DEATHSTAR: Nearby AGB stars with the Atacama Compact Array

I. CO envelope sizes and asymmetries: A new hope for accurate mass-loss-rate estimates^⋆

¹ Theoretical Astrophysics, Division for Astronomy and Space Physics, Department of Physics and Astronomy, Uppsala University, Box 516 751 20 Uppsala, Sweden
e-mail: sofia.ramstedt@physics.uu.se
² Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
³ Department of Physics and Astronomy, Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁴ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussels, Belgium
⁵ Rochester Institute of Technology, Rochester, NY, USA
⁶ Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
⁷ Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA
⁸ Instituto de Física Fundamental (IFF-CSIC), Serrano 123, Madrid, CP 28006, Spain
⁹ Jet Propulsion Laboratory, MS 183-900, California Institute of Technology, Pasadena, CA 91109, USA
¹⁰ Jodrell Bank Centre for Astrophysics, Alan Turing Building, University of Manchester, Manchester M13 9PL, UK

Received: 8 October 2019
Accepted: 22 April 2020

Abstract

Context. This is the first publication from the DEATHSTAR project. The overall goal of the project is to reduce the uncertainties of the observational estimates of mass-loss rates from evolved stars on the Asymptotic Giant Branch (AGB).

Aim. The aim in this first publication is to constrain the sizes of the ¹²CO emitting region from the circumstellar envelopes around 42 mostly southern AGB stars, of which 21 are M-type and 21 are C-type, using the Atacama Compact Array (ACA) at the Atacama Large Millimeter/submillimeter Array. The symmetry of the outflows is also investigated.

Methods. Line emission from ¹²CO J = 2→1 and 3→2 from all of the sources were mapped using the ACA. In this initial analysis, the emission distribution was fit to a Gaussian distribution in the uv-plane. A detailed radiative transfer analysis will be presented in a future publication. The major and minor axis of the best-fit Gaussian at the line center velocity of the ¹²CO J = 2→1 emission gives a first indication of the size of the emitting region. Furthermore, the fitting results, such as the Gaussian major and minor axis, center position, and the goodness of fit across both lines, constrain the symmetry of the emission distribution. For a subsample of sources, the measured emission distribution is compared to predictions from previous best-fit radiative transfer modeling results.

Results. We find that the CO envelope sizes are, in general, larger for C-type than for M-type AGB stars, which is as expected if the CO/H₂ ratio is larger in C-type stars. Furthermore, the measurements show a relation between the measured (Gaussian) ¹²CO J = 2→1 size and circumstellar density that, while in broad agreement with photodissociation calculations, reveals large scatter and some systematic differences between the different stellar types. For lower mass-loss-rate irregular and semi-regular variables of both M- and C-type AGB stars, the ¹²CO J = 2→1 size appears to be independent of the ratio of the mass-loss rate to outflow velocity, which is a measure of circumstellar density. For the higher mass-loss-rate Mira stars, the ¹²CO J = 2→1 size clearly increases with circumstellar density, with larger sizes for the higher CO-abundance C-type stars. The M-type stars appear to be consistently smaller than predicted from photodissociation theory. The majority of the sources have CO envelope sizes that are consistent with a spherically symmetric, smooth outflow, at least on larger scales. For about a third of the sources, indications of strong asymmetries are detected. This is consistent with what was found in previous interferometric investigations of northern sources. Smaller scale asymmetries are found in a larger fraction of sources.

Conclusions. These results for CO envelope radii and shapes can be used to constrain detailed radiative transfer modeling of the same stars so as to determine mass-loss rates that are independent of photodissociation models. For a large fraction of the sources, observations at higher spatial resolution will be necessary to deduce the nature and origin of the complex circumstellar dynamics revealed by our ACA observations.