Nucleosynthesis in AGB stars traced by oxygen isotopic ratios

I. Determining the stellar initial mass by means of the ¹⁷O/¹⁸O ratio ^⋆

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D B2401, 3001 Leuven, Belgium
e-mail: rutger.denutte@ster.kuleuven.be
² Astronomical Institute Anton Pannekoek, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
³ Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, 439 92 Onsala, Sweden
⁴ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁵ Monash Centre for Astrophysics, School of Physics & Astronomy, Monash University, VIC 3800, Australia
⁶ NASA Goddard Space Flight Center, Astrochemistry Laboratory, Code 691.0, Greenbelt, MD 20771, USA
⁷ Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
⁸ Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 27 June 2016
Accepted: 13 December 2016

Abstract

Aims. We seek to investigate the ¹⁷O/¹⁸O ratio for a sample of AGB stars containing M-, S-, and C-type stars. These ratios are evaluated in relation to fundamental stellar evolution parameters: the stellar initial mass and pulsation period.

Methods. Circumstellar ¹³C¹⁶O, ¹²C¹⁷O, and ¹²C¹⁸O line observations were obtained for a sample of nine stars with various single-dish long-wavelength facilities. Line intensity ratios are shown to relate directly to the surface ¹⁷O/¹⁸O abundance ratio.

Results. Stellar evolution models predict the ¹⁷O/¹⁸O ratio to be a sensitive function of initial mass and to remain constant throughout the entire TP-AGB phase for stars initially less massive than 5 M_⊙. This makes the measured ratio a probe of the initial stellar mass.

Conclusions. Observed ¹⁷O/¹⁸O ratios are found to be well in the range predicted by stellar evolution models that do not consider convective overshooting. From this, accurate initial mass estimates are calculated for seven sources. For the remaining two sources, there are two mass solutions, although there is a larger probability that the low-mass solution is correct. Finally, we present hints at a possible separation between M/S- and C-type stars when comparing the ¹⁷O/¹⁸O ratio to the stellar pulsation period.