Magnetic fields play an important role in the evolution of interstellar medium and star formation. As the only direct tracer of interstellar field strength, credible Zeeman measurements remain sparse owing to the lack of suitable Zeeman probes, particularly for cold, molecular gas. Here we report the detection of a magnetic field of +3.8 ± 0.3 μG through the HI narrow self-absorption (HINSA) toward the prestellar core L1544. 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 of the L1544. We find that the molecular envelope traced by HINSA is already magnetically supercritical, with a field strength comparable to that in 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 happen 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.

BIO
Tao-Chung Ching is a research fellow in the Research Institute of Intelligent Computing, Zhejiang Lab. He received his PhD in 2017 from the Institute of Astronomy in the National Tsing-Hua University, Taiwan. During his graduated study, he was also a Predoctoral Fellow of the Harvard-Smithsonian Center for Astrophysics from 2013 to 2015. He worked in National Astronomical Observatories, Chinese Academy of Sciences (NAOC) as a FAST Postdoctoral Fellow from 2017 to 2019 and as a Chinese Academy of Sciences Fellow of Taiwanese Young Talented Scholar from 2019 to 2022. His research interests are interstellar magnetic fields and star formation through observational astronomy from submillimeter to radio wavelengths.

ref：https://www.nature.com/articles/s41586-021-04159-x