Where are the Milky Way’s missing red giants?

Why does the center of the Milky Way appear to be full of young stars but very few old ones? New computer simulations test the theory that the remnants of older, red giant stars are still there—they just aren’t bright enough to be detected with telescopes.

The simulations investigate the possibility that these red giants were dimmed after they were stripped of 10s of percent of their mass millions of years ago during repeated collisions with an accretion disk at the galactic center.

The very existence of the young stars, seen in astronomical observations today, is an indication that such a gaseous accretion disk was present in the galactic center because the young stars are thought to have formed from it as recently as a few million years ago.

The study appears in the Astrophysical Journal. It is the first to run computer simulations on the theory, which was introduced in 2014.

Astrophysicists from Georgia Tech created models of red giants similar to those that are supposedly missing from the galactic center—stars that are more than a billion years old and 10s of times larger in size than the sun. They put them through a computerized version of a wind tunnel to simulate collisions with the gaseous disk that once occupied much of the space within .5 parsecs of the galactic center. They varied orbital velocities and the disk’s density to find the conditions required to cause significant damage to the red giant stars.

“Red giants could have lost a significant portion of their mass only if the disk was very massive and dense,” says Tamara Bogdanovic, the assistant professor who co-led the study. “So dense, that gravity would have already fragmented the disk on its own, helping to form massive clumps that became the building blocks of a new generation of stars.”

These stars are so fast they can escape the Milky Way

The simulations suggest that each of the red giant stars orbited its way into and through the disk as many as dozens of times, sometimes taking as long as days to weeks to complete a single pass-through. Mass was stripped away with each collision as the star blistered the fragmenting disk’s surface.

According to Thomas Forrest Kieffer, the first author on the paper and former Georgia Tech undergraduate, it’s a process that would have taken place 4 to 8 million years ago, which is the same age as the young stars seen in the center of the Milky Way today.

“The only way for this scenario to take place within that relatively short time frame,” Kieffer says, “was if, back then, the disk that fragmented had a much larger mass than all the young stars that eventually formed from it—at least 100 to 1,000 times more mass.”

The impacts also likely lowered the kinetic energy of the red giant stars by at least 20 to 30 percent, shrinking their orbits and pulling them closer to the Milky Way’s black hole. At the same time, the collisions may have torqued the surface and spun up the red giants, which are otherwise known to rotate relatively slowly in isolation.

From lumpy images, a view of the Milky Way’s black hole

“We don’t know very much about the conditions that led to the most recent episode of star formation in the galactic center or whether this region of the galaxy could have contained so much gas,” Bogdanovic says. “If it did, we expect that it would presently house under-luminous red giants with smaller orbits, spinning more rapidly than expected. If such population of red giants is observed, among a small number that are still above the detection threshold, it would provide direct support for the star-disk collision hypothesis and allow us to learn more about the origins of the Milky Way.”

Partial funding came from the Alfred P. Sloan Foundation and the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

Source: Georgia Tech

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