First BISTRO observations of the dark cloud Taurus L1495A-B10: the role of the magnetic field in the earliest stages of low-mass star formation

TL;DR

This study uses polarisation observations to explore magnetic fields in a Taurus molecular cloud, revealing a transitional evolutionary stage in low-mass star formation and confirming a long-standing theoretical prediction.

Contribution

It introduces an observational evidence of a transitional stage between magnetic field-dominated and matter-dominated evolution in star-forming clouds, confirming a 50-year-old theoretical prediction.

Findings

Magnetic fields are roughly perpendicular to filaments in dense cores.

Large-scale magnetic field orientation is uncorrelated with core structures.

First measurement of critical density transition consistent with theoretical prediction.

Abstract

We present BISTRO Survey 850 {\mu}m dust emission polarisation observations of the L1495A-B10 region of the Taurus molecular cloud, taken at the JCMT. We observe a roughly triangular network of dense filaments. We detect 9 of the dense starless cores embedded within these filaments in polarisation, finding that the plane-of-sky orientation of the core-scale magnetic field lies roughly perpendicular to the filaments in almost all cases. We also find that the large-scale magnetic field orientation measured by Planck is not correlated with any of the core or filament structures, except in the case of the lowest-density core. We propose a scenario for early prestellar evolution that is both an extension to, and consistent with, previous models, introducing an additional evolutionary transitional stage between field-dominated and matter-dominated evolution, observed here for the first time. In this scenario, the cloud collapses first to a sheet-like structure. Uniquely, we appear to be seeing this sheet almost face-on. The sheet fragments into filaments, which in turn form cores. However, the material must reach a certain critical density before the evolution changes from being field-dominated to being matter-dominated. We measure the sheet surface density and the magnetic field strength at that transition for the first time and show consistency with an analytical prediction that had previously gone untested for over 50 years (Mestel 1965).