Most isolated massive stars in empty space, eight or more times the mass of our sun, are ejected by the activity of their own star clusters.
Massive stars normally reside in clusters. Isolated massive stars are called massive field stars. Two papers published by students at the University of Michigan examined most of these stars in the Small Magellanic Cloud, a dwarf galaxy near the Milky Way.
The studies, which appear in the same issue of The Astrophysical Journal, reveal how these massive field stars originate or become so isolated. Understanding how massive field stars are isolated, whether they form in isolation or become isolated by being ejected from a star cluster, will help astronomers investigate the conditions under which massive stars form. Understanding this and the formation of clusters is critical to understanding how galaxies evolve.
"About a quarter of all massive stars appear to be isolated, and that's our big question," Johnny Dorigo Jones, a recent college student, said in a statement. "How are they isolated and how did they get there?"
Dorigo Jones shows in his article that the vast majority of massive field stars are "fugitives" or stars ejected from clusters. Graduate student Irene Vargas-Salazar searched for massive field stars that may have formed in relative isolation by looking for evidence of small clusters around them. That means these relatively isolated stars could have formed together with these smaller stars. But he found very few of these weak groups.
"Because massive stars require a large amount of material to form, there are generally many smaller stars around them," Vargas-Salazar said. "My project specifically asks how many of these massive field stars could have formed in the field."
Dorigo Jones examined how massive field stars are ejected from clusters. Look at the two different mechanisms that produce leaks: dynamic ejection and binary supernova ejection. In the first, massive stars are ejected from their clusters, up to half a million miles per hour, due to the unstable orbital configurations of the stellar groups. In the second, a massive star is ejected when a binary pair has a star that explodes and launches its companion into space.
"By having the speeds and masses of our stars, we can compare the distributions of those parameters with the predictions of the model to determine certain contributions of each of the ejection mechanisms, "said Dorigo Jones.
He found that dynamic ejections (ejections caused by unstable orbital configurations) were about 2 to 3 times more numerous than supernova ejections. But Dorigo Jones also found the first observational data showing that a large fraction of the massive star field comes from a combination of dynamic and supernova ejections.
"These have been studied in the past, but now we have established the first observation restrictions on the number of these two-step fugitives, "he said."The way we came to that conclusion is that, essentially, we are seeing that the stars that track supernova ejections in our sample are too numerous and too fast compared to the model's predictions. "
The researchers found that potentially up to half of the stars initially thought to be from supernova ejecta they were first dynamically expelled.
Vargas-Salazar's findings also support the idea that most massive field stars are fugitives, but she observed opposite conditions: she looked for massive field stars that formed in relative isolation in small clusters of smaller stars, where it is the massive target star, called the "tip of the iceberg, or TIB clusters. It did so using two algorithms, "friends of friends" and "closest neighbors", to search for those clusters around 310 massive field stars in the Small Magellanic Cloud.
The "friends of friends" algorithm measures the number density of stars by counting how many stars are at a specific distance from the target star and then doing the same for those stars in turn. The more compact the stars are, the more likely they are to be a cluster. The "nearest neighbors" algorithm measures the numerical density of stars between the target star and its 20 closest companions. The more compact and dense the group, the more likely they are to be groupings, Vargas-Salazar said.
Using statistical tests, Vargas-Salazar compared these observations with three random field data sets and compared known fugitive massive stars with non-fugitive ones. Discovered that only a few massive field stars appeared to have TIB clusters around them, which suggests that very few were actually trained in the field. The equilibrium of the field stars must have originated as fugitives.
"In the end, we showed that 5% or less of the stars had TIB clusters. Instead, our findings imply that most of the stars in the field samples could be out of control, "Vargas-Salazar said.