Spatially resolving the inhomogeneous structure of the dynamical atmosphere of Betelgeuse with VLTI/AMBER

K Ohnaka,R Petrov,A Chelli,T Driebe,M Benisty,K.-H Hofmann,G Weigelt,F Vakili,Ph Stee,D Schertl,F Millour

doi:10.1051/0004-6361/200912247

Abstract

We present spatially resolved high-spectral resolution K-band observations of the red supergiant Betelgeuse (alpha Ori) using AMBER at the Very Large Telescope Interferometer (VLTI). Betelgeuse was observed between 2.28 and 2.31 micron using baselines of 16, 32, and 48m with spectral resolutions of 4800 -- 12000. Spectrally dispersed interferograms have been obtained in the 2nd, 3rd, and 5th lobes, which represents the highest spatial resolution (9 mas) achieved for Betelgeuse, corresponding to 5 resolution elements over its stellar disk. The AMBER data in the continuum can be reasonably fitted by a uniform disk with a diameter of 43.19+/-0.03 mas or a limb-darkening disk with 43.56+/-0.06 mas. The K-band interferometric data taken at various epochs suggest that Betelgeuse seen in the continuum shows much smaller deviations from the above uniform/limb-darkened disk than predicted by 3-D convection simulations. On the other hand, our AMBER data in the CO lines reveal that the blue and red wings of the CO lines originate in spatially distinct regions over the stellar disk, indicating an inhomogeneous velocity field. Our AMBER data in the CO lines can be roughly explained by a simple model, in which a patch of CO gas is moving outward or inward at velocities of 10--15 km s^-1, while the CO gas in the remaining region in the atmosphere is moving in the opposite direction at the same velocities. The AMBER data are also consistent with the presence of warm molecular layers at ~1.4--1.5 Rstar with a CO column density of ~1 x 10^20 cm^-2. Our AMBER observations of Betelgeuse are the first spatially resolved study of the so-called macroturbulence in a stellar atmosphere other than the Sun. The spatially resolved CO gas motion is likely to be related to convective motion or intermittent mass ejections in clumps or arcs.

Highlights

Red supergiants (RSGs) experience slow, intensive mass loss up to 10−4 M yr−1
Our AMBER data in the CO lines can be roughly explained by a simple model, in which a patch of CO gas is moving outward or inward with velocities of 10−15 km s−1, while the CO gas in the remaining region in the atmosphere is moving in the opposite direction at the same velocities
The visibilities and differential phases (DPs) on the middle and longest baselines, as well as the closure phase (CP) were derived from the binned data, while the results on the shortest baseline were derived from the data with a spectral resolution of 12 000

Summary

Introduction

Red supergiants (RSGs) experience slow, intensive mass loss up to 10−4 M yr−1. Despite its importance in stellar evolution and in the chemical enrichment of the interstellar matter, the mass loss mechanism in RSGs is not well understood. While radiation pressure on dust grains is often considered to be the driving mechanism of mass loss in cool evolved stars, it is not clear where and how dust forms in RSGs and how mass outflows are initiated. Winds (Airapetian et al 2000; Schröder & Cuntz 2005, 2007), a combination of Alfvén waves and the wave damping due to dust (Vidotto & Janteco-Pereira 2006), and convective turbulence combined with radiation pressure on molecules (Josselin & Plez 2007). The UV observations of the M supergiant Betelgeuse (α Ori) with the Hubble Space Telescope revealed that the hot Article published by EDP Sciences

Results

Discussion

Conclusion