Parsec-scale cosmic-ray ionisation rate in Orion
- Autor(en)
- Andrea Socci, Giovanni Sabatini, Marco Padovani, Stefano Bovino, Alvaro Hacar Gonzalez
- Abstrakt
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 H
2 (ζH
2
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 ζH
2
ion at parsec scales towards the Orion Molecular Clouds OMC-2 and OMC-3. We explore the change in ζH
2
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 ζH
2
ion values and exploit its connection with the physical properties of the interstellar medium. Methods. The most recent ζH
2
ion estimates are based on o-H
2D
+, which is a direct product of the interaction between cosmic rays and H
2 in cold clouds. Since observations of o-H
2D
+ are challenging, we proxy its abundance through CO depletion by employing C
18O (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 ζH
2
ion in these two regions. Results. The C
18O depletion correlates with both the total column density of H
2 and the N
2H
+ 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 ζH
2
ion ~ 5 × 10
-18-10
-16s
-1. These values agree well with previous estimates based on o-H
2D
+ observations. The ζH
2
ion also shows a functional dependence on the column density of H
2 across a full order of magnitude (~10
22-10
23 cm
-2). The estimated values of ζH
2
ion decrease overall for increasing N(H
2), 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-H
2D
+. 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 ζH
2
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 ζH
2
ion at parsec scales as well.
- Organisation(en)
- Institut für Astrophysik
- Externe Organisation(en)
- INAF - Osservatorio Astrofisico di Arcetri, Università degli Studi di Roma La Sapienza, INAF - Istituto di Radioastronomia, Universidad de Concepción
- Journal
- Astronomy & Astrophysics
- Band
- 687
- Anzahl der Seiten
- 14
- ISSN
- 0004-6361
- DOI
- https://doi.org/10.1051/0004-6361/202449960
- Publikationsdatum
- 07-2024
- Peer-reviewed
- Ja
- ÖFOS 2012
- 103004 Astrophysik, 103003 Astronomie
- Schlagwörter
- ASJC Scopus Sachgebiete
- Astronomy and Astrophysics, Space and Planetary Science
- Link zum Portal
- https://ucrisportal.univie.ac.at/de/publications/84ef506e-4052-49ce-8040-71a9123678de