A star’s interstellar medium relies on magnetic fields, which are vital but “hidden” components. The lack of experimental probes for studying interstellar magnetic fields can be attributed to the secrecy that surrounds them.
While Michael Faraday was using coils in the basement of the Royal Institution in the early nineteenth century to investigate the link between magnetism and electricity, astronomers today are still unable to deploy coils light-years away.
An international team led by Dr. LI Di from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) has obtained accurate magnetic field strength in molecular cloud L1544, a region of the interstellar medium that appears to be ready to form stars. The team used the Five-hundred-meter Aperture Spherical radio Telescope (FAST).
The authors used the so-called HI Narrow Self-Absorption (HINSA) approach, which was first proposed by LI Di and Paul Goldsmith in 2003 based on data from the Arecibo Observatory. The sensitivity of FAST enabled the detection of the HINSA’s Zeeman effect with great clarity. The findings show that such clouds reach a supercritical state, or are prepared for collapse, sooner than conventional models predict.
“FAST’s design of focusing radio waves on a cable-driven cabin results in clean optics, which has been vital to the success of the HINSA Zeeman experiment,” says Dr. LI.
The findings of the study were published in Nature.
Currently, the Zeeman effect, which is the splitting of a spectral line into many components of frequency when in the presence of a magnetic field, is the only method for directly determining the strength of the interstellar magnetic field. The intergalactic Zeeman effect is a very minor phenomenon. A few billionths of a percent of the inherent frequencies of the emission lines can be explained by a frequency shift originating in the relevant clouds.
According to the findings of the 2003 study, the spectra of molecular clouds contain an atomic-hydrogen characteristic known as HINSA, which is created when hydrogen atoms collide with hydrogen molecules and cool, thus producing an HINSA feature. Since the discovery of the Zeeman effect by the Arecibo telescope, the Zeeman effect for HINSA has been hailed as a promising probe of the magnetic field in molecular clouds.
When compared to molecular tracers, HINSA has a line strength that is 5–10 times greater. The HINSA molecule also has a reasonably significant sensitivity to magnetic fields and, in contrast to other molecular tracers, is relatively resistant to astrochemical fluctuations.
The magnetic field strength in L1544, according to FAST’s HINSA measurements, is approximately 4 µGauss, which is 6 million times less than the magnetic field strength on Earth. A combination of quasar (active supermassive blackhole) absorption and hydroxyl emission showed a coherent magnetic field structure with identical orientation and magnitude throughout the cold neutral medium, the molecular envelope, and the dense core.
As a result, in contrary to the usual view, the change from magnetic subcriticality to supercriticality – that is, when the field can and cannot support the cloud against gravity, respectively – occurs in the envelope rather than the core.
The mechanism by which the interstellar magnetic field diminishes to allow cloud collapse is still a mystery in the science of star formation. Ambipolar diffusion – the decoupling of neutral particles from plasma – in cloud cores has long been the most frequently proposed solution to the problem.
The coherence of the magnetic field shown by the HINSA Zeeman effect indicates that dissipation of the magnetic field happens during the development of the molecular envelope, and that this dissipation may occur through a different mechanism than ambipolar diffusion, according to the findings.
Source: 10.1038/s41586-021-04159-x
Image Credit: NAOC
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