Letter to the Editor

Inhomogeneities in molecular layers of Mira atmospheres^⋆,⋆⋆

¹ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
e-mail: mwittkow@eso.org
² US Naval Observatory, 3450 Massachusetts Avenue, NW, Washington, DC 20392-5420, USA
³ Department of Physics and Astronomy, Macquarie University NSW 2109, Australia
⁴ Australian Astronomical Observatory, PO Box 296, Epping, NSW 1710, Australia
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Zentrum für Astronomie der Universität Heidelberg (ZAH), Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁷ Sydney Institute for Astronomy, School of Physics, University of Sydney, Sydney NSW 2006, Australia
⁸ South African Astronomical Observatory (SAAO), PO Box 9, 7935 Observatory, South Africa
⁹ Astrophysics, Cosmology and Gravitation Centre, Astronomy Dept., University of Cape Town, 7701 Rondebosch, South Africa
¹⁰ Research School of Astronomy and Astrophysics, The Australian National University, Canberra, Australia
¹¹ School of Physics & Astronomy, University of Manchester, PO Box 88, Manchester M60 1QD, UK

Received: 6 June 2011
Accepted: 2 July 2011

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 H₂O 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 H₂O 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 H₂O 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.