Astronomers have found an explanation for the strange occurrence of massive stars located far from their birthplace in the disk of our Milky Way Galaxy: the secret seems to lie in the merging of medium-mass stars, specifically HD 93521, a Population I massive O-type star 3,000 light-years above the galactic plane that was ejected on a 39-million year journey either through dynamical interactions or a result of a supernova in a binary system.
Among the brightest objects in our galaxy
Stars more massive than the Sun have very hot cores that drive nuclear energy generation at very high rates. They are among the brightest objects in our galaxy. But because they burn through their hydrogen fuel so quickly, their lifetimes are relatively short, perhaps 10 million years compared to 10 billion years for the Sun, which means that there is little time for them to stray too far from their birthplace. Most massive stars are found in the flat disk part of our galaxy, where gas clouds are dense enough to promote star birth and where astronomers find young clusters of massive stars. Specifically, 80%- 90% of massive stars are located in their birth clusters or within a few hundred light years of the midplane of our Milky Way’s disk.
A topic of active debate
“Astronomers are finding massive stars far away from their place of origin, so far, in fact, that it takes longer than the star’s lifetime to get there,” said Georgia State astronomer Douglas Gies. “How this could happen is a topic of active debate among scientists.”
The massive star known as HD93521 that lies about 3,600 light years above the galaxy’s disk is a poster child for this phenomenon. A new study by Gies and other astronomers from Georgia State reveals a profound discrepancy: the flight time to reach this location far exceeds the predicted age of this massive star.
Gaia spacecraft data
The astronomers used a new distance estimate from the European Space Agency’s Gaia spacecraft together with an investigation of the star’s spectrum to determine the star’s mass and age as well as its motion through space. They find that HD93521 has a mass about 17 times larger than the Sun’s, and this leads to a predicted age of about 5 million years. On the other hand, the motion of the star indicates that its journey from the disk has taken much longer, about 39 million years.
A binary mystery?
The Georgia State astronomers explain this strange difference between the star’s lifetime and travel time by suggesting that HD93521 left the disk as two lower-mass and longer-lived stars, rather than the single massive star we see today.
One of the fastest rotating stars in the galaxy
The clue to the mystery is that HD93521 is one of the fastest rotating stars in the galaxy. Stars can spin up through stellar mergers where two close orbiting stars can grow over time and collide to form one star. “HD93521 probably began life as a close pair of medium-mass stars that were fated to engulf each other and create the single, fast-spinning star we see today,” Gies said. Such intermediate mass stars live long enough to match the long flight time of HD93521.
Binary star mergers a common occurrence
Binary star mergers are actually a common occurrence in our Galaxy. About 40% of intermediate-mass stars will interact with a stellar companion, and a sizable fraction of those will merge into a single star.
HD93521 is not the only case of a massive star found so far away from its birthplace. Georgia State graduate student Peter Wysocki is investigating an example of a distant massive binary pair that is probably representative of the stage just before a merger. This star is known as IT Librae, and it has an orientation that creates mutual eclipses as the two stars pass in front of each other. An investigation of the variations in the light output and motions detected in the spectra leads to estimates of the stellar masses.
Higher-mass star is actually older than it appears
Wysocki finds a similar conundrum from the mass results — the predicted age is much less than IT Librae’s travel time from the disk. But the study also reveals that the lower-mass star in the pair has already begun to transfer much of its mass to the higher-mass star, initiating the process that may eventually lead to a merger. This means that the higher-mass star is actually older than it appears, having begun life as a lower-mass star.
These distant massive stars provide striking evidence that close pairs of stars can merge to make even larger stars, Gies said, and they are key clues about how rapidly rotating massive stars are able to create black holes with large spins.
Reference: “The Transformative Journey of HD 93521” by Douglas R. Gies, Katherine Shepard, Peter Wysocki and Robert Klement, 31 January 2022, The Astronomical Journal.
The infrared image at the top of the page, which is 3.2 light-years across, shows the central region of the massive cluster of stars at the Milky Way’s core, about 27,000 light-years from Earth (ESO, Stefan Gillessen et al).
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Georgia State University