The polar wind of the fast rotating Be star Achernar - VINCI/VLTI interferometric observations of an elongated polar envelope

Context. Be stars show evidence of mass loss and circumstellar envelopes (CSE) from UV resonance lines, near-IR excesses, and the presence of episodic hydrogen emission lines. The geometry of these envelopes is still uncertain, although it is often assumed that they are formed by a disk around the stellar equator and a hot polar wind. Aims. We probe the close environment of the fast rotating Be star Achernar at angular scales of a few milliarcseconds (mas) in the infrared, in order to constrain the geometry of a possible polar CSE. Methods. We obtained long-baseline interferometric observations of Achernar with the VINCI/VLTI beam combiner in the H and K bands, using various telescope configurations and baseline lengths with a wide azimuthal coverage. Results. The observed visibility measurements along the polar direction are significantly lower than the visibility function of the photosphere of the star alone, in particular at low spatial frequencies. This points to the presence of an asymmetric diffuse CSE elongated along the polar direction of the star. To our data, we fit a simple model consisting of two components: a 2D elliptical Gaussian superimposed on a uniform ellipse representing the distorted photosphere of the fast rotating star. Conclusions. We clearly detected a CSE elongated along the polar axis of the star, as well as rotational flattening of the stellar photosphere. For the uniform-ellipse photosphere we derive a major axis of theta(eq) = 2.13 +/- 0.05 mas and a minor axis of theta(pol) = 1.51 +/- 0.02 mas. The relative near-IR flux measured for the CSE compared to the stellar photosphere is f = 4.7 +/- 0.3%. Its angular dimensions are loosely constrained by the available data at rho(eq) = 2.7 +/- 1.3 mas and rho(pol) = 17.6 +/- 4.9mas. This CSE could be linked to free-free emission from the radiative pressure driven wind originating from the hot polar caps of the star.