A dynamical analysis of two groups of OH/IR stars.

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Stars: Agb And Post-Agb, Stars: Kinematics, Galaxy: Kinematics And Dynamics, Galaxy: Structure, Galaxy: Center

Scientific paper

Two samples of OH/IR stars in our Galaxy are compared; one very close to the Galactic Centre (GC), the other in the Bulge and the Disk. The two are very distinct in density distribution, kinematics and astrophysical properties. Part of the observed kinematics is explained by assuming an extended mass concentration in the centre. We describe the samples in terms of their analytic dynamical distribution function within the potential and compare them globally. The distribution function is derived using a quadratic programming method, fitting orbital families to the observed quantities. The stars at the GC have higher rotation and lower velocity dispersion than those in the Disk and the Bulge. The distribution function in (E,L_z_) of the centre stars is strongly concentrated at high energy circular orbits. The stars at the GC also have higher metallicity on average. We conclude that the two samples consist of distinct populations, one that extends over the whole Galaxy and shows signs of gradual evolution. It is seen in both samples. The other is only seen near the galactic centre, has high rotation and does not show signs of long evolution. Presumably this is a younger population, formed in a distinct event at the GC.

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