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, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>ζ</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> , and how it decreases with shielding column. At a reference depth of N (H 2 ) = 3 × 10 21 cm −2 , we infer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>ζ</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>1.4</mml:mn> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>16</mml:mn> </mml:mrow> </mml:msup> </mml:math> s −1 , a factor of ≈3 higher than the average value derived from H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> 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.