First ALMA Maps of Cosmic-Ray Ionization Rate in High-mass Star-forming Regions

Abstract

Low-energy cosmic rays (<1 TeV) are a pivotal source of ionization of the interstellar medium, where they play a central role in determining the gas chemical composition and drastically influence the formation of stars and planets. Over the past few decades, H _3 ^+ absorption line observations in diffuse clouds have provided reliable estimates of the cosmic-ray ionization rate relative to H _2 ( ${\zeta }_{{{\rm{H}}}_{2}}^{\mathrm{ion}}$ ). However, in denser clouds, where stars and planets form, this method is often inefficient due to the lack of H _3 ^+ rotational transitions. The ${\zeta }_{{{\rm{H}}}_{2}}^{\mathrm{ion}}$ estimates are, therefore, still provisional in this context and represent one of the least understood components when it comes to defining general models of star and planet formation. In this Letter, we present the first high-resolution maps of the ${\zeta }_{{{\rm{H}}}_{2}}^{\mathrm{ion}}$ in two high-mass clumps obtained with a new analytical approach recently proposed to estimate the ${\zeta }_{{{\rm{H}}}_{2}}^{\mathrm{ion}}$ in the densest regions of molecular clouds. We obtain $\langle {\zeta }_{{{\rm{H}}}_{2}}^{\mathrm{ion}}\rangle $ that span from 3 × 10 ^−17 to 10 ^−16 s ^−1 , depending on the different distribution of the main ion carriers, in excellent agreement with the most recent cosmic-ray propagation models. The cores belonging to the same parental clump show comparable ${\zeta }_{{{\rm{H}}}_{2}}^{\mathrm{ion}}$ , suggesting that the ionization properties of prestellar regions are determined by global rather than local effects. These results provide important information for the chemical and physical modeling of star-forming regions.

Keywords

• Cosmic rays
• Astrochemistry
• Interstellar line emission
• Star forming regions
• Interstellar medium
• Massive stars