EDP Sciences
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Volume 480, Number 1, March II 2008
Page(s) 149 - 155
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20078773

A&A 480, 149-155 (2008)
DOI: 10.1051/0004-6361:20078773

Resolving the ionized wind of the post-red supergiant IRC +10 420 with VLTI/AMBER

W. J. de Wit1, R. D. Oudmaijer1, M. A. T. Groenewegen2, M. G. Hoare1, and F. Malbet3

1  School of Physics & Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
    e-mail: w.j.m.dewit@leeds.ac.uk
2  Institute for Astronomy, University of Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
3  Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France

(Received 1 October 2007 / Accepted 29 November 2007)

Aims.The paper investigates the milli-arcsecond scale structure of the present-day wind of the post-red supergiant IRC +10 420.
Methods.We use three telescopes of the VLT Interferometer in combination with the AMBER near-infrared beam combiner to measure spectrally dispersed correlated fluxes in the K-band around the Br$\gamma$ transition. The resulting visibilities are compared with predictions of various simple models in order to infer the size of the observed emission region.
Results.The Br$\gamma$ line is resolved by VLTI+AMBER on all three baselines, with the maximum projected baseline extending 69 m and a PA ranging between 10° and 30°. A differential phase between line and continuum is detected on the longest baseline. The best fitting model, a Gaussian flux distribution, implies a size of the Br$\gamma$ emission region of 3.3 milli-arcsec (FWHM). The size of the continuum emission is not constrained by the observations. From a comparison of the AMBER equivalent width of Br$\gamma$ with measurements from various epochs, we find that the stellar photosphere contributes about 60% of the total continuum light at 2.2 $\mu$m. The remaining 40% continuum emission originates from structures larger than the 66 mas AMBER field of view. This independently confirms similar results made by previous studies. If the Br$\gamma$ emission is optically thin, then the observations do not allow to make any inferences about the shape of the line forming region. However, there is indirect evidence that the hydrogen recombination line emission is optically thick. In that case, using simple arguments, we find that the line emitting region is elongated. This is because the spectrum indicates that the projected line emitting area is about twice that of the stellar surface. A circular Br$\gamma$ line emitting area, however, would have a size an order of magnitude larger using the size of the emission region measured along our baseline. We briefly mention the possibilities whether such a structure is due to a bi-polar flow or a circumstellar disk.

Key words: stars: evolution -- stars: mass-loss -- supergiants -- stars: individual: IRC +10 420 -- techniques: interferometric

© ESO 2008