Evidence for the gravity-driven and magnetically-regularized gas flows feeding the massive protostellar cluster in Cepheus A

Abstract

The hierarchical interplay among gravity, magnetic fields, and turbulence in forming massive protostellar clusters remains elusive. We present high-resolution ($\sim$14 arcsec $\simeq$ 0.05 pc) 850 $\mu$m dust polarization and C$^{18}$O line observations of Cepheus A using JCMT SCUBA-2/POL-2 and HARP. Our analysis reveals aligned gravitational (G), magnetic (B), and velocity fields (K), with an energy hierarchy of $E_{\mathrm{G}}$ > $E_{\mathrm{B}}$ > $E_{\mathrm{K}}$. Gravity, as the primary driver, induces gas flows and drags in B-field lines. Magnetic tension, as a secondary force, regulates turbulence, enabling ordered flows with an accretion rate of $\sim$ 2.1 $\pm$ 0.4 $\times$ 10$^{-4}$ M$_\odot$ yr$^{-1}$. This challenges the conventional view of B-fields resisting collapse in the clump/hub scale, instead showing cooperation with gravity. The $\sim$0.6 pc clump-scale B-field (with mean PA $\sim$ 45{\deg}) aligns coherently with fields at cloud ($\sim$5 pc), core ($\sim$0.05 pc), and disk ($\sim$2000 AU) scales, offering new insights into the role of magnetic fields in multiscale star formation dynamics.