Kinematics of young stellar objects in NGC 2024 based on infrared proper motions

¹ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Wien, Austria
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
³ University of Vienna, Research Network Data Science at Uni Vienna, Kolingasse 14-16, 1090 Wien, Austria
⁴ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁵ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁶ Institut universitaire de France (IUF), 1 rue Descartes, 75231 Paris Cedex 05, France
⁷ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 October 2025
Accepted: 27 November 2025

Abstract

The most recently formed young stellar objects (YSOs) in active star-forming regions are excellent tracers of their parent cloud motion. Their positions and dynamics provide insights into cluster formation and constrain kinematic decoupling timescales between stars and gas. However, because of their strong extinction and young age, embedded YSOs are mainly visible at infrared wavelengths, and are thus absent from astrometric surveys such as Gaia. We measured the proper motions of 6769 sources toward the NGC 2024 cluster in the Flame Nebula (d ~ 420 pc) using multi-epoch near-infrared observations from three ESO public surveys: VISIONS, VHS, and the VISTA/VIRCAM science verification program. Cross-validation of our results with Gaia using optically visible stars shows excellent agreement, with uncertainties on the same order of magnitude. For 362 YSO candidates identified from the literature, we derived proper motions on the order of <5 mas yr⁻¹ with mean measurement uncertainties of 0.22 mas yr⁻¹, or 0.44 km s⁻¹. This is the first homogeneous proper motion measurement of this quality for more than half of these stars. For Class I and flat-spectrum sources, our results provide a >13-fold increase in available proper motion measurements. We analyzed the positional and kinematic differences between YSO classes and confirmed a previously reported inside-out age segregation from younger to older stars, likely driven by an outward movement of older stars. No evidence of prolonged hierarchical assembly was found. Instead, the results support a rapid (<1 Myr) cluster collapse into a centrally concentrated system. This scenario also accounts for the observed slightly higher 1D velocity dispersion of Class I sources relative to Class flat objects. YSO radial velocities generally align with the gas velocities measured from the molecular transitions of ¹²CO(3 − 2), HNC(1 − 0), HCN(1 − 0), and show a weaker correlation with N₂H⁺(1 − 0). Some Class II and III objects already appear to be decoupling.