New insights into the dust formation of oxygen-rich AGB stars ^⋆,⋆⋆

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: ikarovic@mpia.de
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
³ Max-Planck-Institut für Radioastronomie, auf dem Hügel 69, 53121 Bonn, Germany
⁴ United States Naval Observatory, 3450 Massachusetts Avenue, Washington DC 20392-5420, NW, USA Present address: National Science Foundation, 4201 Wilson Boulevard, Arlington, VA 22230, USA
⁵ Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
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, NSW, 2006 Sydney, Australia

Received: 26 July 2013
Accepted: 30 September 2013

Abstract

Context. Asymptotic giant branch (AGB) stars are one of the major sources of dust in the universe. The formation of molecules and dust grains and their subsequent expulsion into the interstellar medium via strong stellar winds is under intense investigation. This is in particular true for oxygen-rich stars, for which the path of dust formation has remained unclear.

Aims. We conducted spatially and spectrally resolved mid-infrared multi-epoch interferometric observations to investigate the dust formation process in the extended atmospheres of oxygen-rich AGB stars.

Methods. We observed the Mira variable AGB stars S Ori, GX Mon, and R Cnc between February 2006 and March 2009 with the MIDI instrument at the VLT interferometer. We compared the data to radiative transfer models of the dust shells, where the central stellar intensity profiles were described by dust-free dynamic model atmospheres. We used Al₂O₃ and warm silicate grains, following earlier studies in the literature.

Results. Our S Ori and R Cnc data could be well described by an Al₂O₃ dust shell alone, and our GX Mon data by a mix of an Al₂O₃ and a silicate shell. The best-fit parameters for S Ori and R Cnc included photospheric angular diameters Θ_Phot of 9.7 ± 1.0 mas and 12.3 ± 1.0 mas, optical depths τ_V(Al₂O₃) of 1.5 ± 0.5 and 1.35 ± 0.2, and inner radii R_in of 1.9 ± 0.3 R_Phot and 2.2 ± 0.3 R_Phot, respectively. Best-fit parameters for GX Mon were Θ_Phot = 8.7 ± 1.3 mas, τ_V(Al₂O₃) = 1.9 ± 0.6, R_in(Al₂O₃) = 2.1 ± 0.3 R_Phot, τ_V(silicate)= 3.2 ± 0.5, and R_in(silicate)= 4.6 ± 0.2 R_Phot. Our data did not show evidence of intra-cycle and cycle-to-cycle variability or of asymmetries within the error-bars and within the limits of our baseline and phase coverage.

Conclusions. Our model fits constrain the chemical composition and the inner boundary radii of the dust shells, as well as the photospheric angular diameters. Our interferometric results are consistent with Al₂O₃ grains condensing close to the stellar surface at about 2 stellar radii, co-located with the extended atmosphere and SiO maser emission, and warm silicate grains at larger distances of about 4–5 stellar radii. We verified that the number densities of aluminum can match that of the best-fit Al₂O₃ dust shell near the inner dust radius in sufficiently extended atmospheres, confirming that Al₂O₃ grains can be seed particles for the further dust condensation. Together with literature data of the mass-loss rates, our sample is consistent with a hypothesis that stars with low mass-loss rates form primarily dust that preserves the spectral properties of Al₂O₃, and stars with higher mass-loss rate form dust with properties of warm silicates.