Discovery of magnetic fields in the very young, massive stars W601 (NGC 6611) and OI 201 (NGC 2244)

Abstract

Context: Recent spectropolarimetric observations of Herbig Ae/Be stars have yielded new arguments in favour of a fossil origin for the magnetic fields of intermediate mass stars. Aims: To study the evolution of these magnetic fields, and their impact on the evolution of the angular momentum of these stars during the pre-main sequence phase, we observed Herbig Ae/Be members of young open clusters of various ages.Methods. We obtained high-resolution spectropolarimetric observations of Herbig Ae/Be stars belonging to the young open clusters NGC 6611 ( Myr), NGC 2244 (~ Myr), and NGC 2264 (~ Myr), using ESPaDOnS at the Canada-France-Hawaii Telescope.Results. Here we report the discovery of strong magnetic fields in two massive pre-main sequence cluster stars. We detected, for the first time, a magnetic field in a pre-main sequence rapid rotator: the Herbig B1.5e star W601 (NGC 6611; km s-1). Our spectropolarimetric observations yield a longitudinal magnetic field larger than 1 kG, and imply a rotational period shorter than 1.7 days. The spectrum of this very young object (age Myr) shows strong and variable lines of He and Si. We also detected a magnetic field in the B1 star OI 201 (NGC 2244; km s -1). The Stokes profile of this star does not vary over 5 days, suggesting a long rotational period, a pole-on orientation, or aligned magnetic and rotation axes. OI 201 is situated near the Zero-Age Main Sequence on the HR diagram, and exhibits normal chemical abundances and no spectrum variability.. © 2008 ESO.