The ALMA detection of CO rotational line emission in AGB stars in the Large Magellanic Cloud

¹ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussels, Belgium
e-mail: martin.groenewegen@oma.be
² Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
³ Department of Physics and Astronomy G. Galilei, University of Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁴ Cornell Center for Astrophysics & Planetary Science, Cornell Univ., Ithaca, NY 14853-6801, USA
⁵ Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA
⁶ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
⁷ Institute of Astronomy, Department of Physics and Astronomy, University of Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁸ Astronomy Department, University of Cape Town, 7701 Rondebosch, South Africa
⁹ South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa
¹⁰ Lennard-Jones Laboratories, Keele University, Staffordshire ST5 5BG, UK
¹¹ Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
¹² School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff, CF24 3AA, UK
¹³ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹⁴ Jet Propulsion Laboratory, MS 183-900, California Institute of Technology, Pasadena, CA 91109, USA
¹⁵ Research School of Astronomy & Astrophysics, The Australian National University, Canberra, ACT 2611, Australia
¹⁶ Department of Physical Sciences, The Open University, MK7 6AA, Milton Keynes, UK
¹⁷ Observational Cosmology Lab, Code 665, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁸ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹⁹ European Southern Observatory, Alonso de Córdova 3107, Santiago, Chile
²⁰ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
²¹ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
²² East Asian Observatory, 660 N. A’ohoku Place, University Park, Hilo, Hawaii 96720, USA
²³ Institute of Astronomy & Astrophysics, Academia Sinica, 11F, Astronomy-Mathematics Building, No. 1, Roosevelt Rd., Sec 4, 10617 Taipei, Taiwan, R.O.C.

Received: 26 August 2016
Accepted: 29 September 2016

Abstract

Context. Low- and intermediate-mass stars lose most of their stellar mass at the end of their lives on the asymptotic giant branch (AGB). Determining gas and dust mass-loss rates (MLRs) is important in quantifying the contribution of evolved stars to the enrichment of the interstellar medium.

Aims. This study attempts to spectrally resolve CO thermal line emission in a small sample of AGB stars in the Large Magellanic Cloud (LMC).

Methods. The Atacama Large Millimeter Array was used to observe two OH/IR stars and four carbon stars in the LMC in the CO J = 2−1 line.

Results. We present the first measurement of expansion velocities in extragalactic carbon stars. All four C stars are detected and wind expansion velocities and stellar velocities are directly measured. Mass-loss rates are derived from modelling the spectral energy distribution and Spitzer/IRS spectrum with the DUSTY code. The derived gas-to-dust ratios allow the predicted velocities to agree with the observed gas-to-dust ratios. The expansion velocities and MLRs are compared to a Galactic sample of well-studied relatively low MLRs stars supplemented with extreme C stars with properties that are more similar to the LMC targets. Gas MLRs derived from a simple formula are significantly smaller than those derived from dust modelling, indicating an order of magnitude underestimate of the estimated CO abundance, time-variable mass loss, or that the CO intensities in LMC stars are lower than predicted by the formula derived for Galactic objects. This could be related to a stronger interstellar radiation field in the LMC.

Conclusions. Although the LMC sample is small and the comparison to Galactic stars is non-trivial because of uncertainties in their distances (hence luminosities), it appears that for C stars the wind expansion velocities in the LMC are lower than in the solar neighbourhood, while the MLRs appear to be similar. This is in agreement with dynamical dust-driven wind models.