An Early Transition to Magnetic Supercriticality in Star Formation

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.

3 figures, 1 table, Nature accepted

References in corpus (8)

• Theory of Star Formation
• Observing the gas temperature drop in the high-density nucleus of L 1544
• The dynamics of collapsing cores and star formation
• The Transition from Atomic to Molecular Hydrogen in Interstellar Clouds: 21cm Signature of the Evolution of Cold Atomic Hydrogen in Dense Clouds
• The central 1000 AU of a pre-stellar core revealed with ALMA. I. 1.3 mm continuum observations
• Review of Zeeman Effect Observations of Regions of Star Formation
• The Magnetic Field of L1544: I. Near-Infrared Polarimetry and the Non-Uniform Envelope
• The role of magnetic field in molecular cloud formation and evolution

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• Role of Filamentary Structures in the Formation of Two Dense Cores, L1544 and L694-2
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• Accurate polarization calibration of FAST spectral data for measurements of Zeeman splittings of OH megamasers in IRAS 02524+2046