Models for the Gravitational Field of the Galactic Bar: An Application to Stellar Orbits in the Galactic Plane and Orbits of Some Globular Clusters

We built three models for the gravitational field of the Galactic bar. Thesemodels are an inhomogeneous ellipsoid, an inhomogeneous prolate spheroid, and asuperposition of four inhomogeneous ellipsoids. Among the three models, thesuperposition provides our best approximation to the observed boxy massdistribution of the Galactic bar. Adding the bar component to an axisymmetricGalactic model, we have calculated stellar midplane orbits and orbits of someglobular clusters with known kinematical data. For all models we find a seculardispersion effect upon the orbital energy and angular momentum, as measured inthe Galactic inertial frame. This effect might be relevant to explain theorbital prograde-retrograde distribution of globular clusters. For the stellarkinematics, we study the connection between the sense of orbital motion in themidplane and the onset of chaos in the presence of the bar. In the inner regionof the bar, chaos is induced by an axisymmetric central component (bulge) andit arises in orbits that change its orbital sense from prograde to retrogradeand vice versa as seen from an inertial reference frame. Outside the barregion, chaos appears only in prograde orbits. Our results concerning suchconnection are consistent and extend those obtained for midplane orbits in thepresence of only a spiral pattern in the axisymmetric Galactic model.Comment: 28 pages and 16 figures. Submitted to Ap