Direct measurement of the size and shape of the present-day stellar wind of η Carinae

Abstract

We present new high angular resolution observations at near-IR wavelengths of the core of the Luminous Blue Variable η 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 η Carinae. The present-day stellar wind of η Carinae is resolved on a scale of several stellar radii. Assuming spherical symmetry, we find a mass loss rate of 1.6×10-3 M⊙/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 deprojected 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. η 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 Ω limit.