Magnetic, chemical and rotational properties of the Herbig Ae/Be binary system HD 72106

Abstract

Recently, magnetic fields have been detected in a handful of pre-main sequence Herbig Ae and Be (HAeBe) stars. This hints at an evolutionary connection between magnetic HAeBe stars and Ap and Bp stars. Ap/Bp stars are magnetic chemically peculiar main sequence stars, whose origins remain poorly understood. In this context the HD 72106 system, a very young double star, is particularly interesting. HD 72106 consists of an intermediate mass primary with a strong magnetic field and an apparently non-magnetic Herbig Ae secondary. In this work we examine 20 high-resolution spectropolarimetric observations of HD 72106A and B, obtained with the ESPaDOnS instrument at the Canada-France-Hawaii Telescope. We find that the HD 72106 system is a true binary, based on the components' positions in space, locations on the H-R diagram, and dynamical properties. For the primary we determine an effective temperature Teff = 11000 ± 1000 K and a mass of 2.4 ± 0.4 M ; while for the secondary we find Teff = 8750 ± 500 K and M = 1.9 ± 0.2 M . Through detailed spectral modeling, strong chemical peculiarities characteristic of Ap/Bp stars are found in the primary. Abundances of 10 times the solar value are found for Fe, Si and Ti, with Cr 100 times solar and Nd 1000 times solar. Helium is found to display less than 1/10 the solar abundance. By contrast, detailed spectrum modeling of the secondary shows that it possesses approximately solar abundances. The rotation period and magnetic field geometry of the primary are investigated in detail. A remarkably short rotation period of 0.63995 ± 0.00014 days is derived. A dipole magnetic field geometry is found, with Bp = 1300 ± 100 G, beta = 60 ± 5 , and i = 23 ± 11 . Doppler Imaging of the surface distribution of Si, Ti, Cr, and Fe is performed for the primary, and strong inhomogeneities are found. All four maps present similar abundance patterns, with a large spot near the positive magnetic pole. Implications of these results, particularly with respect to the stage of evolution at which chemical peculiarities arise, are discussed.