Herschel/HIFI observations of O-rich AGB stars: molecular inventory^⋆,⋆⋆

¹ Onsala Space Observatory, Dept. Earth and Space Science, Chalmers University of Technology, 439 92 Onsala, Sweden
e-mail: kay.justtanont@chalmers.se
² Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
³ University of Bonn, Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ Observatorio Astronómico Nacional (IGN), Alfonso XII N°3, 28014 Madrid, Spain
⁵ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁶ Observatorio Astronómico Nacional, Ap 112, 28803 Alcalá de Henares, Spain
⁷ European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁸ CAB, INTA-CSIC, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
¹⁰ Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
¹¹ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
¹² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹³ Johns Hopkins University, Baltimore, MD 21218, USA
¹⁴ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
¹⁵ N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
¹⁶ European Southern Observatory, Karl Schwarzschild Str. 2, Garching bei München, Germany

Received: 20 June 2011
Accepted: 14 November 2011

Abstract

Aims. Spectra, taken with the heterodyne instrument, HIFI, aboard the Herschel Space Observatory, of O-rich asymptotic giant branch (AGB) stars which form part of the guaranteed time key program HIFISTARS are presented. The aim of this program is to study the dynamical structure, mass-loss driving mechanism, and chemistry of the outflows from AGB stars as a function of chemical composition and initial mass.

Methods. We used the HIFI instrument to observe nine AGB stars, mainly in the H₂O and high rotational CO lines. We investigate the correlation between line luminosity, line ratio and mass-loss rate, line width and excitation energy.

Results. A total of nine different molecules, along with some of their isotopologues have been identified, covering a wide range of excitation temperature. Maser emission is detected in both the ortho- and para-H₂O molecules. The line luminosities of ground state lines of ortho- and para-H₂O, the high-J CO and NH₃ lines show a clear correlation with mass-loss rate. The line ratios of H₂O and NH₃ relative to CO J = 6−5 correlate with the mass-loss rate while ratios of higher CO lines to the 6−5 is independent of it. In most cases, the expansion velocity derived from the observed line width of highly excited transitions formed relatively close to the stellar photosphere is lower than that of lower excitation transitions, formed farther out, pointing to an accelerated outflow. In some objects, the vibrationally excited H₂O and SiO which probe the acceleration zone suggests the wind reaches its terminal velocity already in the innermost part of the envelope, i.e., the acceleration is rapid. Interestingly, for R Dor we find indications of a deceleration of the outflow in the region where the material has already escaped from the star.