Near-IR spectroscopy of OH/IR stars in the Galactic centre

¹ Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium e-mail: evelien@ster.kuleuven.be
² CNRS UMR6091, Observatoire de Besançon, BP 1615, 25010 Besançon, France
³ Itittut für Astronomie, Türkenschanzstrasse 17, 1180 Wien, Austria
⁴ Observatoire Astronimique, 11 rue de l'Université, 67000 Strasbourg, France

Received: 13 March 2006
Accepted: 18 April 2006

Abstract

Context.Based on the expansion velocities of their circumstellar shells, Galactic centre (GC) OH/IR stars can be divided into two groups that are kinematically different and therefore believed to have evolved from different stellar populations.

Aims.We studied the metallicity distribution of the OH/IR stars population in the GC on the basis of a theoretical relation between EW(Na), EW(Ca), and EW(CO) and the metallicity.

Methods.For 70 OH/IR stars in the GC, we obtained near-IR spectra. The equivalent line-widths of , , (2, 0), and the curvature of the spectrum around 1.6 μm due to water absorption were determined.

Results.The near-IR spectrum of OH/IR stars is influenced by several physical processes. OH/IR stars are variable stars suffering high mass-loss rates. The dust that is formed around the stars strongly influences the near-IR spectra and reduces the equivalent line-widths of and . A similar effect is caused by the water content in the outer atmosphere of the OH/IR star. Because of these effects, it is not possible to determine the metallicities of these stars with our low-resolution near-infrared spectroscopy.