^{+1.2}_{-1.0}$ kG. Strong emission - varying according to the magnetic period - is detected in the H$\alpha$ profile. This is consistent with the presence of a centrifugal magnetosphere surrounding the rapidly rotating magnetic C1 component.