K. Ohnaka, K.-H. Hofmann, D. Schertl, G. Weigelt, and some other AMBER consortium members
High-spectral resolution imaging of the dynamical atmosphere of the red supergiant Antares in the CO first overtone lines with VLTI/AMBER
Aims. We present aperture-synthesis imaging of the red supergiant Anares (α Sco) in the CO first overtone lines. Our goal is to probe the structure and dynamics of the outer atmosphere.
Methods. Antares was observed between 2.28 and 2.31 µm with VLTI/AMBER with spectral resolutions of up to 12000 and angular resolutions of down to 7.2 mas at two epochs with a time interval of one year.
Results. The reconstructed images in individual CO lines reveal that the star appears differently in the blue wing, line center, and red wing. In 2009, the images in the line center and red wing show an asymmetrically extended component, while the image in the blue wing shows little trace of it. In 2010, however, the extended component appears in the line center and blue wing, and the image in the red wing shows only a weak signature of the extended component. Our modeling of these AMBER data suggests the presence of an outer atmosphere (MOLsphere) extending to 1.3-1.4 R★ with CO column densities of 5 × 1019 − 1 × 1020 cm-2 and a temperature of ∼2000 K. The CO line images observed in 2009 can be explained by a model, in which a large patch or clump of CO gas is infalling only with 0-5 km s-1. In contrast to the images in the CO lines, the AMBER data in the continuum show only a slight deviation from limb-darkened disks and only marginal time variations. We derive a limb-darkened disk diameter of 37.38 ± 0.06 mas and a power-law-type limb-darkening parameter of (8.7 ± 1.6) × 10-2 from the 2009 data and 37.31 ± 0.09 mas and (1.5 ± 0.2) × 10-1 from the 2010 data. Combining these angular diameters with the measured parallax and bolometric flux, we obtain an effective temperature of 3660 ± 120 K and a luminosity of log L★⁄L☉ = 4.88 ± 0.23. Comparison with theoretical evolutionary tracks suggests a mass of 15 ± 5 M☉.
Conclusions. The properties of the outer atmosphere of Antares are similar to that of another well-studied red supergiant, Betelgeuse. The density of the extended outer atmosphere of Antares and Betelgeuse is higher than predicted by the current 3-D convection simulations by at least six orders of magnitude, implying that convection alone cannot explain the formation of the extended outer atmosphere.
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