The polar wind of the fast rotating Be star Achernar: VINCI-VLTI interferometric observations of an elongated polar envelopeReport as inadecuate

The polar wind of the fast rotating Be star Achernar: VINCI-VLTI interferometric observations of an elongated polar envelope - Download this document for free, or read online. Document in PDF available to download.

1 LESIA - Laboratoire d-études spatiales et d-instrumentation en astrophysique 2 LUAN - Laboratoire Universitaire d-Astrophysique de Nice 3 LG - Laboratoire Gemini

Abstract : 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, though it is often assumed that they are formed by a disk around the stellar equator and a hot polar wind. We probe the close environment of the fast rotating Be star Achernar at angular scales of a few mas in the infrared, in order to constrain the geometry of a possible polar CSE. 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 large azimuthal coverage. 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 at the presence of an asymmetric diffuse CSE elongated along the polar direction of the star. We fit to our data 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. We clearly detect a CSE elongated along the polar axis of the star, as well as the rotational flattening of the stellar photosphere. The relative near-IR flux measured for the CSE compared to the stellar photosphere is 5\%. Its angular dimensions are loosely constrained by the available data$2.7 \pm 1.3$ mas and $17.6 \pm 4.9$ mas. This CSE could be linked to free-free emission from the radiative pressure driven wind originating from the hot polar caps of the star.

Keywords : techniques: high angular resolution techniques: interferometric stars: emission-line Be stars: mass-loss stars: rotation stars: individual: Achernar

Author: Pierre Kervella - Armando Domiciano de Souza -



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