Suppressed Cosmic Ray Energy Densities in Molecular Clouds From Streaming Instability-Regulated Transport

TL;DR

This study uses advanced simulations to show that streaming instability-regulated cosmic ray transport significantly reduces CR energy in molecular clouds, affecting ionization rates and star formation efficiency.

Contribution

First numerical simulations including explicit cosmic ray transport in star-forming molecular clouds, revealing the impact of streaming instability on CR energy attenuation and star formation.

Findings

CR energy is strongly attenuated due to streaming instability.

Low CR ionization rates limit CR impact on star formation in typical environments.

High-CR environments show elevated ionization and increased star formation efficiency.

Abstract

Cosmic rays (CRs) are the primary driver of ionization in star forming molecular clouds (MCs). Despite their potential impacts on gas dynamics and chemistry, no simulations of star cluster formation following the creation of individual stars have included explicit cosmic ray transport (CRT) to date. We conduct the first numerical simulations following the collapse of a $2000 M_{\odot}$ MC and the subsequent star formation including CRT using the STARFORGE framework implemented in the GIZMO code. We show that when CR-transport is streaming-dominated, the CR energy in the cloud is strongly attenuated due to energy losses from the streaming instability. Consequently, in a Milky Way like environment the median CR ionization rate (CRIR) in the cloud is low ($ \zeta \lesssim 2 \times 10^{-19} \rm s^{-1}$) during the main star forming epoch of the calculation and the impact of CRs on the star formation in the cloud is limited. However, in high-CR environments, the CR distribution in the cloud is elevated ($\zeta \lesssim 6 \times 10^{-18}$), and the relatively higher CR pressure outside the cloud causes slightly earlier cloud collapse and increases the star formation efficiency (SFE) by $50 \%$ to $\sim 13 \%$. The initial mass function (IMF) is similar in all cases except with possible variations in a high-CR environment. Further studies are needed to explain the range of ionization rates observed in MCs and explore star formation in extreme CR environments.