Letter to the Editor

The Hα line forming region of AB Aurigae spatially resolved at sub-AU with the VEGA/CHARA spectro-interferometer

¹ Laboratoire d'Astrophysique de Grenoble (LAOG), Université Joseph-Fourier, UMR 5571 CNRS, BP 53, 38041 Grenoble Cedex 09, France e-mail: Karine.Perraut@obs.ujf-grenoble.fr
² INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Laboratoire Fizeau, OCA/UNS/CNRS UMR 6525, Parc Valrose, 06108 Nice Cedex 2, France
⁴ Centro de Astrofisica, Universidade do Porto, 4150-752 Porto, Portugal
⁵ Université de Lyon, 69003 Lyon, France; Université Lyon 1, Observatoire de Lyon, 9 avenue Charles André, 69230 Saint Genis Laval, France; CNRS, UMR 5574, Centre de Recherche Astrophysique de Lyon; École Normale Supérieure, 69007 Lyon, France
⁶ Georgia State University, PO Box 3969, Atlanta GA 30302-3969, USA
⁷ CHARA Array, Mount Wilson Observatory, Mount Wilson, CA 91023, USA

Received: 2 April 2010
Accepted: 25 May 2010

Abstract

Context. A crucial issue in star formation is to understand the physical mechanism by which mass is accreted onto and ejected by a young star. To derive key constraints on the launching point of the jets and on the geometry of the winds, the visible spectro-polarimeter VEGA installed on the CHARA optical array can be an efficient means of probing the structure and the kinematics of the hot circumstellar gas at sub-AU.

Aims. For the first time, we observed the Herbig Ae star AB Aur in the Hα emission line, using the VEGA low spectral resolution (R = 1700) on two baselines of the array.

Methods. We computed and calibrated the spectral visibilities of AB Aur between 610 nm and 700 nm in spectral bands of 20.4 nm. To simultaneously reproduce the line profile and the inferred visibility around Hα, we used a 1D radiative transfer code (RAMIDUS/PROFILER) that calculates level populations for hydrogen atoms in a spherical geometry and that produces synthetic spectro-interferometric observables.

Results. We clearly resolved AB Aur in the Hα line and in a part of the continuum, even at the smallest baseline of 34 m. The small P-Cygni absorption feature is indicative of an outflow but could not be explained by a spherical stellar wind model. Instead, it favors a magneto-centrifugal X-disk or disk-wind geometry. The fit of the spectral visibilities from 610 to 700 nm could not be accounted for by a wind alone, so another component inducing a visibility modulation around Hα needed to be considered. We thus considered a brightness asymmetry possibly caused by large-scale nebulosity or by the known spiral structures.

Conclusions. Thanks to the unique capabilities of VEGA, we managed to simultaneously record for the first time a spectrum at a resolution of 1700 and spectral visibilities in the visible range on a target as faint as m_V = 7.1. It was possible to rule out a spherical geometry for the wind of AB Aur and provide realistic solutions to account for the Hα emission compatible with magneto-centrifugal acceleration. It was difficult, however, to determine the exact morphology of the wind because of the surrounding asymmetric nebulosity. The study illustrates the advantages of optical interferometry and motivates observations of other bright young stars in the same way to shed light on the accretion/ejection processes.