The OH/IR Star Population in the Galactic Center
The OH/IR stars and M-type Mira variables, that form the topic of this thesis are in two important ways different from OH/IR stars found elsewhere in the Galaxy. Their location in the central few tens of parsecs of the Galactic center allows one to study the dynamics of the Galaxy on that scale, and to investigata a population of evolved stars with a high metal abundance.
The results presented here contribute to both areas; by investigating the possibility to determine proper motions of the stars (and thus individual stellar orbits), as well as by obtaining and analysing a new sample of OH/IR stars. All conclusions are drawn from interferometric observations of molecular maser line emission from the circumstellar envelope surrounding these stars.
In the innermost parts of the Galaxy accurate positions for OH/IR stars, necessary to measure their proper motions, cannot be determined from the easily detectable OH maser emission because of interstellar scattering. Sio masers are better suited, and after a survey for candidate OH/IR stars that have SiO or H2O masers, it is shown that it is difficult, but indeed possible to determine milli-arcsecond accurate stellar positions relative to Sgr A*, the alleged dynamical center f the Galaxy. In the future, proper motions for OH/IR stars with SiO masers can be determined with VLBI techniques, and will be helpful in retrieving the mass-distribution in, and around the Galactic center.
With massive molecular clouds present, the Galactic center is expected to be a site for massive star formation. However, one should be careful to associate H2O masers with such regions. Some H2O masers should be ascribed to OH/IR stars or M-type Mira variables instead.
Much effort has been put in the search for more, faint OH/IR stars in the 1612 MHz OH maser line. A significant contribution is given to the statistical description of Galactic center OH/IR stars, both in kinematical as well as in intrinsic properties. More than 50 previously unknown OH/IR stars were detected in the central 40 parsec, most of them confirmed by the two different OH maser surveys described here. As the distance to the OH/IR stars is almost the same and relatively well known, and because the sample is relatively well studied at infrared wavelengths, the new sample forms a good base to study, for example, the mass-loss mechanism responsible for turning OH/IR stars into planetary nebulae.
A first analysis of intrinsic properties reveals that many OH/IR stars in the Galactic center a formed more than a Giga-year ago in a period of enhanced star formation, possibly a starburst; a striking conclusion that can only be drawn from OH/IR stars because they form the only traceable population older than a few 100 Mega-year in the Galactic center.