Where Are The Milky Way’s Missing Red Giants?

As the star pummels the disk, it develops a bow shock ahead of the star and a cometary tail behind it. The tail is essentially the gas that is stripped from the star as it passes through and loses a portion of its overall mass.

Credit: Georgia Tech

“Red giants could have lost a significant portion of their mass only if the disk was very massive and dense,” said Tamara Bogdanovic, the Georgia Tech 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.”

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 former Georgia Tech undergraduate student Thomas Forrest Kieffer, the first author on the paper, 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.

A sequence of snap shots based on a simulation of a red giant entering and exiting a clump in a fragmenting accretion disk. In this case, it takes four days for the star to travel through the clump (each .1 unit of time is approximately one hour).

Credit: Georgia Tech

“The only way for this scenario to take place within that relatively short time frame,” Kieffer said, “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.

“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 said. “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.”

 

 

Contacts and sources:

Jason Maderer

Georgia Institute of Technology

Citation: “Can Star-Disk Collisions Explain the Missing Red Giants Problem in the Galactic Center,” was published on June 1.

This research was funded, in part, by the Alfred P. Sloan Foundation (grant # BR2013-016) and the National Science Foundation (AST-1333360). 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.