Galactic interstellar ¹⁸O/O ratios – a radial gradient?^*

¹ Joint Astronomy Centre, 660 N. A'ohoku Place, Hilo, HI 96720, USA e-mail: j.wouterloot@jach.hawaii.edu
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ INAF - Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy

Received: 26 June 2007
Accepted: 4 May 2008

Abstract

Context. The determination of interstellar abundances is essential for a better understanding of stellar nucleosynthesis and the “chemical” evolution of the Galaxy.

Aims. The aim is to determine ¹⁸O/¹⁷O abundance ratios across the entire Galaxy. These provide a measure of the amount of enrichment by high-mass versus intermediate-mass stars.

Methods. Such ratios, derived from the C¹⁸O and C¹⁷O –0 lines alone, may be affected by systematic errors. Therefore, the C¹⁸O and C¹⁷O (1–0), (2–1), and (3–2), as well as the ¹³CO(1–0) and (2–1) lines, were observed towards 18 prominent galactic targets (a total of 25 positions). The combined dataset was analysed with a large velocity gradient model, accounting for optical depth effects.

Results. The data cover galactocentric radii between 0.1 and 16.9 kpc (solar circle at 8.5 kpc). Near the centre of the Galaxy, ¹⁸O/¹⁷O = 2.88 ± 0.11. For the galactic disc out to a galactocentric distance of ~10 kpc, ¹⁸O/¹⁷O = 4.16 ± 0.09. At ~16.5 kpc from the galactic centre, ¹⁸O/¹⁷O = 5.03 ± 0.46. Assuming that ¹⁸O is synthesised predominantly in high-mass stars ( ), while C¹⁷O is mainly a product of lower mass stars, the ratio from the inner Galaxy indicates a dominance of CNO-hydrogen burning products that is also apparent in the carbon and nitrogen isotope ratios. The high ¹⁸O/¹⁷O value of the solar system (5.5) relative to that of the ambient interstellar medium suggests contamination by nearby high-mass stars during its formation. The outer Galaxy poses a fundamental problem. High values in the metal-poor environment of the outer Galaxy are not matched by the low values observed towards the even more metal-poor Large Magellanic Cloud. Apparently, the outer Galaxy cannot be considered as an intermediate environment between the solar neighbourhood and the interstellar medium of small metal-poor galaxies. The apparent ¹⁸O/¹⁷O gradient along the galactic disc and the discrepancy between outer disc and LMC isotope ratios may be explained by different ages of the respective stellar populations. More data from the central and far outer parts of the Galaxy are, however, needed to improve the statistical significance of our results.