HD 156324: a tidally locked magnetic triple spectroscopic binary with a disrupted magnetosphere

Abstract

HD 156324 is an SB3 (B2V/B5V/B5V) system in the Sco OB4 association. The He-strong primary possesses both a strong magnetic field, and H$\hat{I}\pm$ emission believed to originate in its Centrifugal Magnetosphere (CM). We analyse a large spectroscopic and high-resolution spectropolarimetric dataset. The radial velocities (RVs) indicate that the system is composed of two sub-systems, which we designate A and B. Period analysis of the RVs of the three components yields orbital periods $P_{\mathrm{orb}}=1.5806(1)$~d for the Aa and Ab components, and 6.67(2)~d for the B component, a PGa star. Period analysis of the longitudinal magnetic field \bz~and H$\hat{I}\pm$ equivalent widths, which should both be sensitive to the rotational period $P_{\mathrm{rot}}$ of the magnetic Aa component, both yield $\sim$1.58~d. Since $P_{\mathrm{orb}}=P_{\mathrm{rot}}$ Aa and Ab must be tidally locked. Consistent with this, the orbit is circularized, and the rotational and orbital inclinations are identical within uncertainty, as are the semi-major axis and the Kepler corotation radius. The star's H$\hat{I}\pm$ emission morphology differs markedly from both theoretical and observational expectations in that there is only one, rather than two, emission peaks. We propose that this unusual morphology may be a consequence of modification of the gravitocentrifugal potential by the presence of the close stellar companion. We also obtain upper limits on the magnetic dipole strength $B_{\mathrm{d}}$ for the Ab and B components, respectively finding $B_{\mathrm{d}}<2.6$~kG and $<0.7$~kG.

15 pages, 10 figures, 3 tables, accepted for publication in MNRAS (reference MN-17-3873-MJ.R1)