Magnetic fields and chemical peculiarities of the very young intermediate-mass binary system HD 72106

TL;DR

This study investigates the magnetic field and chemical peculiarities of the young binary system HD 72106, revealing a strong magnetic field in the primary and chemical overabundances characteristic of a Bp star, providing insights into early magnetic star evolution.

Contribution

It presents the first detailed magnetic and chemical analysis of a very young Herbig Ae/Be binary, demonstrating the presence of a strong magnetic field and chemical peculiarities in the primary star.

Findings

Magnetic field detected in the primary star but not in the secondary.

Primary star shows strong chemical peculiarities typical of Bp stars.

Magnetic field geometry modeled as a dipole with 1230 G strength.

Abstract

The recently discovered magnetic Herbig Ae and Be stars may provide qualitatively new information about the formation and evolution of magnetic Ap and Bp stars. We have performed a detailed investigation of one particularly interesting binary system with a Herbig Ae secondary and a late B-type primary possessing a strong, globally ordered magnetic field. Twenty high-resolution Stokes V spectra of the system were obtained with the ESPaDOnS instrument mounted on the CFHT. In these observations we see clear evidence for a magnetic field in the primary, but no evidence for a magnetic field in the secondary. A detailed abundance analysis was performed for both stars, revealing strong chemical peculiarities in the primary and normal chemical abundances in the secondary. The primary is strongly overabundant in Si, Cr, and other iron-peak elements, as well as Nd, and underabundant in He. The primary therefore appears to be a very young Bp star. In this context, line profile variations of the primary suggest non-uniform lateral distributions of surface abundances. Interpreting the 0.63995 +/- 0.00009 day variation period of the Stokes I and V profiles as the rotational period of the star, we have modeled the magnetic field geometry and the surface abundance distributions of Si, Ti, Cr and Fe using Magnetic Doppler Imaging. We derive a dipolar geometry of the surface magnetic field, with a polar strength of 1230 G and an obliquity of 57 degrees. The distributions Ti, Cr and Fe are all qualitatively similar, with an elongated patch of enhanced abundance situated near the positive magnetic pole. The Si distribution is somewhat different, and its relationship to the magnetic field geometry less clear.