Cosmic rays (CRs) influence the evolution of molecular clouds and play a crucial role in star formation. However, measuring the CRIR, which quantifies how often CRs ionize gas molecules, has been challenging. This is largely because it relies on the detection of low-energy CRs that are difficult to observe directly due to the Solar wind's interference.

The Voyager probes provided a unique opportunity by measuring the CR spectrum beyond the heliosphere, but their data only represents a small region of the local interstellar medium. Most CRIR estimates rely on indirect methods, including the absorption of specific ions like H3+, which are produced by CRs interacting with molecular hydrogen (H2). For years, these measurements produced CRIR estimates that were far higher than those suggested by the Voyager data, creating a long-standing mystery.

A breakthrough came with new 3D dust extinction maps from the Gaia satellite, which provided detailed insights into the distribution of dust and gas in molecular clouds. Armed with this data, the MPE team reanalyzed the H3+ observations using simulations to model the structure of individual clouds and the corresponding CRIR. Their results aligned much closer with the CR spectrum measured by Voyager, resolving the decade-long discrepancy.

"One of the astonishing outcomes of our analysis is that the re-evaluated values of CRIR are an order of magnitude lower than the previous estimates, which actually brings our results into agreement with the CR spectrum measured by the Voyager probes," said Alexei Ivlev, one of the lead authors of the study.

Kedron Silsbee noted the broader impact of the findings on astrochemical models: "In addition to the impressive results on the CRIR, this work represents a major step forward in the realism of astrochemical modeling. These are the first simulations to incorporate the actual gas density distribution. I anticipate that combining astrochemical simulations with accurate determinations of the density structure and the radiation field will result in many more exciting advances in the coming years."

Further analysis also revealed a related discovery: earlier estimates of gas densities in diffuse molecular clouds were significantly overstated. Prof. David Neufeld from Johns Hopkins University revisited the methods used to estimate gas densities, identifying errors in the rates of molecular carbon (C2) excitation used in previous calculations. His revisions brought the gas density estimates into agreement with the extinction map data.

Reflecting on these changes, Neufeld said, "Initially, I had been quite skeptical of the lower density estimates that emerged from the dust extinction maps, because they were inconsistent with what we thought we knew. But when I looked more closely at the methods used previously to evaluate the gas density from observations of C2, I found that they had yielded density estimates that were far too high."

The revised CRIR and gas density values will have significant implications for our understanding of molecular clouds, star formation, and the broader dynamics of the Milky Way.

"CRs are fundamental ingredients for the dynamical evolution of interstellar molecular clouds, where stars and planets form, and for chemical evolution in space, where the precursors of pre-biotic molecules form. It is thus crucial for astrophysics and astrochemistry to know the CRIR, making this one of our long-standing scientific goals," added Paola Caselli, director at the Center for Astrochemical Studies at MPE.

The research also suggests the need for a re-evaluation of previous CRIR estimates in molecular clouds, considering recent advancements in understanding gas distribution within the Milky Way.

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