Spatially resolved, high-spectral resolution observation of the K giant Aldebaran in the CO first overtone lines with VLTI/AMBER

Aims. We present a high-spatial and high-spectral resolution observation of the well-studied K giant Aldebaran with AMBER at the Very Large Telescope Interferometer (VLTI). Our aim is to spatially resolve the outer atmosphere (so-called MOL sphere) in individual CO first overtone lines and derive its physical properties, which are important for understanding the mass-loss mechanism in normal (i.e., non-Mira) K-M giants. Methods. Aldebaran was observed between 2.28 and 2.31 mu m with a projected baseline length of 10.4 m and a spectral resolution of 12 000. Results. The uniform-disk diameter observed in the CO first overtone lines is 20-35% larger than is measured in the continuum. We have also detected a signature of inhomogeneities in the CO-line-forming region on a spatial scale of similar to 45 mas, which is more than twice as large as the angular diameter of the star itself. While the MARCS photospheric model reproduces the observed spectrum well, the angular size in the CO lines predicted by the MARCS model is significantly smaller than observed. This is because the MARCS model with the parameters of Aldebaran has a geometrical extension of only similar to 2% (with respect to the stellar radius). The observed spectrum and interferometric data in the CO lines can be simultaneously reproduced by placing an additional CO layer above the MARCS photosphere. This CO layer is extended to 2.5 +/- 0.3 R-star with CO column densities of 5 x 10(19) - 2 x 10(20) cm(-2) and a temperature of 1500 +/- 200 K. Conclusions. The high spectral resolution of AMBER has enabled us to spatially resolve the inhomogeneous, extended outer atmosphere (MOLsphere) in the individual CO lines for the first time in a K giant. Our modeling of the MOLsphere of Aldebaran suggests a rather small gradient in the temperature distribution above the photosphere up to 2-3 R-star.