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|Title:||Oxygen isotopic ratios in intermediate-mass red giants||Authors:||Lebzelter, T.
Hinkle, K. H.
|Issue Date:||2015||Journal:||ASTRONOMY & ASTROPHYSICS||Number:||578||First Page:||A33||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. <BR /> Aims: By measuring <SUP>16</SUP>O/<SUP>17</SUP>O and <SUP>16</SUP>O/<SUP>18</SUP>O in red giants of known mass we compare the isotope ratios with predictions from stellar and galactic evolution modelling. <BR /> 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<SUB>☉</SUB>. 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. <BR /> Results: The observed <SUP>16</SUP>O/<SUP>17</SUP>O 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 <SUP>16</SUP>O/<SUP>18</SUP>O 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 <SUP>12</SUP>C/<SUP>13</SUP>C ratios of the K giants studied implies the absence of extra-mixing in these objects. <BR /> Conclusions: A comparison with galactic chemical evolution models indicates that the <SUP>16</SUP>O/<SUP>18</SUP>O abundance ratio underwent a faster decrease than predicted. To explain the observed ratios, the most likely scenario is a higher initial <SUP>18</SUP>O abundance combined with a lower initial <SUP>16</SUP>O abundance. Comparing the measured <SUP>18</SUP>O/<SUP>17</SUP>O ratio with the corresponding value for the interstellar medium points towards an initial enhancement of <SUP>17</SUP>O as well. Limitations imposed by the observations prevent this from being a conclusive result.||Acknowledgments:||We are indebted to Larry Nittler for providing a computer-readable table with his derivation of the O/O in molecular clouds. We thank the anonymous referee for the very valuable comments. T.L. and W.N. were supported by Austrian Science Fund FWF under project number P23737-N16. W.N. was also supported by project P21988-N16. B.A. acknowledges the support from the project STARKEY funded by the ERC Consolidator Grant, G.A. No. 615604. This paper is based on observations obtained at the Kitt Peak National Observatory (NOAO Prop. ID:2013B-0218; PI: T. Lebzelter). The Kitt Peak National Observatory is part of the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation.||URI:||http://hdl.handle.net/20.500.12386/24184||URL:||https://www.aanda.org/articles/aa/abs/2015/06/aa25832-15/aa25832-15.html||ISSN:||0004-6361||DOI:||10.1051/0004-6361/201525832||Bibcode ADS:||2015A&A...578A..33L||Fulltext:||open|
|Appears in Collections:||1.01 Articoli in rivista|
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