“Crossing Failure”: The Arduous Journey of Low-Energy Cosmic Rays in the Birthplace of Stars

Professor Yang Ruizhi and associate researcher Liu Bing of the University of Science and Technology of China, Li Guangxing, professor of Yunnan University, Cui Yudong, doctor of Sun Yat-sen University, Emmadeona Wilhelmi, doctor of the German DESY Institute, and Felix Aharonian, professor of the MPIK Research Institute, and other experts in the field of astrophysics cooperated to analyze and study the gamma radiation of the giant molecular clouds Taurus and Perseus close to the earth. It is found that the dense molecular cloud mass has a strong “shielding” effect on low-energy cosmic rays, and the density of cosmic rays at the dense mass of molecular clouds is significantly lower than the average value in the interstellar medium. As an important observation, the research results were recently published in the international journal Nature Astronomy.

The ability of cosmic rays of different energies to enter the dense core of molecular clouds is also different, high-energy ones can easily penetrate, and low-energy ones are mostly shielded (cartoon schematic, courtesy of China University of Science and Technology).

Cosmic rays are relativistic charged particles from outer space, and the gamma rays produced by their interaction with the interstellar medium are excellent probes for tracing their origin and propagation. As an important part of the interstellar medium, cosmic rays contribute about one-third of the energy density of the interstellar medium. Of particular importance is the fact that cosmic rays dominate the heating and ionization of gases in dense molecular cloud clumps, and it is in molecular cloud clumps that star formation and interstellar chemistry associated with the origin of life take place.

If cosmic rays can freely penetrate molecular clouds, then the flow intensity of gamma rays per unit area of different regions of the molecular cloud observed by telescopes is simply positively correlated with the column density of gas, and the energy spectral morphology of gamma rays should also be consistent. Through the research and analysis of the observation data of the Fermi-LAT telescope, the collaborative team led by Yang Ruizhi found that under the assumption of uniform cosmic ray distribution, the gamma-ray residual radiation distribution map of Fermi-LAT obtained (obtained by subtracting the number of photons obtained by fitting the uniform distribution model) showed a significant “hole” structure at the dense molecular cloud mass in the giant molecular clouds Taurus and Perseus. This result means that the actual cosmic ray density at these dense molecular cloud clumps is lower than that of the less dense diffuse regions.

Further, the researchers split the gamma-ray radiation from the molecular cloud into two components corresponding to dense clumps and diffuse gases, and combined gamma ray data and gas density distribution data to derive the energy spectrum distribution of cosmic rays in the two regions. They found that the density of low-energy cosmic rays at dense clumps decreased significantly compared to the distribution of cosmic rays in diffuse gas.

One possible explanation for this spectral signature , the researchers believe that cosmic rays diffuse in molecular cloud clumps are significantly slower than their diffusion in the interstellar medium, so that low-energy cosmic rays lose most of their energy due to intense ionization processes and inelastic scattering processes before penetrating dense clumps, and thus cannot enter the densest regions. Cosmic rays with higher energy can penetrate the dense core region of molecular clouds, while low-energy cosmic rays are “shielded” because they lose energy too quickly. This part of the “shielded” part of the lower energy cosmic rays is also the main component that dominates the ionization and heating of gases, thereby regulating star formation and interstellar chemistry.

This study is the first to measure the density of cosmic rays in dense molecular cloud clumps, and found the modulation effect of molecular cloud density fluctuations on cosmic rays. The findings are expected to have a profound impact on research efforts in areas such as star formation and interstellar chemistry. (Source: Wang Min, China Science News)

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