Astronomy & Astrophysics

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Issue		A&A Volume 481, Number 3, April III 2008
Page(s)		L99 - L102
Section		Letters
DOI		10.1051/0004-6361:200809444
Published online		04 March 2008

A&A 481, L99-L102 (2008)
DOI: 10.1051/0004-6361:200809444

Letter

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

E. Alecian^{1, 2}, G. A. Wade¹, C. Catala², S. Bagnulo³, T. Boehm⁴, D. Bohlender⁵, J.-C. Bouret⁶, J.-F. Donati⁴, C. P. Folsom³, J. Grunhut¹, and J. D. Landstreet⁷

¹ Dept. of Physics, Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, ON K7K 7B4, Canada
e-mail: evelyne.alecian@rmc.ca
² Observatoire de Paris, LESIA, Place Jules Janssen, 92195 Meudon Cedex, France
³ Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK
⁴ Laboratoire d'Astrophysique, Observatoire Midi-Pyrénées, 14 avenue Edouard Belin, 31400 Toulouse, France
⁵ National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁶ Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, BP 8, 13376 Marseille Cedex 12, France
⁷ Dept. of Physics & Astronomy, University of Western Ontario, ON N6A 3K7, London, Canada

(Received 23 January 2008 / Accepted 21 February 2008)

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 (<6 Myr), NGC 2244 (~1.9 Myr), and NGC 2264 (~8 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 $10.2~M_{\odot}$ Herbig B1.5e star W601 (NGC 6611; $v\sin i\simeq 190$ 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 $\sim 0.017$ Myr) shows strong and variable lines of He and Si. We also detected a magnetic field in the $12.1~M_\odot$ B1 star OI 201 (NGC 2244; $v\sin i=23.5$ km s^-1). The Stokes V 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.

Key words: stars: magnetic fields -- stars: pre-main sequence -- stars: chemically peculiar -- techniques: polarimetric -- techniques: spectroscopic

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