• DOI:10.1051/0004-6361/201118128
• Corpus ID: 119195226

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

@article{Ohnaka2012SpatiallyRT,
  title={Spatially resolving the outer atmosphere of the M giant BK Virginis in the CO first overtone lines with VLTI/AMBER},
  author={Keiichi Ohnaka and K. H. Hofmann and Dieter Schertl and Gerd Weigelt and Fabien Malbet and F. Massi and Anthony Meilland and P. Stee},
  journal={Astronomy and Astrophysics},
  year={2012},
  volume={537},
  pages={53}
}

• K. Ohnaka, K. Hofmann, +5 authors P. Stee
• Published 25 November 2011
• Physics
• Astronomy and Astrophysics

Context. The mass-loss mechanism in normal K–M giant stars with small variability amplitudes is not yet understood, although the majority among red giant stars are precisely of this type. Aims. We present high-spatial and high-spectral resolution observations of the M7 giant BK Vir with AMBER at the Very Large Telescope Interferometer (VLTI). Our aim is to probe the physical properties of the outer atmosphere by spatially resolving the star in the individual CO first overtone lines. Methods. BK Vir was observed between 2.26 and 2.31 μm using the 16–32–48 m telescope configuration with an angular resolution of 9.8 mas and a spectral resolution of 12 000. Results. The uniform-disk diameters observed in the CO first overtone lines are 12−31% larger than those measured in the continuum. We also detected asymmetry in the CO line-forming region, which manifests itself as non-zero/non-π differential and closure phases. The data taken 1.5 months apart show possible time variation on a spatial scale of 30 mas (corresponding to 3× stellar diameter) at the CO band head. Comparison of the observed data with the MARCS photospheric model shows that whereas the observed CO line spectrum can be satisfactorily reproduced by the model, the angular sizes observed in the CO lines are much larger than predicted by the model. Our model with two additional CO layers above the MARCS photosphere reproduces the observed spectrum and