The APOGEE-2 Survey of the Orion Star Forming Complex II: Six-dimensional structure

Abstract: We present an analysis of spectrosopic and astrometric data from APOGEE-2 and Gaia DR2 to identify structures towards the Orion Complex. By applying a hierarchical clustering algorithm to the 6-dimensional stellar data, we identify spatially and/or kinematically distinct groups of young stellar objects with ages ranging from 1 to 12 Myr. We also investigate the star forming history within the Orion Complex, and identify peculiar sub-clusters. With this method we reconstruct the older populations in the regions that are presently largely devoid of molecular gas, such as Orion C (which includes the $\sigma$ Ori cluster), and Orion D (the population that traces Ori OB1a, OB1b, and Orion X). We report on the distances, kinematics, and ages of the groups within the Complex. The Orion D groups is in the process of expanding. On the other hand, Orion B is still in the process of contraction. In $\lambda$ Ori the proper motions are consistent with a radial expansion due to an explosion from a supernova; the traceback age from the expansion exceeds the age of the youngest stars formed near the outer edges of the region, and their formation would have been triggered when they were half-way from the cluster center to their current positions. We also present a comparison between the parallax and proper motion solutions obtained by Gaia DR2, and those obtained towards star-forming regions by Very Long Baseline Array.

• The paper identifies spatial and kinematic subgroups, uncovering distinct star formation histories in the Orion Complex.
• It employs a hierarchical clustering algorithm on APOGEE-2 and Gaia DR2 data to determine precise distances and age distributions of various subregions.
• The study reveals expansion patterns, including those in λ Orionis, suggesting that past supernova events have influenced the cluster's evolution.

An Analysis of the Six-Dimensional Structure of the Orion Star Forming Complex

The paper investigates the six-dimensional structure of the Orion Star Forming Complex using data acquired from the APOGEE-2 survey and Gaia DR2, revealing distinct and previously unresolved spatial and kinematic properties within this iconic star-forming region. The authors employ a hierarchical clustering algorithm on astrometric and spectroscopic data to map diverse groups of young stellar objects (YSOs) with ages ranging from 1 to 12 Myr, distinguishing their star-forming history and dynamical evolution.

Key findings presented in this work include the identification of coherent spatial and kinematic subgroups, notably dividing the Orion Complex into several distinct regions with different star-forming histories. The paper highlights the existence of distinct stellar populations in the regions referenced as Orion A, B, C, D, and λ Ori, each exhibiting unique positional, kinematic, and age distributions.

Numerical Results and Bold Claims

Among the numerical highlights, the authors report precise distance measurements of key subregions: approximately 389 pc for the Orion A's Orion Nebula Cluster (ONC), up to 443 pc towards the southern region of the same cloud, 407 pc for Orion B, 412 pc for Orion C, and 350 pc for Orion D population. These spatial coordinates are corroborated by both parallax data and stellar kinematics, confirming the hierarchical clustering approach's efficacy in delineating these components.

The investigation reveals that the Orion D region—which encapsulates the historical sub-associations of Ori OB1a and OB1b—emerges as a single expanding entity now largely devoid of the molecular gas. In contrast, Orion C, encompassing the σ Orionis cluster, is characterized by a distinct star-forming history, exhibiting expansion in proper motions and uniform radial velocities.

Interestingly, the λ Orionis region evidences a radial expansion pattern as well, likely indicative of a historical supernova event. The proper motion data suggests a cluster center explosion, triggering outward expansion that aligns with the spatial distribution of younger stellar populations observed along the ring formed by remnant molecular material.

Theoretical and Practical Implications

This work's theoretical implications lie in advancing the understanding of star cluster formation and dispersion processes. By detailing the star formation history and present-day dynamics of these groups, the study offers insights into the potential evolution of molecular cloud complexes into dispersed stellar associations. The elucidation of expansion effects and kinematic structures emphasizes the process through which triggered star formation might occur, particularly in environments influenced by supernova events.

Practically, these findings could enhance the precision of future models of molecular cloud evolution and star cluster dynamics. The detailed 6D mapping paves the way for more focused studies on individual subclusters, allowing for critical examination of specific astrophysical processes at play in similar star-forming environments across the galaxy.

Speculation on Future Developments

From an AI perspective, the increasing incorporation of machine learning algorithms in astrophysical data processing foreshadows enhanced clustering techniques, potentially offering greater efficiency and insights beyond the current hierarchical methodologies. As AI techniques mature, they may allow for real-time data analysis and cluster identification, leading to progressively more intricate mappings of complex molecular regions.

This paper sets a precedent for subsequent research focused on unraveling the intricate structures of star-forming complexes, offering a detailed account of how different environmental factors come together to shape stellar and cluster evolution. Further studies could take advantage of future datasets from upcoming space observatories, integrating even more refined measurements into these models to confirm and extend the conclusions presented here.