The VLTI/MIDI view on the inner mass loss of evolved stars from theHerschelMESS sample

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, π 1 Gruis, 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.

The adventure of carbon stars

“…Our sample consists of stars observed with the Very Large Telescope Interferometer (VLTI) of ESO's Paranal Observatory with the mid-infrared interferometric recombiner (MIDI, Leinert et al 2003) instrument; showing (1) a SiC feature in the visibility spectrum and (2) no evidence of asymmetry from differential phase (Paladini et al 2017). The stars can be grouped into Mira variables (R Lep, R Vol), Semi-regular (Y Pav, AQ Sgr, U Hya), and Irregular (X TrA) stars (Samus et al 2009).…”

“…For two stars, namely R Vol and U Hya, we are presenting new VLTI/MIDI data observed within the programs 090.D-0410(A) and 086.D-0899(K). For the remaining stars, our data come from archive observations (Paladini et al 2017). …”

“…Details on the data of R Lep, Y Pav, AQ Sgr, and X Tra are given in Paladini et al (2017), where the reader will also find the uv-coverage and the journal of the observations.…”

“…(g) Bergeat & Chevallier (2005). Geometric modeling for the remaining stars in our sample are presented in Paladini et al (2017). For a discussion and interpretation, we refer the reader to the values published in their Table 4.…”

Tomography of silicate dust around M-type AGB stars

“…It will be followed by a series of papers comparing various models with existing spatially resolved interferometric observations for M-type AGB stars (e.g., data acquired within the VLTI/MIDI AGB Large Program; Paladini et al 2017).…”