Orbits of selected globular clusters in the Galactic bulge

TL;DR

This study analyzes the orbits of eight globular clusters in the Galactic bulge, emphasizing the importance of the Galactic bar in their dynamical behavior and revealing diverse orbital characteristics including chaotic motions.

Contribution

It provides detailed orbital analysis of bulge globular clusters using proper motion data and models including the Galactic bar, highlighting the bar's role in their dynamics.

Findings

Most clusters have eccentric orbits within 1.5 kpc of the Galactic plane.

Seven clusters share orbital properties with the bar/bulge region.

NGC 6540 exhibits a thick-disc orbital signature.

Abstract

We present orbit analysis for a sample of 8 inner bulge globular clusters, together with one reference halo object. We used proper motion values derived from long time base CCD data. Orbits are integrated in both an axisymmetric model and a model including the Galactic bar potential. The inclusion of the bar proved to be essential for the description of the dynamical behavior of the clusters. We use the Monte Carlo scheme to construct the initial conditions for each cluster, taking into account the uncertainties in the kinematical data and distances. The sample clusters show typically maximum height to the Galactic plane below 1.5 kpc, and develop rather eccentric orbits. Seven of the bulge sample clusters share the orbital properties of the bar/bulge, having perigalactic and apogalatic distances, and maximum vertical excursion from the Galactic plane inside the bar region. NGC 6540 instead shows a completely different orbital behaviour, having a dynamical signature of the thick-disc. Both prograde and prograde-retrograde orbits with respect to the direction of the Galactic rotation were revealed, which might characterize a chaotic behaviour.