EDP Sciences
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Volume 437, Number 3, July III 2005
Page(s) 957 - 965
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20052737

A&A 437, 957-965 (2005)
DOI: 10.1051/0004-6361:20052737

Oxygen isotopic ratios in galactic clouds along the line of sight towards Sagittarius B2

E. T. Polehampton1, J.-P. Baluteau2 and B. M. Swinyard3

1  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
    e-mail: epoleham@mpifr-bonn.mpg.de
2  Laboratoire d'Astrophysique de Marseille, CNRS & Université de Provence, BP 8, 13376 Marseille Cedex 12, France
    e-mail: Jean-Paul.Baluteau@oamp.fr
3  Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK
    e-mail: B.M.Swinyard@rl.ac.uk

(Received 21 January 2005 / Accepted 31 March 2005 )

As an independent check on previous measurements of the isotopic abundance of oxygen through the Galaxy, we present a detailed analysis of the ground state rotational lines of 16OH and 18OH in absorption towards the giant molecular cloud complex, Sagittarius B2. We have modelled the line shapes to separate the contribution of several galactic clouds along the line of sight and calculate 16OH/18OH ratios for each of these features. The best fitting values are in the range 320-540, consistent with the previous measurements in the Galactic Disk but slightly higher than the standard ratio in the Galactic Centre. They do not show clear evidence for a gradient in the isotopic ratio with galactocentric distance. The individual 16OH column densities relative to water give ratios of [ H2O/OH] = 0.6-1.2, similar in magnitude to galactic clouds in the sight lines towards W51 and  W49 . A comparison with CH indicates [OH/CH] ratios higher than has been previously observed in diffuse clouds. We estimate OH abundances of 10-7-10-6 in the line of sight features.

Key words: infrared: ISM -- ISM: molecules -- Galaxy: abundances -- ISM: individual objects: Sagittarius B2

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© ESO 2005

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