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The mass-loss mechanism in normal K--M giant stars with small variabilityamplitudes is not yet understood, although they are the majority among redgiant stars. We present high-spatial and high-spectral resolution observationsof the 2.3 micron CO lines in the M7 giant BK Vir with a spatial resolution of9.8 mas and a spectral resolution of 12000, using AMBER at the Very LargeTelescope Interferometer (VLTI). The angular diameters observed in the CO linesare 12--31% larger than those measured in the continuum. We also detectedasymmetry in the CO line-forming region. The data taken 1.5 months apart showpossible time variation on a spatial scale of 30 mas (corresponding to 3 xstellar diameter) at the CO band head. Comparison of the observed data with theMARCS photospheric model shows that whereas the observed CO line spectrum canbe well reproduced by the model, the angular sizes observed in the CO lines aremuch larger than predicted by the model. Our model with two additional COlayers above the MARCS photosphere reproduces the observed spectrum andinterferometric data in the CO lines simultaneously. This model suggests thatthe inner CO layer at ~1.2 stellar radii is very dense and warm with a COcolumn density of ~10^{22} cm^{-2} and temperatures of 1900--2100K, while theouter CO layer at 2.5--3.0 stellar radii is characterized by column densitiesof 10^{19}--10^{20} cm^{-2} and temperatures of 1500--2100K. Our AMBERobservations of BK Vir have spatially resolved the extended molecular outeratmosphere of a normal M giant in the individual CO lines for the first time.The temperatures derived for the CO layers are higher than or equal to theuppermost layer of the MARCS photospheric model, implying the operation of someheating mechanism in the outer atmosphere.

Spatially resolving the outer atmosphere of the M giant BK Vir in the CO first overtone lines with VLTI/AMBER