Cosmic ray induced ionisation of a molecular cloud shocked by the W28 supernova remnant

Abstract

Cosmic rays are an essential ingredient in the evolution of the interstellar medium, as they dominate the ionisation of the dense molecular gas, where stars and planets form. However, since they are efficiently scattered by the galactic magnetic fields, many questions remain open, such as where exactly they are accelerated, what is their original energy spectrum, and how they propagate into molecular clouds. In this work we present new observations and discuss in detail a method that allows us to measure the cosmic ray ionisation rate towards the molecular clouds close to the W28 supernova remnant. To perform these measurements, we use CO, HCO${}^{+}$, and DCO${}^{+}$ millimetre line observations and compare them with the predictions of radiative transfer and chemical models away from thermodynamical equilibrium. The CO observations allow us to constrain the density, temperature, and column density towards each observed position, while the DCO${}^{+}$/HCO${}^{+}$ abundance ratios provide us with constraints on the electron fraction and, consequently, on the cosmic ray ionisation rate. Towards positions located close to the supernova remnant, we find cosmic ray ionisation rates much larger ($\gtrsim 100$) than those in standard galactic clouds. Conversely, towards one position situated at a larger distance, we derive a standard cosmic ray ionisation rate. Overall, these observations support the hypothesis that the $\gamma$ rays observed in the region have a hadronic origin. In addition, based on CR diffusion estimates, we find that the ionisation of the gas is likely due to $0.1-1$ GeV cosmic rays. Finally, these observations are also in agreement with the global picture of cosmic ray diffusion, in which the low-energy tail of the cosmic ray population diffuses at smaller distances than the high-energy counterpart.

Accepted to A\&A

References in corpus (6)

• Detection of the Characteristic Pion-Decay Signature in Supernova Remnants
• Observations of the young supernova remnant RX J1713.7-3946 with the Fermi Large Area Telescope
• Discovery of 35 New Supernova Remnants in the Inner Galaxy
• The effect of uncertainties on chemical models of dark clouds
• Gamma ray emission from SNR RX J1713.7-3946 and the origin of galactic cosmic rays
• AGILE detection of GeV gamma-ray emission from the SNR W28

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