We have used the galaxy modelling algorithm gravpot16, to explore the most probable orbital elements of a sample of 64 selected N-rich stars across the Milky Way. We use the newly measured proper motions from Gaia Data Release 2 with existing line-of-sight velocities from the second generation of the Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) and spectrophotometric distance estimations from starhorse. We adopted a set of high-resolution particle simulations evolved in the same steady-state Galactic potential model with a bar, in order to identify the groups of N-rich stars that have a high probability of belonging to the bulge/bar, disc and stellar halo component. We find that the vast majority of the N-rich stars show typically maximum height from the Galactic plane below 3 kpc, and develop eccentric orbits (e > 0.5), which means that these stars appear to have bulge/bar-like and/or halo-like orbits. We also show that ∼66 per cent of the selected N-rich stars currently reside in the inner Galaxy inside the corotation radius, whilst ∼ 14 per cent are in halo-like orbits. Among the N-rich stars in the inner Galaxy, ∼ 27 per cent share orbital properties in the boundary between bulge/bar and disc, depending on the bar pattern speeds. Our dynamical analysis also indicates that some of the N-rich stars are likely to be halo interlopers, which suggests that halo contamination is not insignificant within the bulge area.

We investigate regular and chaotic two-dimensional (2D) and three-dimensional (3D) orbits of stars in models of a galactic potential consisting in a disk, a halo and a bar, to find the origin of boxy components, which are part of the bar or (almost) the bar itself. Our models originate in snapshots of an N -body simulation, which develops a strong bar. We consider three snapshots of the simulation and for the orbital study we treat each snapshot independently, as an autonomous Hamiltonian system. The calculated corotation-to-bar-length ratios indicate that in all three cases the bar rotates slowly, while the orientation of the orbits of the main family of periodic orbits changes along its characteristic. We characterize the orbits as regular, sticky, or chaotic after integrating them for a 10 Gyr period by using the GALI 2 index. Boxiness in the equatorial plane is associated either with quasi-periodic orbits in the outer parts of stability islands, or with sticky orbits around them, which can be found in a large range of energies. We indicate the location of such orbits in diagrams, which include the characteristic of the main family. They are always found about the transition region from order to chaos. By perturbing such orbits in the vertical direction we find a class of 3D non-periodic orbits, which have boxy projections both in their face-on and side-on views.

The PERLAS potential has been successfully used in many studies related with the dynamics of the spiral arms on the equatorial plane of normal (non-barred) spiral galaxies. In the present work we extend these studies by investigating the three-dimensional dynamics of the spiral arms in the same type of potential. We consider a typical open, logarithmic, spiral pattern of pitch angle 25 • and we examine the stellar orbits that can support it as the ratio of the masses of the spiral over the disk component (M s /M d ) varies. We indicate the families of "three-dimensional" periodic orbits that act as the backbone of the spiral structure and we discuss their stability in the models we present. We study further the quasi-periodic and non-periodic orbits in general that follow spiral-supporting orbits as the M s /M d ratio increases. We find that a bisymmetric spiral with 25 • pitch angle is better supported by orbits in models with 0.03 M s /M d 0.07. In these cases a strong spiral pattern is supported between the radial 2:1 and 4:1 resonances, while local enhancements of the imposed spirals are encountered in some models between 4:1 and corotation. A characteristic bar-like structure is observed in all models at radii smaller than the radius of the 2:1 resonance.

We present a preliminary analysis of the effect of dynamical friction on the orbits of part of the globular clusters in our Galaxy. Our study considers an anisotropic velocity dispersion field approximated using the results of studies in the literature. An axisymmetric Galactic model with mass components consisting of a disc, a bulge, and a dark halo is employed in the computations. We provide a method to compute the dynamical friction acceleration in ellipsoidal, oblate, and prolate velocity distribution functions with similar density in velocity space. Orbital properties, such as mean time-variations of perigalactic and apogalactic distances, energy, and z-component of angular momentum, are obtained for globular clusters lying in the Galactic region R ≲ 10 kpc, |z| ≲ 5 kpc, with R, z cylindrical coordinates. These include clusters in prograde and retrograde orbital motion. Several clusters are strongly affected by dynamical friction, in particular Liller 1, Terzan 4, Terzan 5, NGC 6440, and NGC 6553, which lie in the Galactic inner region. We comment on the more relevant implications of our results on the dynamics of Galactic globular clusters, such as their possible misclassification between the categories ‘halo’, ‘bulge’, and ‘thick disc’, the resulting biasing of globular-cluster samples, the possible incorrect association of the globulars with their parent dwarf galaxies for accretion events, and the possible formation of ‘nuclear star clusters’.