A new look at the molecular gas in M42 and M43; possible evidence for cloud-cloud collision which triggered formation of the OB stars in the Orion Nebula Cluster (1701.04669v4)

Abstract: The Orion Nebula Cluster toward the HII region M42 is the most outstanding young cluster at the smallest distance 410pc among the rich high-mass stellar clusters. By newly analyzing the archival molecular data of the 12CO(J=1-0) emission at 21" resolution, we identified at least three pairs of complementary distributions between two velocity components at 8km/s and 13km/s. We present a hypothesis that the two clouds collided with each other and triggered formation of the high-mass stars, mainly toward two regions including the nearly ten O stars, theta1 Ori and theta2 Ori, in M42 and the B star, NU Ori, in M43. The timescale of the collision is estimated to be ~0.1Myr by a ratio of the cloud size and velocity corrected for projection, which is consistent with the age of the youngest cluster members less than 0.1Myr. The majority of the low-mass cluster members were formed prior to the collision in the last one Myr. We discuss implications of the present hypothesis and the scenario of high-mass star formation by comparing with the other eight cases of triggered O star formation via cloud-cloud collision.

Analysis of Cloud-Cloud Collision in M42 and M43 and its Implications for Star Formation in the Orion Nebula Cluster

This paper by Fukui et al. offers a detailed examination of the molecular gas in M42 and M43, focusing on the possibility of cloud-cloud collisions (CCC) as a trigger for the formation of high-mass stars in the Orion Nebula Cluster (ONC). By analyzing archival $^{12}$CO($J$ = 1--0) emission data, the paper provides insights into the dynamics and interactions of molecular clouds within these regions, contributing to our understanding of star formation processes.

Observational Findings and Hypotheses

The paper identifies two primary velocity components at 8 km/s and 13 km/s in the region, suggesting that these represent distinct molecular clouds. Through observational data, the authors identify at least three pairs of complementary distributions between these velocity components, implying a potential CCC event. This hypothesis posits that the collision between these clouds may have triggered the formation of high-mass stars, notably near two regions containing nearly ten O stars, $\theta^1$ Ori and $\theta^2$ Ori, in M42, and the B star, NU Ori, in M43.

The collision is estimated to have occurred approximately 0.1 Myr ago, deduced from the ratio of cloud size to the velocity corrected for projection effects. This is congruent with the age of the youngest cluster members, suggesting a relatively recent stellar formation event resulting from the collision.

Implications and Theoretical Context

The proposed CCC as a mechanism for star formation ties into broader theoretical efforts to understand high-mass star formation. Traditionally, theories have included monolithic collapse and competitive accretion, yet these often fail to account convincingly for observed phenomena when contrasted with empirical data from various regions, including the ONC. The CCC model provides a framework where the supersonic collision generates enough shock-induced turbulence to enhance mass accretion rates substantially. This aligns with theoretical models that suggest CCCs increase cloud density and the effective sound speed in the interface layer, making high-mass star formation feasible by overcoming radiative pressure feedback.

Comparative Cases

The paper further discusses this scenario by comparing it with eight other cases of suspected triggered O star formation via CCC, providing a more comprehensive backdrop for the analysis of the ONC. These comparisons underscore the importance of understanding relative velocities, cloud densities, and dispersal timescales to evaluate whether CCC is a common phenomenon or merely an exceptional case in specific environments.

Future Directions

The hypothesis presented in this paper opens avenues for extended analyses of molecular gas distributions and star formation histories in other high-mass star-forming regions. The ability to detect CCC and its effects observationally can provide significant insights into the overall process of star formation on a galactic scale. Future developments might focus on enhancing observational capabilities, perhaps using higher resolution and sensitivity instruments to identify further evidence of CCC events and refine our understanding of their mechanics and prevalence.

Conclusion

This paper by Fukui et al. provides a compelling case for the role of cloud-cloud collisions in star formation within the Orion Nebula Cluster. While more research is needed to confirm the prevalence of CCCs across different astrophysical environments, the findings contribute to shifting paradigms about the mechanisms driving high-mass star formation and the evolutionary processes of molecular clouds in our galaxy.