2026/05/30 by Ka Wai Ho, Munan Gong, Kedron Silsbee +1 · 1 voice
Physics and Astronomy · #astro-ph.GA
arxiv published 2026/05/30 · arxiv updated 2026/07/10
The cosmic ray ionization rate (CRIR) is a key parameter governing the physical, chemical and thermal evolution of the interstellar medium. The primary technique for measuring the CRIR in diffuse molecular clouds relies on observations of \rm H3+. Previous analyses of these observations have derived the CRIR under the assumption of steady-state chemistry. Here, we investigate the effect of time-dependent chemistry on the inferred CRIR from \rm H3+ observations. We perform 3D MHD simulations with coupled chemistry and driven turbulence. Following procedures similar to those used in the literature to analyze \rm H3+ observations, we conduct mock CRIR measurements by post-processing our simulations with different values of the CRIR to obtain steady-state abundances of \rm H2 and \rm H3+. By comparing those with the abundances from time-dependent chemistry, we determine the best-fitting value of the CRIR. We find that the abundances of both \rm H2 and \rm H3+ are higher in time-dependent chemistry simulations than in the steady-state case, especially in low-density regions. Furthermore, the inferred CRIR under the steady-state assumption is a factor of ∼ 2-5 higher than the true CRIR, with a median value of ζinferred/ζtrue ≈ 3. This bias increases with stronger magnetic fields, weaker FUV radiation fields, and stronger turbulence. Accounting for time-dependent chemistry, we report an average CRIR per \rm H2 of ζH2 = 2× 10-17~\mathrms-1 from the \rm H3+ observations. The CRIR is consistent with a constant value over the column density range of N=(2-6)×1021~\mathrmcm-2.