Study Shows How Massive Stars Become Isolated in the Universe

Two studies performed at the University of Michigan have shown how certain giant stars—that is, stars whose mass is eight or more times that of the Sun—become separated in the universe: generally, the star clusters of these massive stars push them out.

Massive star cluster called Westerlund 1. Image Credit: ESA/Hubble and NASA.

Giant stars often exist in clusters. Separated massive stars are known as field massive stars. The two articles, published by students from the University of Michigan, assessed a majority of these stars in a dwarf galaxy, called the Small Magellanic Cloud, which is close to the Milky Way.

Both studies, which appeared in the same issue of The Astrophysical Journal, have shown the evolution of these field massive stars, or how they become extremely isolated.

Interpreting the way field massive stars become separated—whether they become separated by being expelled from a cluster of stars or whether they develop in isolation—will allow astronomers to study the conditions in which giant stars are developed.

Figuring out this phenomenon and the formation of clusters is crucial for interpreting the evolution of galaxies.

About a quarter of all massive stars appear to be isolated, and that’s our big question,” stated Johnny Dorigo Jones, a recent undergraduate. “How they’re found to be isolated, and how they got there.”

Jones has described in his article that the huge majority of field massive stars are actually “runaways,” or stars that are expelled from clusters. Irene Vargas-Salazar, a graduate student, searched for field massive stars that could have developed in relative isolation by looking for proof of small clusters around them. This implies that such comparatively isolated stars could have developed along with these smaller stars. However, Vargas-Salazar found only a few numbers of these faint clusters.

Because massive stars require a lot of material to form, there are usually a lot of smaller stars around them. My project asks specifically how many of these field massive stars could have formed in the field.

Irene Vargas-Salazar, Graduate Student, University of Michigan

Jones studied how clusters eject the field massive stars. He examined the two varied mechanisms that create runaways—binary supernova ejection and dynamical ejection.

In the first runaway, a giant star is expelled when a binary pair has a single star that bursts and shoots out its companion into space. In the second runaway, the giant stars are expelled from their clusters—by up to half a million miles for each hour—due to unstable orbital configurations of stellar groups.

By having the velocities and the masses of our stars, we’re able to compare the distributions of those parameters to the model predictions to determine the certain contributions from each of the ejection mechanisms,” added Jones.

Jones also found that dynamical ejections—that is, ejections induced by unstable orbital configurations—were around two to three times more abundant when compared to supernova ejections. However, Jones also found the initial observational data that revealed that a large part of the field massive stars emerged from a combination of both supernova and dynamical ejections.

These have been studied in the past but we have now set the first observational constraints on the numbers of these two-step runaways. The way we reach that conclusion is we’re essentially seeing that the stars that trace the supernova ejections in our sample are a bit too numerous and too fast compared to the model predictions. You can imagine this being remedied by these stars being reaccelerated upon a supernova kick, having first been dynamically ejected.

Johnny Dorigo Jones, Undergraduate, University of Michigan

The investigators found that probably around 50% of the stars, which were initially believed to be from supernova ejections, were initially dynamically ejected.

The findings made by Vargas-Salazar also support the concept that a majority of field massive stars are runaways; however, she also studied the opposite conditions: she searched for field massive stars that developed in relative isolation in minute clusters of smaller stars, where the giant target star, known as the “tip of the iceberg, or TIB, clusters.

Vargas-Salazar achieved this feat by using a pair of algorithms— “nearest neighbors” and “friends-of-friends”—to look for those clusters surrounding 310 field massive stars existing in the Small Magellanic Cloud.

To quantify the number density of the stars, the “friends-of-friends” algorithm counts the number of stars located at a particular distance from the target star and, in turn, does the same for those stars. If the stars are tightly packed, they are more likely to be a cluster.

The “nearest neighbors” algorithm quantifies the number density of stars present between the target star and its closest 20 companions. If the group is more compact and denser, they are more likely to be clusters, added Vargas-Salazar.

Vargas-Salazar used statistical tests and subsequently evaluated those observations against three random-field datasets and also evaluated the familiar runaway massive stars against nonrunaways.

She observed that just a handful of the field massive stars seemed to have TIB clusters around them, indicating that very few stars actually developed in the field. It is believed that the balance of the field massive stars have evolved as runaways.

In the end, we showed that 5% or less of the stars had TIB clusters. Instead, our findings imply that the majority of stars in field samples could be runaways. Our findings are actually supporting the result that Johnny found, wrapped in a neat little bow.

Irene Vargas-Salazar, Graduate Student, University of Michigan

The findings made by Vargas-Salazar gives a partial answer to the question about the formation of massive stars, stated Sally Oey, the senior author of both articles, and a professor of astronomy at the University of Michigan.

Johnny and Irene’s work are flip sides of the same coin,” added Oey. “Irene’s numbers are consistent with Johnny’s in that the vast majority of field massive stars are runaways, but that a few are not. This is a critical finding for understanding how massive stars and clusters form, and in what conditions.”

The work performed by Dorigo Jones and Vargas-Salazar was funded by the National Science Foundation.

Journal Reference:

Jones, J. D., et al. (2020) Runaway OB Stars in the Small Magellanic Cloud: Dynamical versus Supernova Ejections. The Astrophysical Journal.


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