Reprinted from "Heart of the Machine"
Edit | Du Wei, Chen Ping
This is one of the important scientific research achievements that scientists have once again achieved relying on the "Chinese Heavenly Eye".
The "500-meter Aperture Spherical Radio Telescope (FAST)", located in the Dawotai Depression of Pingtang County, Qiannan Buyi and Miao Autonomous Prefecture, Guizhou Province, is known as the "Eye of Heaven in China". Since opening to operation in China on January 11, 2020, it has become the world's largest and most sensitive radio telescope, which indicates that humans can explore the deeper unknowns of the universe.
In just two years, China Sky Eye has discovered about 500 pulsars, making it the most efficient device in the world since its operation. The annual observation time is more than 5300 hours, which is far more efficient than the international counterparts expect.
On January 6, "China's Sky Eye" made significant progress in measuring interstellar magnetic fields in neutral hydrogen spectral lines and appeared on the cover of Nature. This is a batch of important scientific research achievements that scientists have once again achieved relying on "China's Heavenly Eye".
The study adopts the original neutral hydrogen narrow line self-absorption method, and for the first time uses this method to detect the Zeeman effect, and obtains high-confidence interstellar magnetic field measurements with a strength of 3.8±0.3 micro gauss, which provides important observational evidence for solving the "magnetic flux problem", one of the three classical problems of star formation. The study was done by an international cooperation team led by Li Jing, a researcher at the National Astronomical Observatories of the Chinese Academy of Sciences.
Magnetic field problem is the key to star formation, we take the sun as an example, the sun before becoming a star, can be seen as a molecular cloud, but the molecular cloud to form a high density, high temperature star, will be subject to magnetic field resistance. As molecular clouds aggregate to a dense cloud nucleus, the magnetic flux increases, and the resistance caused by the magnetic field increases, which does not allow it to continue to contract.
Li's team observed molecular clouds through FAST and found that the magnetic flux had been canceled out before the molecular cloud contracted to a dense state, rather than the prediction of the previous bipolar dissipation (after the molecular cloud became dense, the magnetic flux increased by the contrarian increase was canceled out by bipolar dissipation), which provided important observational evidence for solving the "magnetic flux problem", one of the classic problems of star formation.
Using HINSA technology, the precise magnetic field strength of L1544 is obtained
The researchers have developed a technique called HI narrow self-absorption (HINSA) to provide probes that transition from HI to H_2. HINSA tracks cold atomic hydrogen mixed well with H_2 and provides the necessary H1 cooling by collision (not available in CNMs).
HINSA is close to H_2 forms a steady state between destruction and is independent of gas density, so transitions near critical density can be detected. Although the Zeeman effect of the HI self-absorption signature has been previously reported, the wide-line width of the absorbing component is most associated with the diffusion of atomic gases. Considering that HINSA typically has a higher luminance temperature than most molecular lines, it is not affected by losses and can be detected over a wide H_2 density range.
The researchers say HINSA is a very promising molecular gas Zeeman probe.
L1544 is a region of interstellar medium about to form stars, in which HINSA is characterized by a strong absorption inclination angle and narrow lines that are almost hot at temperatures below 15 K. The non-hot width and centroid velocity of HINSA are very close to the emission lines of OH, CO, and CO molecules, and their column densities are strongly correlated, indicating that a significant portion of atomic hydrogen is located in the cooled, well-concealed portion of L1544.
Therefore, the researchers hypothesized that the column density sampled by HINSA was similar to the density obtained from dust, although the surface area covered by HINSA was much larger, as shown in Figure 1a below. Previous OH Zeeman detection using the Arecibo telescope near the center of L1544 resulted in a magnetic field strength of B_los = +10.8 ± 1.7 μG, where B-los is the magnetic field component distributed along the line of sight.
In contrast, OH Zeeman's observation of four envelope locations 6.0' (0.24 pc) from the center of L1544 using the Green Bank Telescope (GBT) resulted in edge detection of B_los = +2 ± 3 μG, but the structure of the envelope field was undecided.
As shown in Figure 1 below, the researchers detected Zeman splitting in a 2.9′ (0.12 pc) beam 3.6' (0.15 pc) from the center of L1544 and close to the peak density of the HINSA column using the Chinese Sky Eye FAST to obtain the precise magnetic field strength of L1544.
Figure 2 below shows the spectrum of the Stokes I(v) and V(v) parameters, with v representing speed. Figure 2a decomposes I(v) into a foreground HINSA component, a background WNM component, and three CNM components between HINSA and WNM. Figure 2b shows the Zeyman split of HINSA and the population Zeyman distribution of 5 components.
Figure 3 below shows the Seyman split and B_los of five components, HINSA, CNM1, CNM2, CNM3, and WNM.
The above is only a brief introduction to the research proposal method, readers who are interested in and understand the field of astronomy should move to the original paper.
Reference Links:
http://news.cyol.com/gb/articles/2022-01/06/content_aM47gtBVx.html
https://wap.peopleapp.com/article/6447406/6332134
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