Oxygen isotopic ratios in intermediate-mass red giants

Author(s)

Thomas Lebzelter, Oscar Straniero, Kenneth H. Hinkle, Walter Nowotny-Schipper, Bernhard Aringer

Abstract

Context. The abundances of the three main isotopes of oxygen are altered in the course of the CNO-cycle. When the first dredge-up mixes the burning products to the surface, the nucleosynthesis processes can be probed by measuring oxygen isotopic ratios.

Aims: By measuring ¹⁶O/¹⁷O and ¹⁶O/¹⁸O in red giants of known mass we compare the isotope ratios with predictions from stellar and galactic evolution modelling.

Methods: Oxygen isotopic ratios were derived from the K-band spectra of six red giants. The sample red giants are open cluster members with known masses of between 1.8 and 4.5 M_⊙. The abundance determination employs synthetic spectra calculated with the COMARCS code. The effect of uncertainties in the nuclear reaction rates, the mixing length, and of a change in the initial abundance of the oxygen isotopes was determined by a set of nucleosynthesis and mixing models using the FUNS code.

Results: The observed ¹⁶O/¹⁷O ratios are in good agreement with the model results, even if the measured values do not present clear evidence of a variation with the stellar mass. The observed ¹⁶O/¹⁸O ratios are clearly lower than the predictions from our reference model. Variations in nuclear reaction rates and mixing length parameter both have only a very weak effect on the predicted values. The ¹²C/¹³C ratios of the K giants studied implies the absence of extra-mixing in these objects.

Conclusions: A comparison with galactic chemical evolution models indicates that the ¹⁶O/¹⁸O abundance ratio underwent a faster decrease than predicted. To explain the observed ratios, the most likely scenario is a higher initial ¹⁸O abundance combined with a lower initial ¹⁶O abundance. Comparing the measured ¹⁸O/¹⁷O ratio with the corresponding value for the interstellar medium points towards an initial enhancement of ¹⁷O as well. Limitations imposed by the observations prevent this from being a conclusive result.

Organisation(s)

Department of Astrophysics

External organisation(s)

Università degli Studi di Teramo, National Optical Astronomy Observatory, Tucson

Journal

Astronomy & Astrophysics

Volume

578

No. of pages

8

ISSN

0004-6361

DOI

https://doi.org/10.1051/0004-6361/201525832

Publication date

06-2015

Peer reviewed

Yes

Austrian Fields of Science 2012

103003 Astronomy, 103004 Astrophysics

Keywords

ASJC Scopus subject areas

Astronomy and Astrophysics, Space and Planetary Science

Portal url

https://ucrisportal.univie.ac.at/en/publications/oxygen-isotopic-ratios-in-intermediatemass-red-giants(04a41e31-f0be-4344-9c77-6f708340bab0).html