Inhomogeneities in molecular layers of Mira atmospheres

Abstract

Aims. We investigate the structure and shape of the photospheric and molecular layers of the atmospheres of four Mira variables. Methods. We obtained near-infrared K-band spectro-interferometric observations of the Mira variables R Cnc, X Hya, W Vel, and RW Vel with a spectral resolution of about 1500 using the AMBER instrument at the VLTI. We obtained concurrent JHKL photometry using the the Mk II instrument at the SAAO. Results. The Mira stars in our sample are found to have wavelength-dependent visibility values that are consistent with earlier low-resolution AMBER observations of S Ori and with the predictions of dynamic model atmosphere series based on self-excited pulsation models. The corresponding wavelength-dependent uniform disk (UD) diameters show a minimum near the near-continuum bandpass at 2.25 μm. They then increase by up to 30% toward the H 2 O band at 2.0 μm and by up to 70% at the CO bandheads between 2.29 μm and 2.48 μm. The dynamic model atmosphere series show a consistent wavelength-dependence, and their parameters such as the visual phase, effective temperature, and distances are consistent with independent estimates. The closure phases have significantly wavelength-dependent and non-zero values at all wavelengths indicating deviations from point symmetry. For example, the R Cnc closure phase is 110 • ± 4 • in the 2.0 μm H 2 O band, corresponding for instance to an additional unresolved spot contributing 3% of the total flux at a separation of ∼4 mas. Conclusions. Our observations are consistent with the predictions of the latest dynamic model atmosphere series based on self-excited pulsation models. The wavelength-dependent radius variations are interpreted as the effect of molecular layers lying above the photosphere. The wavelength-dependent closure phase values are indicative of deviations from point symmetry at all wavelengths, thus a complex non-spherical stratification of the extended atmosphere. In particular, the significant deviation from point symmetry in the H 2 O band is interpreted as a signature on large scales (there being a few across the stellar disk) of inhomogeneities or clumps in the water vapor layer. The observed inhomogeneities might possibly be caused by pulsation-and shock-induced chaotic motion in the extended atmosphere.

Fundamental properties and atmospheric structure of the red supergiant VY Canis Majoris based on VLTI/AMBER spectro-interferometry

“…4). photosphere (Wittkowski et al 2008(Wittkowski et al , 2011. Our VY CMa visibility curves show maxima near 2.25 μm and 1.70 μm, where the intensity of molecular bands is lowest.…”

What causes the large extensions of red supergiant atmospheres?

“…We also want to compare the behavior of RSG stars and Mira stars, which also exhibit extended molecular layers (cf., e.g., Wittkowski et al 2011). We used the ratio of the observed visibilities in the continuum band (average between 2.27 and 2.28 μm) and the first (2−0) CO line at 2.294 μm as an observational indication of the contribution from extended atmospheric CO layers.…”

“…Figure 10 shows the resulting ratios (V 2 cont /V 2 CO ) for our sources vs. log(L/L ) and ΔV, considering the RSGs by Wittkowski et al (2012), Arroyo-Torres et al (2013) Arroyo- Torres et al (2014). Results for Mira stars obtained by Wittkowski et al (2011) are also shown for comparison. Figure 11 shows the same visibility ratio but only for the RSG sample.…”

“…Self-excited pulsation models of Mira-variable AGB stars (Ireland et al 2004a(Ireland et al ,b, 2008Scholz et al 2014) have been successful to describe interferometric observations of these sources, including their extended atmospheric molecular layers (Woodruff et al 2009;Wittkowski et al 2011;Hillen et al 2012). The observed visibility spectra of our RSG sources show similar features as those of Mira stars, in particular at the CO bandheads (see Sect.…”

“…Observations of Mira variable stars using the IOTA or VLTI interferometers show evidence of molecular layers lying above the photospheric layers (e.g., Perrin et al 2004;Wittkowski et al 2008Wittkowski et al , 2011). Theoretical dynamic model atmospheres (Ireland et al 2004a(Ireland et al ,b, 2008Scholz et al 2014) can explain reasonably well these molecular layers for Miras.…”