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Astron. Astrophys. 329, 991-1009 (1998)


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The nature of OH/IR stars in the galactic centre

J.A.D.L. Blommaert 1, 2, 3, W.E.C.J. van der Veen 4, H.J. Van Langevelde 5, H.J. Habing 1 and L.O. Sjouwerman 6

1 Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands
2 ISO Science Operations Centre, Astrophysics Division, Space Science Dept. of ESA, Villafranca, P.O. Box 50727, E-28080 Madrid, Spain
3 CNRS, Institut d'Astrophysique de Paris, 98bis, Blvd Arago, F-75014 Paris, France
4 Columbia University, Dept. Of Astronomy, 538 West 120 [FORMULA] Street, New York, NY 10027, USA
5 Joint Institute for VLBI in Europe, P.O. Box 2, 7990 AA Dwingeloo, The Netherlands
6 Onsala Space Observatory, S-439 92 Onsala, Sweden

Received 22 January 1997 / Accepted 5 August 1997

Abstract

We report on infrared observations of stars in a field of 30 [FORMULA] near the galactic centre. All these objects were previously detected as OH (1612 MHz) maser sources. For a large fraction of these stars variability data are available from a VLA monitor programme. This makes it possible to correct the IR measurements for variability. Corrections for interstellar extinction are also applied. The resulting infrared colours, periods and luminosities are compared with results for other samples of OH/IR stars and it is shown that the galactic centre stars are similar to the same type of objects in the bulge of the Galaxy but that more luminous (and thus younger) stars exist in the centre. The question of the existence of two distinct populations of OH/IR stars near the galactic centre is addressed, but the limited number of stars inhibits a firm conclusion. We do find that when the sample is divided according to OH expansion velocity, the dust-to-gas mass loss ratio, which depends on metallicity, is on average twice as high for the high expansion velocity group as for the low expansion velocity stars. The luminosities and the number density of the low expansion velocity stars are consistent with them being an extension of the bulge population, whereas the high expansion velocity group contains brighter sources and is more likely to be a population intrinsic to the galactic centre. The previously proposed period-luminosity (PL-) relation for OH/IR stars can be studied with this sample. We find that the OH/IR stars significantly deviate from the PL-relation and argue that the OH/IR stars have evolved away from the PL-relation. It was found that non-variable OH/IR stars are often associated with peculiar IR sources.

Key words: stars: AGB and post – AGB – stars: evolution – Galaxy: center – infrared: stars

Send offprint requests to: J. Blommaert (Villafranca, Spain)

© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997
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