An Early Transition to Magnetic Supercriticality in Star Formation

TL;DR

This study detects magnetic fields in a prestellar core and finds that the molecular envelope becomes magnetically supercritical earlier in star formation than previously thought, indicating an early flux reduction.

Contribution

It provides the first direct measurement of magnetic fields in a prestellar core using HINSA and reveals early magnetic supercriticality during star formation transition.

Findings

Magnetic field of +3.8 μG detected in L1544 core.

Molecular envelope is magnetically supercritical despite high density.

Magnetic flux reduction occurs earlier than classical models suggest.

Abstract

Magnetic fields play an important role in the evolution of interstellar medium and star formation. As the only direct probe of interstellar field strength, credible Zeeman measurements remain sparse due to the lack of suitable Zeeman probes, particularly for cold, molecular gas. Here we report the detection of a magnetic field of $+$3.8 $\pm$ 0.3 $\mu$G through the HI narrow self-absorption (HINSA) toward L1544, a well-studied prototypical prestellar core in an early transition between starless and protostellar phases characterized by high central number density and low central temperature. A combined analysis of the Zeeman measurements of quasar HI absorption, HI emission, OH emission, and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope. The molecular envelope traced by HINSA is found to be magnetically supercritical, with a field strength comparable to that of the surrounding diffuse, magnetically subcritical CNM despite a large increase in density. The reduction of the magnetic flux relative to the mass, necessary for star formation, thus seems to have already happened during the transition from the diffuse CNM to the molecular gas traced by HINSA, earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes.