Vol. 619
In section 8. Stellar atmospheres

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

by K. Ohnaka and C. A. L. Morales Marín, A&A 620, A23

This work observed the well known K1.5 giant Arcturus (α Boo) with ESO VLTI/AMBER in the CO first overtone lines near 2.3 μm with a high spectral resolution of 12 000. These spectro-interferometric observations resolved the multicomponent outer atmosphere of this star. The observations show that it appears to be significantly larger than predicted, and includes an extended atmospheric component that is not accounted for by current photospheric models of this star. The authors find that the observations can be explained by a model with two additional CO layers above the photosphere. In this model, the inner CO layer is located just above the photosphere (1.04 ± 0.02 R), with a temperature of 1600 ± 400 K and a CO column density of 10(20±0.3) cm−2. The outer CO layer is found to be extended - even up to 2.6 ± 0.2 R with a temperature of 1800 ± 100 K and a CO column density of 10(19±0.15) cm−2. Thus these AMBER observations show that a quasistatic cool molecular component of this star extends out to 2–3 R, within which region the chromospheric wind steeply accelerates.