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

Abstract

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 interferometric data in the CO lines simultaneously. This model suggests that the inner CO layer at ∼1.2 R is very dense and warm with a CO column density of ∼10 22 cm −2 and temperatures of 1900−2100 K, while the outer CO layer at 2.5−3.0 R is characterized by column densities of 10 19 −10 20 cm −2 and temperatures of 1500−2100 K. Conclusions. Our AMBER observations of BK Vir have spatially resolved the extended molecular outer atmosphere 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, the uppermost layer of the MARCS photospheric model, implying the operation of some heating mechanism in the outer atmosphere. This study also illustrates that testing photospheric models only with the spectra of strong molecular or atomic features can be misleading.

What causes the large extensions of red supergiant atmospheres?

“…We also observe a monotonic decrease beyond 2.3 μm, which may be caused by pseudocontinuum contributions from CO or by contributions from water vapor. We observed the same phenomenon previously for the RSGs VY CMa (Wittkowski et al 2012), AH Sco, UY Sct, and KW Sgr (Arroyo-Torres et al (2013), as well as for the smallamplitude pulsating red giants RS Cap (Marti-Vidal et al 2011), BK Vir (Ohnaka et al 2012), α Tau (Ohnaka 2013), and β Peg (Arroyo-Torres et al 2014).…”

High spectral resolution imaging of the dynamical atmosphere of the red supergiant Antares in the CO first overtone lines with VLTI/AMBER

“…While the presence of the MOLsphere is confirmed by the IR interferometric imaging of Antares and Betelgeuse in the individual CO lines, there are still problems with the MOLsphere, as mentioned in Ohnaka et al (2012). The additional TiO absorption originating in the MOLsphere makes the TiO bands too strong compared to the observed spectra of RSGs (Hron et al 2010).…”

“…However, the spectrum of α Cen A shows weak CO first overtone lines. In this case, as described in Ohnaka et al (2012), the calibrated spectrum of the science target is derived as F sci = F sci obs /(F cal obs /F cal ), where F sci(cal) and F sci(cal) obs denote the true and observed (i.e., including the atmospheric transmission and the detector's response) spectra of the science target (or calibrator), respectively. We used the high resolution solar spectrum presented by Wallace & Hinkle (1996) to estimate the true spectrum of the calibrator α Cen A, because this star has the same spectral type as the Sun.…”

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

“…AMBER is a spectro-interferometric instrument at VLTI and can provide high spatial resolutions and high spectral resolutions of up to 12 000 (Petrov et al 2007) by combining three 8.2 m Unit Telescopes (UTs) or 1.8 m Auxiliary Telescopes (ATs). AMBER measures the visibility, which is the amplitude of the Fourier transform of the object's intensity distribution on the sky, as well as the closure phase and differential phase.…”