Title: Zeeman effect of HI narrow self-absorption line, a new systematic probe of magnetic field in molecular cloud

Abstract: Magnetic fields have 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 owing to the lack of suitable Zeeman probes, particularly for cold, molecular gas. I report the detection of a magnetic field of +3.8 ± 0.3 microgauss through the HI narrow self-absorption (HINSA) using the Five-hundred-meter Aperture Spherical Telescope (FAST) towards a well-studied prototypical prestellar core L1544. The molecular envelope traced by the HINSA is found to be magnetically supercritical, with a field strength comparable to that of the surrounding diffuse, magnetically subcritical cold neutral medium despite a large increase in density. The reduction of the magnetic flux relative to the mass, which is necessary for star formation, thus seems to have happened earlier than envisioned in the classical picture where magnetically supercritical cores form out of magnetically subcritical envelopes. In the talk, I will also show the FAST HINSA Zeeman survey toward sources from low-mass cores to massive star-forming regions.