JWST Observations of Cosmic-Ray-excited H ₂ 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 (B68) that reveal the spatially resolved signature of cosmic-ray-excited molecular hydrogen (CRXH 2 ) emissions for the first time. Following up on our initial detection of CRXH 2 emissions from B68, we now exploit JWST’s sensitivity and spatial multiplexing to map CRXH 2 rovibrational lines across 16 sight lines through the cloud. By disentangling the CRXH 2 and UV-pumped H 2 components, we isolate the para-H 2 -dominated spectrum attributable to cosmic-ray (CR) excitation. We find that there are significant spatial variations in the ratio of the CRXH 2 line intensity to the line-of-sight H 2 column density; these cannot be accounted for by dust extinction alone and demonstrate a clear attenuation of the CR flux with increasing shielding column. Modeling B68 as a Bonnor–Ebert sphere, we constrain both the unshielded CR ionization rate, ζ H 2 , and how it decreases with shielding column. At a reference depth of N (H 2 ) = 3 × 10 21 cm −2 , we infer ζ H 2 ≈ 1.4 × 1 0 − 16 s −1 , a factor of ≈3 higher than the average value derived from H 3 + absorption studies. These results provide the most direct probe to date of CR penetration into cold, dense gas, offering new constraints on both the microphysics of CR–H 2 interactions and the attenuation of low-energy CRs in molecular clouds. Our findings establish CRXH 2 emission as a powerful new diagnostic of the CR environment in interstellar space.