Dark Dragon Breaks Magnetic Chain: Dynamical Substructures of IRDC G28.34 Form in Supported Environments
arxiv(2024)
摘要
We have comprehensively studied the multi-scale physical properties of the
infrared dark cloud (IRDC) G28.34 (the Dragon cloud) with dust polarization and
molecular line data from Planck, FCRAO-14m, JCMT, and ALMA. We find that the
averaged magnetic fields of clumps tend to be either parallel with or
perpendicular to the cloud-scale magnetic fields, while the cores in clump MM4
tend to have magnetic fields aligned with the clump fields. Implementing the
relative orientation analysis (for magnetic fields, column density gradients,
and local gravity), Velocity Gradient Technique (VGT), and modified
Davis-Chandrasekhar-Fermi (DCF) analysis, we find that: G28.34 is located in a
trans-to-sub-Alfvénic environment (ℳ_A=0.74 within r=15
pc); the magnetic field is effectively resisting gravitational collapse in
large-scale diffuse gas, but is distorted by gravity within the cloud and
affected by star formation activities in high-density regions; and the
normalized mass-to-flux ratio tends to increase with increasing density and
decreasing radius. Considering the thermal, turbulent, and magnetic supports,
we find that the environmental gas of G28.34 is in a super-virial (supported)
state, the infrared dark clumps may be in a near-equilibrium state, and core
MM4-core4 is in a sub-virial (gravity-dominant) state. In summary, we suggest
that magnetic fields dominate gravity and turbulence in the cloud environment
at large scales, resulting in relatively slow cloud formation and evolution
processes. Within the cloud, gravity could overwhelm magnetic fields and
turbulence, allowing local dynamical star formation to happen.
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