Orbital analysis of stars in the nuclear stellar disc of the Milky Way

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, Laboratoire Lagrange, CNRS, Blvd de l’Observatoire, 06304 Nice, France
² Division of Astrophysics, Department of Physics, Lund University, Box 43 22100 Lund, Sweden
e-mail: niels.nieuwmunster@oca.eue-mail: lsmith@ast.cam.ac.uke-mail: jason.sanders@ucl.ac.uk
³ Department of Physics, University of Surrey, Guildford GU2 7XH, UK
⁴ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁵ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁷ School of Physics & Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁸ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁹ Department of Physics and Astronomy, University College London, London WC1E 6BT, UK

Received: 18 December 2023
Accepted: 21 February 2024

Abstract

Context. While orbital analysis studies were so far mainly focused on the Galactic halo, it is possible now to do these studies in the heavily obscured region close to the Galactic Centre.

Aims. We aim to do a detailed orbital analysis of stars located in the nuclear stellar disc (NSD) of the Milky Way allowing us to trace the dynamical history of this structure.

Methods. We integrated orbits of the observed stars in a non-axisymmetric potential. We used a Fourier transform to estimate the orbital frequencies. We compared two orbital classifications, one made by eye and the other with an algorithm, in order to identify the main orbital families. We also compared the Lyapunov and the frequency drift techniques to estimate the chaoticity of the orbits.

Results. We identified several orbital families as chaotic, z-tube, x-tube, banana, fish, saucer, pretzel, 5:4, and 5:6 orbits. As expected for stars located in a NSD, the large majority of orbits are identified as z-tubes (or as a sub-family of z-tubes). Since the latter are parented by x₂ orbits, this result supports the contribution of the bar (in which x₂ orbits are dominant in the inner region) in the formation of the NSD. Moreover, most of the chaotic orbits are found to be contaminants from the bar or bulge which would confirm the predicted contamination from the most recent NSD models.

Conclusions. Based on a detailed orbital analysis, we were able to classify orbits into various families, most of which are parented by x₂-type orbits, which are dominant in the inner part of the bar.