Spatially resolved high-resolution spectroscopy of the 2.3 micron CO lines in the red supergiant Betelgeuse with VLTI/AMBER

We present spatially resolved high-spectral resolution observations of the 2.3 micron CO first overtone lines in the red supergiant Betelgeuse using AMBER at the Very Large Telescope Interferometer (VLTI). For the first time, we have succeeded in reconstructing spatially resolved spectra of the individual CO lines for the inhomogeneous, dynamical atmosphere with a spectral resolution of 6000 and a spatial resolution of 9.8 mas — the highest spatial resolution achieved for Betelgeuse. The observed spatially resolved spectrum reveals a pronounced, asymmetrically extended atmosphere up to at least 1.3 stellar radii. Moreover, the extended atmosphere appears differently across the CO line profiles because of the vigorous motions of a large CO gas clump with velocities of 20—30 km/s. Comparison between the CO line data taken in 2008 and 2009 shows a significant change in the dynamics of the atmosphere. In contrast to the CO line data, the continuum data taken in 2008 and 2009 reveal no or only marginal time variations, much smaller than the maximum variation predicted by the current 3-D convection simulations. Our AMBER observations suggest the following new picture: the material within 1.5 stellar radii is strongly stirred possibly by magnetohydrodynamical processes or pulsation and may be violently flung out rather than spilling out in an ordered, spherical fashion as often assumed.