Star-Disk Collisions Most Likely to Cause the 'Disappearance' of Red Giants in the Milky Way

Astrophysicists from the Georgia Institute of Technology have devised a new computer simulation showing that repeated collision with an accretion disk at the galactic center stripped the mass of red giant stars million years ago, making them dim and more difficult to detect.

Red giant stars are stars that are more than a billion years old and are ten times larger than the sun.

Their findings, published in The Astrophysical Journal, suggest that each of the red giant stars has orbited its way into and through the disk a dozen times, completing a single pass-through after days or weeks. And during its orbit, the mass of the red giant stars were stripped away every time it collides with high density clumps. This process is believed to occur 4 to 8 million years ago.

For their simulation, the researchers put models of red giant stars through three-dimensional hydrodynamic simulations to replicate its collision with the gaseous disk that once occupied much of the space within .5 parsecs of the galactic center. The researchers also use varying orbital velocities and disk's density to determine the necessary conditions to cause a significant damage to the giant red stars.

The researchers discovered that red giant stars can only be stripped of their mass if the disk very massive, at least 100 to 1,000 times more mass than the all the young stars that eventually formed from it. The disks also need to be very dense, enough to have the gravity fragment the disk on its own, in order for the red giant star to lose significant portion of its mass.

Their simulation also revealed that impacts from the collision were likely to reduce the kinetic energy of the red giant stars by 20 to 30 percent, causing their orbits to shrink and to pull them closer to the Milky Way's black hole.

According to the researchers, understanding the conditions leading to the "disappearance" of red giant stars might provide more information about the origins of our galaxy.

In a press release, Tamara Bogdanovic, an assistant professor at Georgia Tech, noted that that there is still no concrete evidence showing very large amount of gas in the galactic center. However, if such gas is detected, researchers will most likely observe under-luminous red giants spinning more rapidly in smaller orbits. The detection of dimmed red giants could provide direct support to the star-disk collission, potentially opening new doors in the study of Milky Way's origin.