China's FAST Telescope Detected Accurate Field Strength in a Molecular Cloud

A team of researchers from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) has successfully measured the magnetic field strength in molecular cloud L1544.

This is an area of the interstellar medium that appears to be ripe for star formation, and it's basically a nursery for hundreds of baby stars. The achievement was made possible by China's Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the research team using the so-called HI Narrow Self-Absorption (HINSA) technique, which was first presented in 2003 by Dr. Di Li, who also led this project, and Paul Goldsmith, based on Arecibo data.

The scientists discovered an atomic-hydrogen signature dubbed HINSA in the spectra of molecular clouds two decades ago, which is created by hydrogen atoms cooled by collisions with hydrogen molecules. As a result, for HINSA, the Zeeman effect, which is the splitting of a spectral line into numerous components of frequency in the presence of a magnetic field, has been identified as a promising probe of the magnetic field in molecular clouds.

The sensitivity of FAST enabled a clear observation of the Zeeman effect, and these results, which were published in the journal Nature on Thursday, indicate that such clouds reach a supercritical state, prepared for collapse, sooner than standard models predict.

The magnetic field strength at L1544 is estimated to be around 4 Gauss, or 6 million times less than that of Earth, according to FAST's HINSA measurements. A combined investigation of quasar absorption and hydroxyl emission revealed a coherent magnetic field structure with identical orientation and magnitude throughout the cold neutral medium, the molecular envelope, and the dense core. The fact that the magnetic field strength wasn't stronger than in the outer layer is important, as "If the standard theory worked, the magnetic field needs to be much stronger to resist a 100-fold increase in cloud density. That didn’t happen,” explained Di Li, per Science.

And, according to Paola Caselli from the Max Planck Institute for Extraterrestrial Physics, who wasn't a part of the research, this is a "very big statement" that could drastically alter the way we view star formation because "the paper basically says that gravity wins in the cloud: That’s where stars start to form, not in the dense core."