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

Abstract

We built three models for the gravitational field of the Galactic bar. These models are an inhomogeneous ellipsoid, an inhomogeneous prolate spheroid, and a superposition of four inhomogeneous ellipsoids. Among the three models, the superposition provides our best approximation to the observed boxy mass distribution of the Galactic bar. Adding the bar component to an axisymmetric Galactic model, we have calculated stellar midplane orbits and orbits of some globular clusters with known kinematical data. For all models we find a secular dispersion effect on the orbital energy and angular momentum, as measured in the Galactic inertial frame. This effect might be relevant to explain the orbital prograde-retrograde distribution of globular clusters. For the stellar kinematics, we study the connection between the sense of orbital motion in the midplane and the onset of chaos in the presence of the bar. In the inner region of the bar, chaos is induced by an axisymmetric central component (bulge), and it arises in orbits that change its orbital sense from prograde to retrograde and vice versa as seen from an inertial reference frame. Outside the bar region, chaos appears only in prograde orbits. Our results concerning such a connection are consistent and extend those obtained for midplane orbits in the presence of only a spiral pattern in the axisymmetric Galactic model.

Open clusters towards the Galactic centre: chemistry and dynamics

“…The characteristics of the bar model are: i) the bar length of 3.14 kpc; ii) axial ratios 10:3.75:2.56; iii) a mass of 9.0 × 10 10 M ; iv) an angular speed of 60 km s −1 /kpc: and v) an initial angle with respect to Sun-GC direction of 20 • . We used the same parameter values as Pichardo et al (2004) or Allen et al (2008) for their more sophisticated models of the Galactic bar. For the axisymmetric background potential we kept the Allen & Santillan (1991) model, except for the mass of the bulge of 4.26 × 10 9 M odot (the bar replaces 70% of bulge mass).…”

Absolute proper motion of the Galactic open cluster M 67

“…We employed a Galactic potential that includes the axisymmetric model of Allen & Santillán (1991) and the bar and spiral arms models of Pichardo et al (2003Pichardo et al ( , 2004. The axisymmetric background potential of Allen & Santillán (1991) has been scaled to give a rotation velocity of 254 km s −1 at the solar position, based on the most-recent radio astrometry observations by Reid et al (2009).…”

“…The orbital self-consistency of the spiral arms was tested through the reinforcement of the spiral potential by the stellar orbits (Patsis et al 1991). For an extensive description of the models, see Pichardo et al (2003Pichardo et al ( , 2004. Table 2 gives the local standard of rest (LSR) initial velocity (U, V, W) and the corresponding cylindrical components (Π, Θ) for three different heliocentric distances of M 67 (central, minimum, and maximum).…”

Classifying orbits in galaxy models with a prolate or an oblate dark matter halo component

“…In addition, families of regular orbits are often used as the basic tool in constructing a dynamical model for describing the main properties of galaxies. Over the last several decades, a huge amount of research work has been devoted to understanding the orbital structure in different types of galaxy models (e.g., Pfenniger 1984;Contopoulos & Grosbøl 1989;Sellwood & Wilkinson 1993;Pfenniger 1996;Ollé & Pfenniger 1998;Pichardo et al 2004). However, the vast majority of the existing literature deals only either with the distinction between regular and chaotic motion (e.g., Manos & Athanassoula 2011;Bountis et al 2012;Manos et al 2013) or the detection of periodic orbits and the analysis of their stability (e.g., Skokos et al 2002a,b;Kaufmann & Patsis 2005).…”