Herschel^⋆ observations of extreme OH/IR stars

The isotopic ratios of oxygen as a sign-post for the stellar mass^⋆⋆

¹ Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: kay.justtanont@chalmers.se
² University College London, Dept. of Physics & Astronomy, Gower Street, London, WC1E 6BT, UK
³ Astronomy and Astrophysics Research Group, Dep. of Physics and Astrophysics, V.U. Brussel, Pleinlaan 2, 1050 Brussels, Belgium
⁴ Flemish Institute of Technical Research, VITO, 2400 Mol, Belgium
⁵ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁶ University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Wien, Austria
⁷ School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff, CF24 3AA, UK
⁸ Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire OX11 0QX, UK
⁹ European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
¹⁰ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
¹¹ Sterrenkundig Instituut Anton Pannekoek, Universiteit van Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands

Received: 7 April 2015
Accepted: 11 May 2015

Abstract

Aims. The late stages of stellar evolution are mainly governed by the mass of the stars. Low- and intermediate-mass stars lose copious amounts of mass during the asymptotic giant branch (AGB) which obscure the central star making it difficult to study the stellar spectra and determine the stellar mass. In this study, we present observational data that can be used to determine lower limits to the stellar mass.

Methods. Spectra of nine heavily reddened AGB stars taken by the Herschel Space Observatory display numerous molecular emission lines. The strongest emission lines are due to H₂O. We search for the presence of isotopologues of H₂O in these objects.

Results. We detected the ¹⁶O and ¹⁷O isotopologues of water in these stars, but lines due to H₂¹⁸O are absent. The lack of ¹⁸O is predicted by a scenario where the star has undergone hot-bottom burning which preferentially destroys ¹⁸O relative to ¹⁶O and ¹⁷O. From stellar evolution calculations, this process is thought to occur when the stellar mass is above 5 M_⊙ for solar metallicity. Hence, observations of different isotopologues of H₂O can be used to help determine the lower limit to the initial stellar mass.

Conclusions. From our observations, we deduce that these extreme OH/IR stars are intermediate-mass stars with masses of ≥5 M_⊙. Their high mass-loss rates of ~10^-4M_⊙ yr^-1 may affect the enrichment of the interstellar medium and the overall chemical evolution of our Galaxy.