VLTI/AMBER spectro-interferometric imaging of VX Sagittarii's inhomogenous outer atmosphere

Abstract

Aims. We aim to explore the photosphere of the very cool late-type star VX Sgr and in particular the characterization of molecular layers above the continuum forming photosphere. Methods. We obtained interferometric observations with the VLTI/AMBER interferometer using the fringe tracker FINITO in the spectral domain 1.45−2.50 μm with a spectral resolution of ≈35 and baselines ranging from 15 to 88 m. We performed independent image reconstruction for different wavelength bins and fit the interferometric data with a geometrical toy model. We also compared the data to 1D dynamical models of Miras atmosphere and to 3D hydrodynamical simulations of red supergiant (RSG) and asymptotic giant branch (AGB) stars. Results. Reconstructed images and visibilities show a strong wavelength dependence. The H-band images display two bright spots whose positions are confirmed by the geometrical toy model. The inhomogeneities are qualitatively predicted by 3D simulations. At ≈2.00 μm and in the region 2.35−2.50 μm, the photosphere appears extended and the radius is larger than in the H band. In this spectral region, the geometrical toy model locates a third bright spot outside the photosphere that can be a feature of the molecular layers. The wavelength dependence of the visibility can be qualitatively explained by 1D dynamical models of Mira atmospheres. The best-fitting photospheric models show a good match with the observed visibilities and give a photospheric diameter of Θ = 8.82 ± 0.50 mas. The H 2 O molecule seems to be the dominant absorber in the molecular layers. Conclusions. We show that the atmosphere of VX Sgr seems to resemble Mira/AGB star model atmospheres more closely than do RSG model atmospheres. In particular, we see molecular (water) layers that are typical of Mira stars.

The atmospheric structure and fundamental parameters of the red supergiants AH Scorpii, UY Scuti, and KW Sagittarii

“…This has also already been observed for VX Sgr (Chiavassa et al 2010). Although the pulsation amplitudes of RSGs are generally lower than those of Mira variable AGB stars, we speculate that the same process of pulsation-induced shock waves may also lead to the extended molecular layers that we observed here for the RSGs AH Sco, UY Sct, and KW Sgr, and that have also been observed previously for other RSGs.…”

“…Also shown are three additional RSGs that have been observed in a similar way using the VLTI/AMBER instrument, which are VX Sgr (Chiavassa et al 2010), Betelgeuse (Ohnaka et al 2009(Ohnaka et al , 2011, and VY CMa (Wittkowski et al 2012). We calculated the (2011) of 3690 ± 54 K. Likewise, we calculated the properties of VX Sgr using the photospheric angular diameter of 8.82 ± 0.5 mas by Chiavassa et al (2010), the distance of 1570 ± 270 pc by Chen & Shen (2008), and the bolometric flux calculated by us ( f bol = (5.00 ± 1.0) 10 −9 W m 2 ). It has been discussed that VX Sgr may be either a relatively massive AGB star or a red supergiant (Chiavassa et al 2010).…”

“…We calculated the (2011) of 3690 ± 54 K. Likewise, we calculated the properties of VX Sgr using the photospheric angular diameter of 8.82 ± 0.5 mas by Chiavassa et al (2010), the distance of 1570 ± 270 pc by Chen & Shen (2008), and the bolometric flux calculated by us ( f bol = (5.00 ± 1.0) 10 −9 W m 2 ). It has been discussed that VX Sgr may be either a relatively massive AGB star or a red supergiant (Chiavassa et al 2010). Our values suggest that VX Sgr is an RSG, so we used the spectral type of M4 Ia by Humphreys et al (1972) Figure 6 shows the positions of AH Sco, UY Sct, and KW Sgr in the Hertzsprung-Russell (HR) diagram, together with the recent evolutionary tracks from Ekström et al (2012).…”

What causes the large extensions of red supergiant atmospheres?

“…On the other hand, interferometric observations of RSGs also indicate the presence of extended molecular layers (CO and water), which cannot be explained by hydrostatic model atmospheres (Perrin et al 2005;Ohnaka et al 2011Ohnaka et al , 2013Wittkowski et al 2012). The RSG VX Sgr (Chiavassa et al 2010a) showed a good agreement with Mira models, although they have very different stellar parameters than expected for this source.…”

“…In order to understand the processes that may explain the extended molecular layers, the second goal of this paper is to investigate the effects of realistic three-dimensional (3D) radiative hydrodynamical (RHD) simulations of stellar convection as well as of one-dimensional (1D) self-excited pulsation models on the extensions of RSG atmospheres. These processes were previously discussed as possible mechanisms to levitate RSG atmospheres (Chiavassa et al 2010a(Chiavassa et al , 2011a.…”