HD 34736: An intensely magnetised double-lined spectroscopic binary with rapidly-rotating chemically peculiar B-type components

Abstract

We report the results of a comprehensive study of the spectroscopic binary (SB2) system HD 34736 hosting two chemically peculiar (CP) late B-type stars. Using new and archival observational data, we characterise the system and its components, including their rotation and magnetic fields. Fitting of the radial velocities yields $P_{\mathrm{orb}}=83.{}^{\mathrm{d}}219(3)$ and $e=0.8103(3)$. The primary component is a CP He-wk star with $T_{\mathrm{eff}A}=13000\pm 500$ K and ${\upsilon }_{\mathrm{e}}\sin i=75\pm 3$ km/s, while the secondary exhibits variability of Mg and Si lines, and has $T_{\mathrm{eff}B}=11500\pm 1000$ K and ${\upsilon }_{\mathrm{e}}\sin i=110$-180 km/s. TESS and KELT photometry reveal clear variability of the primary component with a rotational period $P_{\mathrm{rot}A}=1.{}^{\mathrm{d}}2799885(11)$, which is lengthening at a rate of $1.26(6)$ s/yr. For the secondary, $P_{\mathrm{rot}B}=0.{}^{\mathrm{d}}5226938(5)$, reducing at a rate of $-0.14(3)$ s/yr. The longitudinal component $⟨B_{\mathrm{z}}⟩$ of the primary's strongly asymmetric global magnetic field varies from $-6$ to +5 kG. Weak spectropolarimetric evidence of a magnetic field is found for the secondary star. The observed X-ray and radio emission of HD 34736 may equally be linked to a suspected T Tau-like companion or magnetospheric emission from the principal components. Given the presence of a possible third magnetically active body, one can propose that the magnetic characteristics of the protostellar environment may be connected to the formation of such systems.

This preprint has 28 pages, 23 figures, and includes the complete version of Table 1. Published in MNRAS

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