B-stars in Orion: clues on the formation of clusters and associations
In the series of papers on the massive star and cluster formation benchmark, the Orion Star Forming Region, (Alves&Bouy 2012, Bouy et al. 2014, and Kubiak et al. 2016) found that despite the 1000+ papers written, most of the young stars in the region are not directly related to the current star formation but lie in the foreground to the cloud, biasing the basics observables in this critical region (ages, age spreads, masses, IMF, etc.). Close determination of the dynamical properties and spatial distributions of OB stars in the whole Orion star-forming region is crucial for understanding how massive star factories operate, and dissolve into the Galactic field. The key to understanding the past and future evolution of associations lies in careful measurements of the positions and velocities of their constituent stars.
The kinematics of this important region was first investigated in 1977 by Warren and Hesser, where some evidence was already presented towards a more complex star formation history of one of the sub-regions. A kinematic study of the Orion Belt sub-region (Blaauw’s OB 1b population) presented by Gieseking1983 and another by Jeffries2006, present clear evidence for the existence of two kinematic components in the vicinity of sigma Ori, unaccounted for in Blaauw’s sequential star formation picture. Recently, and towards another Orion sub-region, Alves&Bouy2012 found an increase in the velocity dispersion for sources located in the vicinity of the Orion Nebula (NGC1980 or $\iota$ Ori cluster), suggesting mixing of different populations (NGC 1980 was found to lie in the foreground of the Orion Nebula, about 20 pc from it). Disentangling these populations of young stars is critical to understand how massive star formation proceeds in Nature, and in particular quantifying the role of feedback (the engine in the sequential model).
We present analysis of precision radial velocities (RV) of mostly B spectral type stars in the Orion Constellation obtained by SES@STELLA. Only a few (typically, two or three) spectroscopic observations per star have been collected, with the main goal of resolving the large-scale kinematic structure of the region and screening for binary systems. We would like to investigate the overall structure and properties of the Orion star forming complex. The key aim was to provide precise radial velocities for the large sample of massive stars in Orion. The primary goal of the survey was to disentangle accurately the different young stellar populations and construct the most accurate star-forming history of this region.