Authors: Michael Y. Grudić, Dávid Guszejnov, Stella SR Offner, Anna L. Rosen, Aman N. Raju, Claude-André Faucher-Giguère, and Philip F. Hopkins
Institution of first author: Carnegie Observatories, Pasadena, CA
Status: Submitted to MNRAS [open access]
Simulating the formation of a star cluster is like following a very complicated soup recipe. Sure, it starts with the basic ingredients – a gas cloud and some heaviness to bring it down, but you can’t just leave it at that. You need to mix it a little, stir in a little turbulence with a healthy spoonful of magnetic fields. Then you’ll need some freshly harvested gas chemistry models to help with heating and cooling, and don’t forget to add them little by little Stellar winds, outflows and radiation along the way. Maybe even add a supernova here or there. And so it continues, adding more and more difficult-to-model ingredients to achieve a final product that meets the requirements. Until recently, many of these ingredients were far from being household items and would be extremely difficult, if not impossible, to ingest. However, modern simulations have incredibly well-stocked pantries. Most modern models have gotten to a point where, while not a perfect replica of clustering, they make a good soup.
Today’s newspaper introduces the latest generation of the STARFORGE simulations, called “The anvil of creation“. This simulation replicates the collapse of a giant molecular cloud into a star cluster (the progression from molecular cloud to star cluster is shown in the movie below) and includes an extremely thorough treatment of “excellent feedback‘ (which describes how young stars shed mass and energy back into the surrounding interstellar medium). This leads to one of the most realistic star cluster formation models ever computed. But what exactly do we mean by “realistic”? The complex interweaving of different physics (magnetic fields, gravity, hydrodynamic turbulence, stellar feedback, etc.) cannot be perfectly reproduced, so star formation simulations must find the right corners to cut. The goal is to simplify the system while preserving the measurable characteristics of real star-forming regions, such as star-forming efficiency (the fraction of gas converted into stars) and the distribution of stellar masses produced (the much-debated Initial mass function). Simulations often succeed in reproducing some of these features but remain inconsistent with others. Today’s newspaper reports that the star cluster in the “Anvil of Creation” shows no major, glaring contradictions to observed trends in star formation – a major victory for this type of simulation.
A sinking feeling
stars are small. Perhaps not SMALL, but compared to the gas clouds from which they are born, it is often best to treat them as infinitesimally small point particles in the simulation. These are often referred to as sink particles. The way these sink particles form (like protostars), affecting their environment in the simulation, evolving with age, and possibly dying as such supernovae is handled by carefully designed “feedback” models that summarize the net effect of all physics occurring at scales smaller than the simulation’s finest resolution. The authors of today’s article implement a very thorough set of these models that govern the behavior of these sink particles. They encompass a wide range of these feedback mechanisms, from bipolar outflows (pillars of high-velocity gas shot out by protostars as they accumulate mass) to ionizing radiation and strong winds generated by massive stars. The authors note that the bipolar drains play the largest role in reinvigorating the cloud (at least early in the simulation). These outflows appear to be crucial in determining the final distribution of stellar masses, which ends up being very close to the observed distribution of true star clusters (see Figure 1). Radiation, winds and supernovae from the most massive stars are responsible for displacing and destroying the remaining gas towards the end of the simulation. It is the combined action of all these mechanisms during the cloud’s intricate evolution that reproduces a “realistic” event of star cluster formation.
The formation of the star cluster takes about 8 million years in the simulation. During this time, the authors observe an accelerated collapse, the start of star formation, and then the evacuation of gas through stellar feedback (see Figure 1). A somewhat surprising finding is how slowly the most massive stars form. Instead of rapidly collapsing from the densest pockets of gas in the cloud, massive stars appear to accumulate their mass slowly over millions of years. Since we can’t sit around and observe the formation of real stars over millions of years, this result could inform how we interpret the effectively static still images we observe of real nearby star-forming regions. The assumptions we make about the age distribution of stars have far-reaching implications for how we interpret the light from unresolved young stellar populations, so a thorough theoretical understanding of how and when different types of stars form is crucial to studying more distant ones galaxies .
So where to go from here? There is a wealth of nuance to explore using detailed and internally consistent simulations such as The Anvil of Creation, including the formation of protoplanetary disks and the effects of various molecular cloud environments and initial conditions. The future of star formation simulations looks bright, emerging through the rapidly disappearing veil of theoretical and computational challenges.
Astrobite edited by Laila Linke
Featured Photo Credit: A still image from the simulation fly-around film released with today’s newspaper, available at http://www.starforge.space/M2e4_fullphysics_flyaround.mp4
About H Perry Hatchfield
I am a PhD student in physics at the University of Connecticut, where I study star formation and gas structure in the galactic center of the Milky Way. To do this, I use radio observations of molecular clouds and hydrodynamic simulations, and my aim is to find ways to compare these two exciting ways of exploring the universe.