First direct detection of a Keplerian rotating disk around the Be star $\alpha$ Arae using the VLTI/AMBER instrument.

Aims. We aim to study the geometry and kinematics of the disk around the Be star $\alpha$ Arae as a function of wavelength, especially across the Br$\gamma$ emission line. The main purpose of this paper is to answer the question about the nature of the disk rotation around Be stars. Methods. We use the VLTI/AMBER instrument operating in the K band which provides a gain by a factor 5 in spatial resolution compared to previous VLTI/MIDI observations. Moreover, it is possible to combine the high angular resolution provided with the (medium) spectral resolution of AMBER to study the kinematics of the inner part of the disk and to infer its rotation law. Results. We obtain for the first time the direct evidence that the disk is in keplerian rotation, answering a question that occurs since the discovery of the first Be star $\gamma$ Cas by father Secchi in 1866. We also present the global geometry of the disk showing that it is compatible with a thin disk + polar enhanced winds modeled with the SIMECA code. We found that the disk around $\alpha$ Arae is compatible with a dense equatorial matter confined in the central region whereas a polar wind is contributing along the rotational axis of the central star. Between these two regions the density must be low enough to reproduce the large visibility modulus (small extension) obtained for two of the four VLTI baselines. Moreover, we obtain that $\alpha$ Arae is rotating very close to its critical rotation. This scenario is also compatible with the previous MIDI measurements.

[1]  R. Yudin.  Statistical analysis of intrinsic polarization, IR excess and projected rotational velocity distributions of classical Be stars , 2001 .

[2]  K. Wood,et al.  The Effect of Multiple Scattering on the Polarization from Axisymmetric Circumstellar Envelopes. II. Thomson Scattering in the Presence of Absorptive Opacity Sources , 1996 .

[3]  K. Gayley,et al.  Effects of Gravity Darkening on Radiatively Driven Mass Loss from Rapidly Rotating Stars , 1998 .

[4]  J. M. Marlborough,et al.  The effects of electron scattering on Balmer emission lines in Be stars. , 1979 .

[5]  David Mouillet,et al.  AMBER : Instrument description and first astrophysical results Special feature AMBER , the near-infrared spectro-interferometric three-telescope VLTI instrument , 2007 .

[6]  Ivan Hubeny,et al.  A computer program for calculating non-LTE model stellar atmospheres , 1988 .

[7]  A. Secchi.  Schreiben des Herrn Prof. Secchi, Dir. der Sternwarte des Collegio Romano, an den Herausgeber , 1866 .

[8]  Karen S. Bjorkman,et al.  Deriving the Geometry of Be Star Circumstellar Envelopes from Continuum Spectropolarimetry. I. The Case of ζ Tauri , 1997 .

[9]  D. Cohen,et al.  ROSAT PSPC Observations of 27 Near-Main-Sequence B Stars , 1997 .

[10]  D. B. Friend,et al.  A rotating, magnetic, radiation-driven wind model applied to Be stars , 1986 .

[11]  David Mozurkewich,et al.  Constraints on the Geometry of Circumstellar Envelopes: Optical Interferometric and Spectropolarimetric Observations of Seven Be Stars , 1997 .

[12]  J. Blondin,et al.  Two-dimensional Hydrodynamical Simulations of Wind-compressed Disks around Rapidly Rotating B Stars , 1994 .

[13]  Steven R. Cranmer,et al.  Inhibition of Wind-Compressed Disk Formation by Nonradial Line Forces in Rotating Hot-Star Winds , 1996 .

[14]  John I. Castor,et al.  Radiation-driven winds in Of stars. , 1975 .

[15]  S. Owocki,et al.  The effect of oblateness and gravity darkening on the radiation driving in winds from rapidly rotating B stars , 1995 .

[16]  O. Struve.  On the Origin of Bright Lines in Spectra of Stars of Class B , 1931 .

[17]  Be-star rotation: how close to critical? , 2003, astro-ph/0312113.

[18]  P. Stee,et al.  Near-IR and visible interferometry of Be stars: Constraints from wind models , 2001 .

[19]  Andrea Richichi,et al.  CHARM: A Catalog of High Angular Resolution Measurements , 2002 .

[20]  Ivan Hubeny,et al.  Non-LTE line-blanketed model atmospheres of hot stars. 1: Hybrid complete linearization/accelerated lambda iteration method , 1995 .

[21]  P. Stee.  Estimation of the mass loss, opening angle and mass of Be circumstellar disks from Brγ continuum emission and interferometric measurements , 2003 .

[22]  A. Evans,et al.  POLARIMETRY OF SOUTHERN PECULIAR EARLY-TYPE STARS , 1998 .

[23]  Pierre Kervella,et al.  The polar wind of the fast rotating Be star Achernar. VINCI/VLTI interferometric observations of an , 2006 .

[24]  E. Tatulli,et al.  Interferometric data reduction with AMBER/VLTI. Principle, estimators, and illustration. , 2007 .

[25]  J. Cassinelli,et al.  Equatorial disk formation around rotating stars due to ram pressure confinement by the stellar wind , 1993 .

[26]  A. Uesugi,et al.  Catalogue of rotational velocities of the stars , 1970 .

[27]  N. Morrell,et al.  High and intermediate-resolution spectroscopy of Be stars - An atlas of H$\gamma$, He I 4471 and Mg II 4481 lines , 2001 .

[28]  O. Chesneau,et al.  First VLTI/MIDI observations of a Be star: Alpha arae , 2005, astro-ph/0501162.

[29]  F. Paresce,et al.  The spinning-top Be star Achernar from VLTI-VINCI , 2003, astro-ph/0306277.

[30]  D. Baade,et al.  Evolution in circumstellar envelopes of Be stars: From disks to rings? ? , 2001 .

[31]  J. Porter,et al.  Classical Be Stars , 2003, 1310.3962.

[32]  K. Wood,et al.  The Effect of Multiple Scattering on the Polarization from Axisymmetric Circumstellar Envelopes. I. Pure Thomson Scattering Envelopes , 1996 .