In a groundbreaking study, an international team of researchers led by David A. Neufeld from Johns Hopkins University has utilized the James Webb Space Telescope (JWST) to observe cosmic-ray excited molecular hydrogen emissions in the dark cloud Barnard 68. The team, which includes scientists from various institutions, has made significant strides in understanding the interaction between cosmic rays and molecular hydrogen, a process crucial for the energy dynamics of interstellar space.
The researchers initially detected cosmic-ray excited molecular hydrogen (CRXH2) emissions from Barnard 68, a starless dark cloud. Leveraging the unprecedented sensitivity and spatial multiplexing capabilities of JWST’s NIRSpec instrument, they mapped CRXH2 rovibrational lines across 16 different sight lines through the cloud. This allowed them to separate the CRXH2 emissions from those pumped by ultraviolet radiation, isolating the spectrum dominated by para-H2 excitation from cosmic rays.
The study revealed significant spatial variations in the ratio of CRXH2 line intensity to the line-of-sight H2 column density. These variations could not be explained by dust extinction alone and indicated a clear attenuation of the cosmic-ray flux as it penetrates deeper into the cloud. By modeling Barnard 68 as a Bonnor-Ebert sphere, the researchers constrained both the unshielded cosmic-ray ionization rate and its decrease with increasing shielding column. At a reference depth of N(H2) = 3 x 10^21 cm^-2, they inferred a cosmic-ray ionization rate of approximately 1.4 x 10^-16 s^-1, which is about three times higher than the average value derived from previous H3+ absorption studies.
This research, published in the Astrophysical Journal Letters, provides the most direct probe to date of cosmic-ray penetration into cold, dense gas. The findings offer new constraints on the microphysics of cosmic-ray-H2 interactions and the attenuation of low-energy cosmic rays in molecular clouds. The study establishes CRXH2 emission as a powerful new diagnostic tool for understanding the cosmic-ray environment in interstellar space.
For the energy sector, understanding cosmic-ray interactions with molecular hydrogen can have implications for space-based solar power systems and the assessment of radiation environments for space infrastructure. The insights gained from this research could contribute to the development of more robust and efficient energy technologies for space exploration and utilization.
This article is based on research available at arXiv.