The VLTI/MIDI^⋆ view on the inner mass loss of evolved stars from the Herschel MESS sample

¹ Institut d’Astronomie et d’Astrophysique, Université libre de Bruxelles, CP 226, Boulevard du Triomphe, 1050 Brussels, Belgium
e-mail: claudia.paladini@ulb.ac.be
² University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Wien, Austria
³ University of Uppsala, Department of Physics and Astronomy, Division of Astronomy and Space Physics, Box 516, 75120 Uppsala, Sweden
⁴ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, boulevard de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
⁵ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁶ National Astronomical Research Institute of Thailand, 191 Siriphanich Bldg., Huay Kaew Rd., Suthep, Muang, 50200 Chiang Mai, Thailand
⁷ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium
⁸ Belgian Institute for Space Aeronomy (BIRA-IASB), Ringlaan-3-Avenue Circulaire, 1180 Brussels, Belgium
⁹ Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
¹⁰ National Taiwan Normal University, Department of Earth Sciences, 88 Sec. 4, Ting-Chou Rd, Wenshan District, 11677 Taipei, Taiwan, ROC

Received: 19 August 2015
Accepted: 4 January 2017

Abstract

Context. The mass-loss process from evolved stars is a key ingredient for our understanding of many fields of astrophysics, including stellar evolution and the chemical enrichment of the interstellar medium (ISM) via stellar yields. Nevertheless, many questions are still unsolved, one of which is the geometry of the mass-loss process.

Aims. Taking advantage of the results from the Herschel Mass loss of Evolved StarS (MESS) programme, we initiated a coordinated effort to characterise the geometry of mass loss from evolved red giants at various spatial scales.

Methods. For this purpose we used the MID-infrared interferometric Instrument (MIDI) to resolve the inner envelope of 14 asymptotic giant branch stars (AGBs) in the MESS sample. In this contribution we present an overview of the interferometric data collected within the frame of our Large Programme, and we also add archive data for completeness. We studied the geometry of the inner atmosphere by comparing the observations with predictions from different geometric models.

Results. Asymmetries are detected for the following five stars: R Leo, RT Vir, π¹Gruis, omi Ori, and R Crt. All the objects are O-rich or S-type, suggesting that asymmetries in the N band are more common among stars with such chemistry. We speculate that this fact is related to the characteristics of the dust grains. Except for one star, no interferometric variability is detected, i.e. the changes in size of the shells of non-mira stars correspond to changes of the visibility of less than 10%. The observed spectral variability confirms previous findings from the literature. The detection of dust in our sample follows the location of the AGBs in the IRAS colour–colour diagram: more dust is detected around oxygen-rich stars in region II and in the carbon stars in region VII. The SiC dust feature does not appear in the visibility spectrum of the U Ant and S Sct, which are two carbon stars with detached shells. This finding has implications for the theory of SiC dust formation.