The origin of hydrogen line emission for five Herbig Ae/Be stars spatially resolved by VLTI/AMBER spectro-interferometry

S. Kraus; K.-H. Hofmann; M. Benisty; J.-P. Berger; O. Chesneau; A. Isella; F. Malbet; A. Meilland; N. Nardetto; A. Natta; T. Preibisch; D. Schertl; M. Smith; P. Stee; E. Tatulli; L. Testi; G. Weigelt

doi:doi:10.1051/0004-6361:200809946

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Issue		A&A Volume 489, Number 3, October III 2008


Page(s)		1157 - 1173
Section		Interstellar and circumstellar matter
DOI		http://dx.doi.org/10.1051/0004-6361:200809946
Published online		09 July 2008

A&A 489, 1157-1173 (2008)
DOI: 10.1051/0004-6361:200809946

The origin of hydrogen line emission for five Herbig Ae/Be stars spatially resolved by VLTI/AMBER spectro-interferometry

S. Kraus¹, K.-H. Hofmann¹, M. Benisty², J.-P. Berger², O. Chesneau³, A. Isella⁴, F. Malbet², A. Meilland¹, N. Nardetto¹, A. Natta⁵, T. Preibisch¹, D. Schertl¹, M. Smith⁶, P. Stee³, E. Tatulli², L. Testi⁷, and G. Weigelt¹

¹ Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: skraus@mpifr-bonn.mpg.de
² Laboratoire d'Astrophysique de Grenoble, UMR 5571 Université Joseph Fourier/CNRS, BP 53, 38041 Grenoble Cedex 9, France
³ UMR 6525 H. Fizeau, Univ. Nice Sophia Antipolis, CNRS, Observatoire de la Côte d'Azur, Av. Copernic, 06130 Grasse, France
⁴ Caltech, MC 105-24, 1200 East California Blvd., Pasadena CA 91125, USA
⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo Fermi 5, 50125 Firenze, Italy
⁶ Centre for Astrophysics & Planetary Science, University of Kent, Canterbury CT2 7NH, UK
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany

Received 10 April 2008 / Accepted 3 July 2008

Abstract
Context. Accretion and outflow processes are of fundamental importance for our understanding of the formation of stars and planetary systems. To trace these processes, diagnostic spectral lines such as the Br $\gamma$ 2.166 $\mu$ m line are widely used, although due to a lack of spatial resolution, the origin of the line emission is still unclear.
Aims. Employing the AU-scale spatial resolution which can be achieved with infrared long-baseline interferometry, we aim to distinguish between theoretical models which associate the Br $\gamma$ line emission with mass infall (magnetospheric accretion, gaseous inner disks) or mass outflow processes (stellar winds, X-winds, or disk winds).
Methods. Using the VLTI/AMBER instrument, we spatially and spectrally ( $\lambda$ $/\Delta\lambda$ = 1500) resolved the inner ( $\la$ 5 AU) environment of five Herbig Ae/Be stars (HD 163296, HD 104237, HD 98922, MWC 297, V921 Sco) in the Br $\gamma$ emission line as well as in the adjacent continuum. From the measured wavelength-dependent visibilities, we derive the characteristic size of the continuum and Br $\gamma$ line-emitting region. Additional information is provided by the closure phase, which we could measure both in the continuum wavelength regime (for four objects) as well as in the spectrally resolved Br $\gamma$ emission line (for one object). The spectro-interferometric data is supplemented by archival and new VLT/ISAAC spectroscopy.
Results. For all objects (except MWC 297), we measure an increase of visibility within the Br $\gamma$ emission line, indicating that the Br $\gamma$ -emitting region in these objects is more compact than the dust sublimation radius. For HD 98922, our quantitative analysis reveals that the line-emitting region is compact enough to be consistent with the magnetospheric accretion scenario. For HD 163296, HD 104237, MWC 297, and V921 Sco we identify an extended stellar wind or a disk wind as the most likely line-emitting mechanism. Since the stars in our sample cover a wide range of stellar parameters, we also search for general trends and find that the size of the Br $\gamma$ -emitting region does not seem to depend on the basic stellar parameters (such as the stellar luminosity), but correlates with spectroscopic properties, in particular with the H $\alpha$ line profile shape.
Conclusions. By performing the first high-resolution spectro-interferometric survey on Herbig Ae/Be stars, we find evidence for at least two distinct Br $\gamma$ line-formation mechanisms. Most significant, stars with a P-Cygni H $\alpha$ line profile and a high mass-accretion rate seem to show particularly compact Br $\gamma$ -emitting regions ( $R_{{\rm Br}\gamma}/R_{{\rm cont}}$ < 0.2), while stars with a double-peaked or single-peaked H $\alpha$ -line profile show a significantly more extended Br $\gamma$ -emitting region (0.6 $\la$ $R_{{\rm Br}\gamma}/R_{{\rm cont}}$ $\la$ 1.4), possibly tracing a stellar wind or a disk wind.

Key words: stars: pre-main sequence -- stars: winds, outflows -- planetary systems: protoplanetary disks -- line: formation -- accretion, accretion disks -- techniques: interferometric

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