J, H, K spectro-interferometry of the Mira variable S Orionis
Wittkowski, M.; Boboltz, D. A.; Driebe, T.; Le Bouquin, J.-B.; Millour, F.; Ohnaka, K.; Scholz, M.
Astronomy and Astrophysics, Volume 479, Issue 1, pp.L21-L24 (2008)
Abstract
Aims.We present J, H, K spectrally dispersed interferometry with a spectral resolution of 35 for the Mira variable S Orionis. We aim at measuring the diameter variation
as a function of wavelength that is expected due to molecular layers lying above the continuum-forming photosphere. Our final goal is a better understanding of the
pulsating atmosphere and its role in the mass-loss process. Methods: Visibility data of S Ori were obtained at phase 0.78 with the VLTI/AMBER instrument using the fringe
tracker FINITO at 29 spectral channels between 1.29 μm and 2.32 μm. Apparent uniform disk (UD) diameters were computed for each spectral channel. In addition, the
visibility data were directly compared to predictions by recent self-excited dynamic model atmospheres. Results: S Ori shows significant variations in the visibility
values as a function of spectral channel that can only be described by a clear variation in the apparent angular size with wavelength. The closure phase values are close
to zero at all spectral channels, indicating the absence of asymmetric intensity features. The apparent UD angular diameter is smallest at about 1.3 μm and 1.7 μm and
increases by a factor of ~1.4 around 2.0 μm. The minimum UD angular diameter at near-continuum wavelengths is Θ_UD=8.1 ± 0.5 mas, corresponding to R˜ 420 R_о. The S Ori
visibility data and the apparent UD variations can be explained reasonably well by a dynamic atmosphere model that includes molecular layers, particularly water vapor and
CO. The best-fitting photospheric angular diameter of the model atmosphere is Θ_Phot=8.3±0.2 mas, consistent with the UD diameter measured at near-continuum wavelengths.
Conclusions: The measured visibility and UD diameter variations with wavelength resemble and generally confirm the predictions by recent dynamic model atmospheres. These
size variations with wavelength can be understood as the effects from water vapor and CO layers lying above the continuum-forming photosphere. The major remaining
differences between observations and model prediction are very likely due to an imperfect match of the phase and cycle combination between observation and available models.
Based on observations made with the VLT Interferometer (VLTI) at Paranal Observatory under program ID 080.D-0691.
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