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

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 $P_{\mathrm{o}\mathrm{r}\mathrm{b}}=83.\text{\hspace{0.17em}⁣}{}^{\mathrm{d}}219(3)$ and $e=0.8103(3)$. The primary component is a CP He-wk star with $T_{\mathrm{e}\mathrm{f}\mathrm{f}A}=13000\pm 500$ K and ${\upsilon }_{\mathrm{e}}\sin i=75\pm 3$ km/s, while the secondary exhibits variability of Mg and Si lines, and has $T_{\mathrm{e}\mathrm{f}\mathrm{f}B}=11500\pm 1000$ 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 $P_{\mathrm{r}\mathrm{o}\mathrm{t}A}=1.\text{\hspace{0.17em}⁣}{}^{\mathrm{d}}2799885(11)$, which is lengthening at a rate of $1.26(6)$ s/yr. For the secondary, $P_{\mathrm{r}\mathrm{o}\mathrm{t}B}=0.\text{\hspace{0.17em}⁣}{}^{\mathrm{d}}5226938(5)$, 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.