Magnetic field Topology and Star Formation in the Cepheus B Filamentary cloud under External Feedback

Abstract

We present a detailed study of the Cep B molecular cloud based on sub-mm dust polarization and $^{13}$CO (J=3--2) spectral line observations obtained with SCUBA-2/POL-2 and HARP on the James Clerk Maxwell Telescope (JCMT). The 850 $μ$m dust continuum map reveals a prominent filamentary structure oriented Northwest--Southeast (NW-SE), with the magnetic field (B-field) displaying a distinct morphology-curving into a bow-like shape near the filament head and aligning along the spine toward the tail. The filament is thermally supercritical, with its line mass exceeding the critical value for an isothermal filament, indicating that self-gravity drives radial contraction. The mass-to-flux ratio suggests that the filament is magnetically subcritical on global scales, implying that B-fields provide significant support against collapse. Despite this, the presence of dense cores and embedded star formation indicates that collapse proceeds locally. The observed core spacing spans a range of values, with the largest separations comparable to the expected fragmentation scale for a self-gravitating filament undergoing sausage instability, suggesting that gravitational instability sets the primary fragmentation scale. Smaller separations and non-uniform spacing may indicate the influence of local variations and hierarchical fragmentation. Overall, Cep B represents a system in which gravity drives fragmentation, B-fields regulate its evolution, and external feedback shapes both its morphology and star formation activity at the head of the filament.