JWST observations of cosmic-ray-excited H2 in Barnard 68: spatial variations and constraints on cosmic-ray attenuation
Abstract
We present James Webb Space Telescope (JWST) NIRSpec observations of the starless dark cloud Barnard 68 that reveal the spatially-resolved signature of cosmic-ray excited molecular hydrogen (CRXH2) emissions for the first time. Following up on our initial detection of CRXH2 emissions from B68 (Bialy et al. 2025), we now exploit JWST's sensitivity and spatial multiplexing to map CRXH2 rovibrational lines across 16 sight lines through the cloud. By disentangling the CRXH2 and UV-pumped H2 components, we isolate the para-H2-dominated spectrum attributable to cosmic-ray excitation. We find that there are significant spatial variations in the ratio of the CRXH2 line intensity to the line-of-sight H2 column density; these cannot be accounted for by dust extinction alone and demonstrate a clear attenuation of the cosmic-ray flux with increasing shielding column. Modeling B68 as a Bonnor-Ebert sphere, we constrain both the unshielded cosmic-ray ionization rate, ζ H2, and how it decreases with shielding column. At a reference depth of N( H2) = 3 × 1021 cm-2, we infer ζ H2 ≈ 1.4 × 10-16 s-1, a factor of ≈ 3 higher than the average value derived from H3+ absorption studies. These results provide the most direct probe to date of cosmic-ray penetration into cold, dense gas, offering new constraints on both the microphysics of CR-H2 interactions and the attenuation of low-energy cosmic rays in molecular clouds. Our findings establish CRXH2 emission as a powerful new diagnostic of the cosmic-ray environment in interstellar space.
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