Parsec-scale cosmic-ray ionisation rate in Orion

¹ Institute for Astronomy (IfA), University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
e-mail: andrea.socci@univie.ac.at
² INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Chemistry Department, Sapienza University of Rome, P.le A. Moro, 00185 Rome, Italy
⁴ INAF, Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁵ Departamento de Astronomía, Facultad Ciencias Físicas y Matemáticas, Universidad de Concepción Av. Esteban Iturra s/n Barrio Universitario, Casilla 160, Concepción, Chile

Received: 13 March 2024
Accepted: 9 April 2024

Abstract

Context. Cosmic rays are a key component of the interstellar medium because they regulate the dynamics and chemical processes in the densest and coldest regions of molecular clouds. Still, the cosmic-ray ionisation rate of H₂ (ζ_H2^ion) is one of the most debated parameters characterising molecular clouds because of the uncertainties in the adopted chemical networks and analysis techniques.

Aims. This work aims to homogeneously estimate the ζ_H2^ion at parsec scales towards the Orion Molecular Clouds OMC-2 and OMC-3. We explore the change in ζ_H2^ion across a whole star-forming region by probing a range of column densities that has never been explored before. The significant increase in statistics obtained by studying an entire region allows us to place stronger constraints on the range of ζ_H2^ion values and exploit its connection with the physical properties of the interstellar medium.

Methods. The most recent ζ_H2^ion estimates are based on o-H₂D⁺, which is a direct product of the interaction between cosmic rays and H₂ in cold clouds. Since observations of o-H₂D⁺ are challenging, we proxy its abundance through CO depletion by employing C¹⁸O (2–1) observations towards OMC-2 and OMC-3, taking advantage of the existing correlation between the two parameters. Using additional observations of HCO⁺ (1–0) and DCO⁺ (3–2), we determine the deuteration fraction, and we finally derive the map of ζ_H2^ion in these two regions.

Results. The C¹⁸O depletion correlates with both the total column density of H₂ and the N₂H⁺ emission across OMC-2 and OMC-3. The obtained depletion factors and deuteration fractions are consistent with previous values obtained in low- and high-mass star-forming regions. These two parameters additionally show a positive correlation in the coldest fields of our maps. We derive cosmic-ray ionisation rates of ζ_H2^ion ~ 5 × 10^-18-10^-16s^-1. These values agree well with previous estimates based on o-H₂D⁺ observations. The ζ_H2^ion also shows a functional dependence on the column density of H₂ across a full order of magnitude (~10²²–10²³ cm⁻²). The estimated values of ζ_H2^ion decrease overall for increasing N(H₂), as predicted by theoretical models.

Conclusions. The results delivered by our approach are comparable with theoretical predictions and previous independent studies. This confirms the robustness of the analytical framework and promotes CO depletion as a viable proxy of o-H₂D⁺. We also explore the main limitations of the method by varying the physical size of the gas crossed by the cosmic rays (i.e. the path length). By employing a path length obtained from low-resolution observations, we recover values of the ζ_H2^ion that are well below any existing theoretical and observational prediction. This discrepancy highlights the need for interferometric observations in order to reliably constrain the ζ_H2^ion at parsec scales as well.