The non-uniform distribution of gas and protostars in molecular clouds is caused by combinations of various physical processes that are difficult to separate. We explore this non-uniform distribution in the M17 molecular cloud complex that hosts massive star formation activity using the 12 CO (J = 1−0) and 13 CO (J = 1 − 0) emission lines obtained with the Nobeyama 45m telescope. Differences in clump properties such as mass, size, and gravitational boundedness reflect the different evolutionary stages of the M17-HII and M17-IRDC clouds. Clumps in the M17-HII cloud are denser, more compact, and more gravitationally bound than those in M17-IRDC. While M17-HII hosts a large fraction of very dense gas (27%) that has column density larger than the threshold of ∼ 1 g cm −2 theoretically predicted for massive star formation, this very dense gas is deficient in M17-IRDC (0.46%). Our HCO + (J = 1 − 0) and HCN (J = 1 − 0) observations with the TRAO 14m telescope, trace all gas with column density higher than 3 × 10 22 cm −2 , confirm the deficiency of high density ( 10 5 cm −3 ) gas in M17-IRDC.Although M17-IRDC is massive enough to potentially form massive stars, its deficiency of very dense gas and gravitationally bound clumps can explain the current lack of massive star formation.

We investigated the effect of magnetic fields on the collision process between dense molecular cores. We performed three-dimensional magnetohydrodynamic simulations of collisions between two self-gravitating cores using the Enzo adaptive mesh refinement code. The core was modeled as a stable isothermal Bonnor–Ebert (BE) sphere immersed in uniform magnetic fields. Collisions were characterized by the offset parameter b, Mach number of the initial core  , magnetic field strength B 0, and angle θ between the initial magnetic field and collision axis. For head-on (b = 0) collisions, one protostar was formed in the compressed layer. The higher the magnetic field strength, the lower the accretion rate. For models with b = 0 and θ = 90°, the accretion rate was more dependent on the initial magnetic field strength compared with b = 0 and θ = 0° models. For off-center (b = 1) collisions, the higher specific angular momentum increased; therefore, the gas motion was complicated. In models with b = 1 and  = 1 , the number of protostars and gas motion highly depended on B 0 and θ. For models with b = 1 and  = 3 , no significant shock-compressed layer was formed and star formation was not triggered.

Star formation can be triggered by compression from shock waves. In this study, we investigated the interaction of hydrodynamic shocks with Bonnor–Ebert spheres using 3D hydrodynamical simulations with self-gravity. Our simulations indicated that the cloud evolution primarily depends on two parameters: shock speed and initial cloud radius. Stronger shocks can compress clouds more efficiently, and when the central region becomes gravitationally unstable, a shock triggers cloud contraction. However, if it is excessively strong, it shreds the cloud more violently and the cloud is destroyed. From simple theoretical considerations, we derived the condition of triggered gravitational collapse, which agreed with the simulation results. Introducing sink particles, we followed the further evolution after star formation. Since stronger shocks tend to shred the cloud material more efficiently, the stronger the shock is, the smaller the final (asymptotic) masses of the stars formed (i.e., sink particles) become. In addition, shocks accelerate clouds, promoting mixing of shock-accelerated interstellar medium gas. As a result, the separation between sink particles and shocked clouds center and their relative speeds increase over time. We also investigated the effect of cloud turbulence on shock–cloud interaction. We observed that cloud turbulence prevents rapid cloud contraction; thus, turbulent clouds are destroyed more rapidly than thermally supported clouds. Therefore, the masses of stars formed become smaller. Our simulations provide a general guide to the evolutionary process of dense cores and Bok globules impacted by shocks.