Spatially resolving the thermally inhomogeneous outer atmosphere of the red giant Arcturus in the 2.3 micron CO lines

Abstract

The outer atmosphere of K giants shows thermally inhomogeneous structures consisting of the hot chromospheric gas and the cool molecular gas. We present spectro-interferometric observations of the multicomponent outer atmosphere of the well-studied K1.5 giant Arcturus (alpha Boo) in the CO first overtone lines near 2.3 micron. We observed Arcturus with the AMBER instrument at the Very Large Telescope Interferometer (VLTI) at 2.28--2.31 micron with a spectral resolution of 12000 and at projected baselines of 7.3, 14.6, and 21.8 m. The high spectral resolution of the VLTI/AMBER instrument allowed us to spatially resolve Arcturus in the individual CO lines. Comparison of the observed interferometric data with the MARCS photospheric model shows that the star appears to be significantly larger than predicted by the model. It indicates the presence of an extended component that is not accounted for by the current photospheric models for this well-studied star. We found out that the observed AMBER data can be explained by a model with two additional CO layers above the photosphere. The inner CO layer is located just above the photosphere, at 1.04 +/- 0.02 stellar radii, with a temperature of 1600 +/- 400 K and a CO column density of 10^{20 +/- 0.3} cm^-2. On the other hand, the outer CO layer is found to be as extended as to 2.6 +/- 0.2 stellar radii with a temperature of 1800 +/- 100 K and a CO column density of 10^{19 +/- 0.15} cm^-2. The properties of the inner CO layer are in broad agreement with those previously inferred from the spatially unresolved spectroscopic analyses. However, our AMBER observations have revealed that the quasi-static cool molecular component extends out to 2--3 stellar radii, within which region the chromospheric wind steeply accelerates.

10 pages, 9 figures, accepted for publication in Astronomy and Astrophysics

References in corpus (13)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• Pulkovo Compilation of Radial Velocities for 35495 Hipparcos Stars in a Common System
• Interferometric data reduction with AMBER/VLTI.Principle,estimators and illustration
• What causes the large extensions of red-supergiant atmospheres? Comparisons of interferometric observations with 1-D hydrostatic, 3-D convection, and 1-D pulsating model atmospheres
• A critical test of empirical mass loss formulae applied to individual giants and supergiants
• J, H, K spectro-interferometry of the Mira variable S Orionis
• Vigorous atmospheric motion in the red supergiant star Antares
• The limb-darkened Arcturus; Imaging with the IOTA/IONIC interferometer
• Inhomogeneities in molecular layers of Mira atmospheres
• Near-infrared spectro-interferometry of Mira variables and comparisons to 1D dynamic model atmospheres and 3D convection simulations
• VLTI/AMBER spectro-interferometry of the late-type supergiants V766 Cen (=HR 5171 A), sigma Oph, BM Sco, and HD 206859
• Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars VI. First chromosphere model of a late-type giant
• VLTI/AMBER observations of cold giant stars: atmospheric structures and fundamental parameters

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