Unusually high-speed stars that comprise around 20 percent of all massive stars in our Milky Way may form due to strong gravitational interactions inside dense, fairly low-mass star clusters.
According to a study published online in Science on Nov. 17, most of these OB runaways are formed by three-body encounters, as opposed to the standard two-body hypothesis based on supernova explosions in binary systems.
The designation OB refers to the classification of their absorption spectra with O stars informally being called “blue” and B stars “blue-white” due to their high surface temperatures, sometimes up to 50,000 °C.
Two astronomers used observations of stars and numerical simulations to reveal that dynamic interactions occur between single stars and binary pairs in the majority of runaway star formations.
The researchers noted that stars with masses 100 times or more than the sun are found around young star clusters like R136 and Westerlund 2.
OB runaways have velocities of 30 kilometers per second more due to one of two mechanisms. Either such stars are launched when their binary companion explodes in a supernova, or by a dynamical slingshot.
Typically the least massive star escapes, meaning that both stars in the ejecting binary tend to be more massive. The system that best produces massive runaways is relatively wide, comprising the most massive stars in a cluster at its core.
The gravothermal collapse of a cluster core naturally produces such a binary, which subsequently condenses by ejecting stars until it undergoes a collision or reaches a critical binding energy whereupon an encounter causes it to be ejected from the cluster.
“The majority of the galactic OB runaways seem to originate from star clusters that experienced core collapse within the first 1 million years of their existence,” concluded the researchers in their report.
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