Decreased Cosmic-Ray Energy Densities in Molecular Clouds due to Streaming Instability

Cosmic rays are a big deal when it comes to star formation in molecular clouds. However, we’ve been missing out on some of their most important features in simulation models—specifically their transport. A new set of simulations following the collapse of a moderate-size molecular cloud and subsequent star formation attempts to fill in these gaps.

The results are fascinating. They show that the way cosmic rays travel, something called cosmic-ray transport (CRT), has a massive effect on their energy distribution within the cloud. In certain conditions, where CRT is streaming-dominated, cosmic rays can lose a lot of energy due to something called the streaming instability. This means that in environments similar to our Milky Way, the median cosmic ray ionization rate in the cloud might be pretty low, which limits their impact on star formation.

But in high-cosmic-ray environments, things get interesting. Here, the concentration of cosmic rays in the cloud is higher, leading to a small boost in the rate of star formation. In these conditions, the pressure exerted by cosmic rays outside the cloud can even hasten the collapse of the cloud, bumping up star formation efficiency by about 50%.

The authors note that while these simulations provide exciting insights, more research is needed to fully understand the diversity of ionization rates seen in molecular clouds and how cosmic rays might influence star formation in extreme environments. This work shows just how intertwined cosmic rays are with the birth of new stars in our universe.