).
Institut für Angewandte Physik,
TU Wien, 1040 Wien, Austria
Instituut voor Sterrenkunde, KU Leuven, 3001 Leuven, Belgium
National Centre for Nuclear Research, 02-093 Warsaw, Poland
INAF – Osservatorio astronomico d'Abruzzo, 64100 Teramo, Italy
Department of Astrophysics, University of Vienna, 1180 Vienna, Austria
Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, School of Natural Sciences, University of Manchester, M13 9PL Manchester, UK
Department of Physical Sciences, The Open University, MK7 6AA Milton Keynes, UK
Hamburger Sternwarte, Universität Hamburg, 21029 Hamburg, Germany
INFN, Sezione di Perugia, Via A. Pascoli, 06123 Perugia, Italy
American Association of Variable Star Observers (AAVSO), Cambridge, MA, USA
Context. The details of the mass-loss process in the late stages
of low- and intermediate-mass stellar evolution are not well understood,
in particular its dependence on stellar parameters.
Mira variables are highly suitable targets for studying this mass-loss process.
Aims. Here, we follow up on our earlier finding that a near-to-mid-infrared (NIR-MIR)
colour versus pulsation period diagram shows two sequences of Miras
that can be distinguished by the third dredge-up (3DUP) indicator technetium in those stars.
While IR colours are good indicators of the dust mass-loss rate (MLR) from Miras,
no corresponding sequences have been found using the gas MLR.
However, investigations of the gas MLR have been hampered by data limitations.
We aim to alleviate these limitations with new observational data.
Methods. We present new optical spectra of a well-selected sample of Miras.
We searched these spectra for absorption lines of Tc and other 3DUP indicators,
and combine our findings with gas MLRs and expansion velocities from the literature.
Furthermore, we extend the analysis of the MIR emission to WISE data
and compare the broadband spectral energy distributions (SEDs) of Miras with and without Tc.
Results. We find no systematic difference in gas MLRs between Miras with and without Tc.
However, the gas envelopes of Tc-poor Miras appear to have
a higher terminal expansion velocity than those of Miras with Tc.
Furthermore, our analysis of the IR photometry strongly corroborates the earlier finding
that Tc-poor Miras have a higher MIR emission than Tc-rich ones, by as much as a factor of two.
We model the IR colours with DARWIN and stationary wind models and conclude that
Miras with and without Tc have different dust content or dust properties.
Conclusions. We discuss several hypotheses and interpretations of the observations
and conclude that the reduction of free oxygen by 3DUP of carbon and iron-depleted dust grains
in Tc-rich stars are the most convincing explanations for our observations.
Corresponding author: Stefan Uttenthaler (uttenthaler).
You can download a PDF file of this open-access article from Astronomy & Astrophysics.