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

TL;DR

This paper uses 6D stellar data from APOGEE-2 and Gaia DR2 to identify and analyze the structure, kinematics, and star formation history of the Orion Complex, revealing distinct groups and their dynamic states.

Contribution

It introduces a hierarchical clustering method applied to 6D data to reconstruct populations and analyze their kinematic and spatial properties within the Orion Complex.

Findings

Identified multiple spatially and kinematically distinct stellar groups.

Reconstructed older stellar populations in gas-depleted regions.

Observed expansion in Orion D and contraction in Orion B.

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.