New observations and models of circumstellar CO line emission of AGB stars in the Herschel SUCCESS programme⋆,⋆⋆,⋆⋆⋆
1 Onsala Space ObservatoryDepartment of Earth and Space Sciences, Chalmers University of Technology, 439 92 Onsala, Sweden
2 European Space Astronomy Centre, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
3 ASTRON, the Netherlands Institute for Radioastronomy, PO Box 2, 7990 AA Dwingeloo, The Netherlands
4 Observatorio Astronómico Nacional (IGN), PO Box 112, 28803 Alcalá de Henares, Spain
5 Observatorio Astronómico Nacional (IGN), Alfonso XII, 3 y 5, 28014 Madrid, Spain
6 Group of Molecular Astrophysics. ICMM. CSIC. C/ Sor Juana Inés de La Cruz N3, 28049 Madrid, Spain
7 Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d’ Hères, France
Received: 9 June 2015
Accepted: 28 June 2015
Context. Asymptotic giant branch (AGB) stars are in one of the latest evolutionary stages of low to intermediate-mass stars. Their vigorous mass loss has a significant effect on the stellar evolution, and is a significant source of heavy elements and dust grains for the interstellar medium. The mass-loss rate can be well traced by carbon monoxide (CO) line emission.
Aims. We present new Herschel/HIFI and IRAM 30 m telescope CO line data for a sample of 53 galactic AGB stars. The lines cover a fairly large range of excitation energy from the J = 1 → 0 line to the J = 9 → 8 line, and even the J = 14 → 13 line in a few cases. We perform radiative transfer modelling for 38 of these sources to estimate their mass-loss rates.
Methods. We used a radiative transfer code based on the Monte Carlo method to model the CO line emission. We assume spherically symmetric circumstellar envelopes that are formed by a constant mass-loss rate through a smoothly accelerating wind.
Results. We find models that are consistent across a broad range of CO lines for most of the stars in our sample, i.e., a large number of the circumstellar envelopes can be described with a constant mass-loss rate. We also find that an accelerating wind is required to fit, in particular, the higher-J lines and that a velocity law will have a significant effect on the model line intensities. The results cover a wide range of mass-loss rates (~10-8 to 2 × 10-5 M⊙ yr-1) and gas expansion velocities (2 to 21.5 km s-1) , and include M-, S-, and C-type AGB stars. Our results generally agree with those of earlier studies, although we tend to find slightly lower mass-loss rates by about 40%, on average. We also present “bonus” lines detected during our CO observations.
Key words: stars: AGB and post-AGB / circumstellar matter / stars: evolution / stars: mass-loss
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Based on observations carried out with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2015