HD 35502: a hierarchical triple system with a magnetic B5IVpe primary

Abstract

We present our analysis of HD~35502 based on high- and medium-resolution spectropolarimetric observations. Our results indicate that the magnetic B5IVsnp star is the primary component of a spectroscopic triple system and that it has an effective temperature of $18.4\pm 0.6\mathrm{kK}$, a mass of $5.7\pm 0.6M_{\odot }$, and a polar radius of $3.0_{-0.5}^{+1.1}{R}_{\odot }$. The two secondary components are found to be essentially identical A-type stars for which we derive effective temperatures ($8.9\pm 0.3\mathrm{kK}$), masses ($2.1\pm 0.2M_{\odot }$), and radii ($2.1\pm 0.4R_{\odot }$). We infer a hierarchical orbital configuration for the system in which the secondary components form a tight binary with an orbital period of $5.66866(6)\mathrm{d}$ that orbits the primary component with a period of over $40\mathrm{yrs}$. Least-Squares Deconvolution (LSD) profiles reveal Zeeman signatures in Stokes $V$ indicative of a longitudinal magnetic field produced by the B star ranging from approximately $-4$ to $0\mathrm{kG}$ with a median uncertainty of $0.4\mathrm{kG}$. These measurements, along with the line variability produced by strong emission in H$\hat{I}\pm$, are used to derive a rotational period of $0.853807(3)\mathrm{d}$. We find that the measured $v\sin i=75\pm 5{\mathrm{km}\mathrm{s}}^{-1}$ of the B star then implies an inclination angle of the star's rotation axis to the line of sight of $24_{-10}^{+6}°$. Assuming the Oblique Rotator Model, we derive the magnetic field strength of the B star's dipolar component ($14_{-3}^{+9}\mathrm{kG}$) and its obliquity ($63\pm 13°$). Furthermore, we demonstrate that the calculated Alfvén radius ($41_{-6}^{+17}{R}_{\ast }$) and Kepler radius ($2.1_{-0.7}^{+0.4}{R}_{\ast }$) place HD~35502's central B star well within the regime of centrifugal magnetosphere-hosting stars.

24 pages, 14 figures, Accepted for publication in MNRAS