Dust and molecular shells in asymptotic giant branch stars^{⋆,⋆⋆,⋆⋆⋆}

Mid-infrared interferometric observations of R Aquilae, R Aquarii, R Hydrae, W Hydrae, and V Hydrae

¹ Zentrum für Astronomie der Universität Heidelberg (ZAH), Landessternwarte, Königstuhl 12, 69120 Heidelberg, Germany
e-mail: rgeisler@ntnu.edu.tw
² National Taiwan Normal University, Department of Earth Sciences, 88 Sec. 4, Ting-Chou Rd, Wenshan District, Taipei, 11677 Taiwan, ROC
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69120 Heidelberg, Germany
⁴ Laboratoire J.-L. Lagrange, Université de Nice Sophia-Antipolis et Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France

Received: 26 September 2011
Accepted: 21 June 2012

Abstract

Context. Asymptotic giant branch (AGB) stars are one of the largest distributors of dust into the interstellar medium. However, the wind formation mechanism and dust condensation sequence leading to the observed high mass-loss rates have not yet been constrained well observationally, in particular for oxygen-rich AGB stars.

Aims. The immediate objective in this work is to identify molecules and dust species which are present in the layers above the photosphere, and which have emission and absorption features in the mid-infrared (IR), causing the diameter to vary across the N-band, and are potentially relevant for the wind formation.

Methods. Mid-IR (8–13 μm) interferometric data of four oxygen-rich AGB stars (R Aql, R Aqr, R Hya, and W Hya) and one carbon-rich AGB star (V Hya) were obtained with MIDI/VLTI between April 2007 and September 2009. The spectrally dispersed visibility data are analyzed by fitting a circular fully limb-darkened disk (FDD).

Results. The FDD diameter as function of wavelength is similar for all oxygen-rich stars. The apparent size is almost constant between 8 and 10 μm and gradually increases at wavelengths longer than 10 μm. The apparent FDD diameter in the carbon-rich star V Hya essentially decreases from 8 to 12 μm. The FDD diameters are about 2.2 times larger than the photospheric diameters estimated from K-band observations found in the literature. The silicate dust shells of R Aql, R Hya and W Hya are located fairly far away from the star, while the silicate dust shell of R Aqr and the amorphous carbon (AMC) and SiC dust shell of V Hya are found to be closer to the star at around 8 photospheric radii. Phase-to-phase variations of the diameters of the oxygen-rich stars could be measured and are on the order of 15% but with large uncertainties.

Conclusions. From a comparison of the diameter trend with the trends in RR Sco and S Ori it can be concluded that in oxygen-rich stars the overall larger diameter originates from a warm molecular layer of H₂O, and the gradual increase longward of 10 μm can be most likely attributed to the contribution of a close Al₂O₃ dust shell. The chromatic trend of the Gaussian FWHM in V Hya can be explained with the presence of AMC and SiC dust. The observations suggest that the formation of amorphous Al₂O₃ in oxygen-rich stars occurs mainly around or after visual minimum. However, no firm conclusions can be drawn concerning the mass-loss mechanism. Future modeling with hydrostatic and self-consistent dynamical stellar atmospheric models will be required for a more certain understanding.