3D dynamics of the Orion cloud complex -- Discovery of coherent radial gas motions at the 100-pc scale

Abstract

We present the first study of the 3D dynamics of the gas in the entire Southern Orion cloud complex. We used the YSO's proper motions from Gaia as a proxy for the gas proper motion, together with gas radial velocities from archival CO data, to compute the space motion of the different star-forming clouds in the complex, including sub-regions in Orion A, Orion B, and two outlying cometary clouds. From the analysis of the cloud's orbits in space and time we find that they were closest about 6 Myr ago and are moving radially away from roughly the same region in space. This coherent 100-pc scale radial motion supports a scenario where the entire complex is reacting to a major feedback event that we name the Orion−6 event. This event, that we tentatively associate with the recently discovered Orion X population, shaped the distribution and kinematics of the gas we observe today, although it is unlikely to have been the sole major feedback event in the region. We argue that the dynamics of most of the YSOs carry the memory of the feedback-driven star formation history in Orion and that the majority of the young stars in this complex are a product of large-scale triggering, that can raise the star formation rate by at least an order of magnitude, as in the case of the Orion A's Head (the Integral Shape Filament). Our results imply that at the genesis of the Orion Nebula Cluster (and NGC 2023/2024 in Orion B) lies a feedback/compression/triggering process.

A gravitational and dynamical framework of star formation: The Orion Nebula

“…As has been previously noted by Kounkel (2020) and Großschedl et al (2020), the young stars within the Orion Complex itself have a strong preference for expansion, most likely due to supernovae feedback.…”

“…The fresh dense molecular gas that is sitting behind the cluster can nonetheless feel gravitational attraction to the cluster, and it is also infalling as it is pulled towards it. It has been previously noted by Kounkel et al (2020) and Großschedl et al (2020) that 6 Myr ago a supernova (or several supernovae) have triggered the global expansion of the Orion Complex. Given that the ONC is being pushed into the Orion A molecular cloud (Großschedl et al 2018) from the direction of the center of the expansion, it is likely that the shockwave from a supernova has swept along the gas through the filamentary cloud, compressing the gas, and jump-starting the formation of the cluster.…”

“…However, these are only a few isolated cases, and it is difficult to infer any particular trend for it. On the contrary, most of the Orion Complex appears to expand with speeds in excess of 6 km s −1 , due to recent turbulence injection from the feedback of massive stars and supernovae (Kounkel 2020;Großschedl et al 2020;Ha et al 2021). On more compact level, Kuhn et al (2019) have analyzed a sample of 28 young clusters; they found that almost none of them appear to have rotational support, and most of them show a preference of expansion with a typical velocity of ∼ 0.5 km s −1 .…”

Orion A's complete 3D magnetic field morphology

“…The Barnard's loop bubble, with an estimated age of 3 × 10 5 yr, is expanding at a velocity of 100 km s −1 (Ochsendorf et al 2015), while the Orion-Eridanus superbubble has an expansion velocity of ∼ 20 km s −1 (Joubaud et al 2019). This bubble is located between 340 pc and 400 pc b from us (Großschedl et al 2020b). Using Barnard's loop and the Orion A cloud's distances, we find that the Barnard's loop bubble is likely in contact with (and has interacted with) Orion A's head but not its tail, as depicted in Fig.…”

Untangling the Galaxy III: Photometric Search for Pre-main Sequence Stars with Deep Learning

“…It should be noted that different populations do appear to have a somewhat different peculiar velocity relative to one another, at least in the proper motion space. Thus most likely, instead of shockwave clearing a gas of a particular population (as has been the case of supernovae in young star forming regions such as Orion, and potentially Vela; Kounkel 2020;Großschedl et al 2020;Cantat-Gaudin et al 2019) the shock front associated with the expanding bubble may have rammed into the neighboring clouds, not dissimilar to a scenario described by Inutsuka et al (2015) in which molecular clouds trace interaction regions between even shorter lived bubbles.…”