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High-resolution observations of gas and dust around Mira using ALMA and SPHERE/ZIMPOL
Chalmers Univ Technol, Dept Space Earth & Environm, Onsala Space Observ, S-43992 Onsala, Sweden.
Chalmers Univ Technol, Dept Space Earth & Environm, Onsala Space Observ, S-43992 Onsala, Sweden.
Chalmers Univ Technol, Dept Space Earth & Environm, Onsala Space Observ, S-43992 Onsala, Sweden.
Sterrewacht Leiden, POB 9513,Niels Bohrweg 2, NL-2300 RA Leiden, Netherlands.
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 620, article id A75Article in journal (Refereed) Published
Abstract [en]

Context. The outflows of oxygen-rich asymptotic giant branch (AGB) stars are thought to be driven by radiation pressure due to the scattering of photons on relatively large grains, with sizes of tenths of microns. The details of the formation of dust in the extended atmospheres of these stars and, therefore, the mass-loss process, is still not well understood.

Aims. We aim to constrain the distribution of the gas and the composition and properties of the dust grains that form in the inner circumstellar environment of the archetypal Mira variable o Cet.

Methods. We obtained quasi-simultaneous observations using ALMA and SPHERE/ZIMPOL on the Very Large Telescope (VLT) to probe the distribution of gas and large dust grains, respectively.

Results. The polarized light images show dust grains around Mira A, but also around the companion, Mira B, and a dust trail that connects the two sources. The ALMA observations show that dust around Mira A is contained in a high-gas-density region with a significant fraction of the grains that produce the polarized light located at the edge of this region. Hydrodynamical and wind-driving models show that dust grains form efficiently behind shock fronts caused by stellar pulsation or convective motions. The distance at which we observe the density decline (a few tens of au) is, however, significantly larger than expected for stellar-pulsation-induced shocks. Other possibilities for creating the high-gas-density region are a recent change in the mass-loss rate of Mira A or interactions with Mira B. We are not able to determine which of these scenarios is correct. We constrained the gas density, temperature, and velocity within a few stellar radii from the star by modelling the CO v = 1, J = 3-2 line. We find a mass (similar to 3.8 +/- 1.3) x 10(-4) M-circle dot to be contained between the stellar millimetre photosphere, R-*(338 GHz) and 4 R-*(338 GHz). Our best-fit models with lower masses also reproduce the (CO)-C-13 v = 0, J = 3-2 line emission from this region well. We find TiO2 and AlO abundances corresponding to 4.5% and <0.1% of the total titanium and aluminium expected for a gas with solar composition. The low abundance of AlO allows for a scenario in which Al depletion into dust happens already very close to the star, as expected from thermal dust emission observations and theoretical calculations of Mira variables. The relatively large abundance of aluminium for a gas with solar composition allows us to constrain the presence of aluminium oxide grains based on the scattered light observations and on the gas densities we obtain. These models imply that aluminium oxide grains could account for a significant fraction of the total aluminium atoms in this region only if the grains have sizes less than or similar to 0.02 mu m. This is an order of magnitude smaller than the maximum sizes predicted by dust-formation and wind-driving models.

Conclusions. The study we present highlights the importance of coordinated observations using different instruments to advance our understanding of dust nucleation, dust growth, and wind driving in AGB stars.

Place, publisher, year, edition, pages
2018. Vol. 620, article id A75
Keywords [en]
stars: AGB and post-AGB, stars: imaging, stars: individual: Mira Ceti, stars: winds, outflows, polarization, circumstellar matter
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-372708DOI: 10.1051/0004-6361/201833643ISI: 000452371000001OAI: oai:DiVA.org:uu-372708DiVA, id: diva2:1276715
Funder
Swedish Research CouncilSwedish National Space BoardAvailable from: 2019-01-08 Created: 2019-01-08 Last updated: 2019-01-08Bibliographically approved

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