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A&A 410, L37-L40 (2003)
DOI: 10.1051/0004-6361:20031500

Letter

Direct measurement of the size and shape of the present-day stellar wind of $\eta$ Carinae

R. van Boekel1, 2, P. Kervella1, M. Schöller1, T. Herbst3, W. Brandner1, 3, A. de Koter2, L. B. F. M. Waters2, 4, D. J. Hillier5, F. Paresce1, R. Lenzen3 and A.-M. Lagrange6

1  European Southern Observatory, Karl-Schwarzschild-Strasse 2, 5748 Garching, Germany
2  Astronomical Institute "Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
4  Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
5  Department of Physics and Astronomy, University of Pittsburgh, 3941 O'Hara Street, Pitssburgh, PA 15260, USA
6  Laboratoire d'Astrophysique de l'Observatoire de Grenoble, Université J. Fourier, CNRS, BP 53, 38041 Grenoble Cedex 9, France

(Received 15 September 2003 / Accepted 23 September 2003)

Abstract
We present new high angular resolution observations at near-IR wavelengths of the core of the Luminous Blue Variable $\eta$ Carinae, using NAOS-CONICA at the VLT and VINCI at the VLT Interferometer (VLTI). The latter observations provide spatial information on a scale of 5 milli-arcsec or ~11 AU at the distance of $\eta$ Carinae. The present-day stellar wind of $\eta$ Carinae is resolved on a scale of several stellar radii. Assuming spherical symmetry, we find a mass loss rate of  $1.6\times10^~M_{\odot}$/yr and a wind clumping factor of 0.26. The VLTI data taken at a baseline of 24 m show that the object is elongated with a de-projected axis ratio of approximately 1.5; the major axis is aligned with that of the large bi-polar nebula that was ejected in the 19th century. The most likely explanation for this observation is a counter-intuitive model in which stellar rotation near the critical velocity causes enhanced mass loss along the rotation axis. This results from the large temperature difference between pole and equator in rapidly rotating stars. $\eta$ Carinae must rotate in excess of 90 percent of its critical velocity to account for the observed shape. The large outburst may have been shaped in a similar way. Our observations provide strong support for the existence of a theoretically predicted rotational instability, known as the $\Omega$ limit.


Key words: stars: circumstellar matter: stellar winds: mass loss -- stars: individual: $\eta$ Carinae

Offprint request: R. van Boekel, vboekel@science.uva.nl

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© ESO 2003