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

TL;DR

This work delivers the first homogeneous infrared proper-motion catalog for the NGC 2024 cluster by combining 11 VISTA epochs from SV, VHS, and VISIONS to trace embedded and optically visible YSOs at $d\sim420$ pc. The authors show an inside-out age gradient with younger stars near the center and older stars in the outskirts, find no evidence for long-lived substructure, and favor rapid cluster collapse or monolithic formation within $<1$ Myr. They also demonstrate that YSO radial velocities largely follow gas velocities from multiple tracers, indicating ongoing star–gas coupling for much of the young population. Overall, the study demonstrates the viability of infrared astrometry to study early cluster assembly and star–gas dynamics, while highlighting the need for more radial-velocity measurements of the youngest objects to fully map the 3D kinematics.

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 insight 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 thus absent from astrometric surveys such as Gaia. We measured the proper motions of 6,769 sources toward the NGC 2024 cluster in the Flame Nebula 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 with mean measurement uncertainties of ~0.22 mas/yr. 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. 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 12CO(3-2), HNC(1-0), HCN(1-0), and show a weaker correlation with N_2H+(1-0). Some Class II and III objects appear to be already decoupling.

Figures (19)

• Figure 1: Outlines of the SV, VHS, and VISIONS tiles covering the Flame Nebula, shown atop the Herschel Gould Belt Survey 350 $\mu$m emission map. The bold squares indicate the cutouts used in this study. The varying tile orientations result from the survey-specific observing strategies.
• Figure 2: Astrometric reduction workflow employed on the dataset level.
• Figure 3: Workflow depicting creation of the master catalog.
• Figure 4: Schematic overview of the contents of the master catalog. The full table is available online at the CDS.
• Figure 5: Infrared proper motion histograms for all 6,769 sources (top) and deviation from Gaia proper motions for 899 shared sources (bottom).