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
e-mail: taissa@chalmers.se
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

Abstract

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.