Evolution of Magnetic Fields in High Mass Star Formation: Linking field geometry and collapse for the W51 e2/e8 cores

TL;DR

This study uses high-resolution polarization observations to reveal hourglass-shaped magnetic fields in collapsing cores of W51 e2/e8, linking magnetic field geometry to star formation processes and core collapse.

Contribution

It provides the first high-resolution polarization maps showing hourglass magnetic fields in massive star-forming cores, supporting magnetic fragmentation and accretion models.

Findings

Magnetic fields are hourglass-shaped near the cores.

Magnetic fields are aligned with ionized accretion flows.

Cores are within a subcritical envelope, supporting magnetic fragmentation.

Abstract

We report our observational results of 870 $\mu$m continuum emission and its linear polarization in the massive star formation site W51 e2/e8. Inferred from the linear polarization maps, the magnetic field in the plane of sky (B$_{\bot}$) is traced with an angular resolution of 0$\farcs$7 with the Submillimeter Array (SMA). Whereas previous BIMA observations with an angular resolution of 3$\arcsec$ (0.1 pc) showed a uniform B field, our revealed B$_{\bot}$ morphology is hourglass-like in the collapsing core near the Ultracompact H II region e2 and also possibly in e8. The decrease in polarization near the continuum peak seen at lower angular resolution is apparently due to the more complex structures at smaller scales. In e2, the pinched direction of the hourglass-like B field morphology is parallel to the plane of the ionized accretion flow traced by H53$\alpha$, suggesting that the massive stars are formed via processes similar to the low mass stars, i.e. accretion through a disk, except that the mass involved is much larger. Furthermore, our finding that the resolved collapsing cores in e2 and e8 lie within one subcritical 0.5 pc envelope supports the scenario of \textit{magnetic fragmentation} via ambipolar diffusion. We therefore suggest that magnetic fields control the dynamical evolution of the envelope and cores in W51 e2 and e8.