A windy day in the Milky Way

Turbulence, or chaotic changes in the pressure and velocity of a fluid, is one of the great mysteries of classical physics. Much of the gas in galaxies is known to be turbulent, but the mechanisms that developed and maintain this turbulence remain areas of active research. While we still don’t know all the details of the physics behind turbulence, a lot of time and effort has gone into identifying statistics that can tell us whether gas is turbulent or not. In other words, we know what turbulence looks like even if we don’t know all the details of how it works. Today’s weather forecast calls for strong winds blowing in from the arXiv as we explore a new paper studying how stellar winds from star clusters can drive such turbulence.

Stellar winds, particularly those from massive stars like O or B types, blow bubbles in the surrounding cold gas by pushing it outwards and leaving a cavity behind. These are analogous to the bubbles we see on Earth that are created by air pushing into some other medium. In the case of a stellar-wind bubble, the “air” is hot stellar wind material. When massive stars are found in a star cluster, their bubbles tend to overlap and form a “superbubble”. One incredible example of this is the Orion Nebula Cluster. The authors of today’s paper run simulations that roughly mimic the stellar profile of the Orion Nebula Cluster, and they too find the creation of large superbubble.

In these simulations, the most massive stars expel high-velocity, hot gas that fills the superbubble and pushes it outwards into cooler gas. This expansion produces a thick shell at an intermediate temperature. Because this shell is more dense than the central hot gas, it is able to cool faster and remain much cooler than the superbubble interior. As the simulations progress, turbulent instabilities appear in the hot gas inside the shell.

Plots of the expanding superbubble created by winds from massive stars.
CREDITS: Gallegos-Garcia et al. 2020

Modeling turbulence is crucial to understanding many processes in galaxy evolution, such as star formation. Through simulations like these, astronomers can get a better idea of exactly why gas in galaxies behaves the way it does and how it can form new stars, solar systems, and even us.

Source: “A Windy Day in the Milky Way” AASNOVA, 15 September 2020

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