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 $P_\mathrm{orb}=83\hbox{$.\!\!^{\mathrm{d}}$}219(3)$ and e = 0.8103(3). The primary component is a CP He-wk star with TeffA = 13000 ± 500 K and υesin i = 75 ± 3 km s−1 , while the secondary exhibits variability of Mg and Si lines, and has TeffB = 11500 ± 1000 K and υesin i = 110–180 km s−1 . TESS and KELT photometry reveal clear variability of the primary component with a rotational period $P_{\mathrm{rot}A}=1\hbox{$.\!\!^{\mathrm{d}}$}279\, 988\, 5(11)$, which is lengthening at a rate of 1.26(6) s yr−1. For the secondary, $P_{\mathrm{rot}B}=0\hbox{$.\!\!^{\mathrm{d}}$}522\, 693\, 8(5)$, reducing at a rate of −0.14(3) s yr−1. The longitudinal component 〈Bz〉 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.