Where have all the red giants gone?
Computer simulationscarried out by researchers from the Georgia Institute of Technologymay have discovered why older stars are suspiciously absent from thecentral region of our galaxy. Present day observations of thegalactic center show it to be populated almost entirely by bright,young stars that are estimated to be only a few million years old,highlighting a startling lack of stellar diversity near the core ofthe Milky Way.
A theory on thedisappearance of the geriatric stars introduced in 2014 suggestedthat the red giants may have been stripped of 10s of percent of theirmass and luminosity as they passed through the plane of a massive,incredibly dense accretion disk.
The young starscurrently inhabiting the region surrounding the galactic center wouldhave been born of the disk, which was thought to stretch around 0.5parsecs (1.6 light-years) fromthe supermassive black hole at the heart of our galaxy known asSagittarius A* (Sgr A*). The disk is thought to have been so densethat it fractured under its own gravity, forming clumps that wouldact as the kick needed to create the next generation of stars.
The new study is thefirst to use detailed computer simulations to test the accretion disktheory. The team created detailed computer analogues of billion yearold red dwarf stars, and ran a series of tests akin to putting thegeriatric bodies through a cosmic wind tunnel as theysimulated the effects of repeated interactions with the disk – see the video below.
The velocity of thestars and density of the disk were varied to allow the team todetermine the conditions needed to diminish the red giants. It wasdiscovered that in order to account for the mass loss proposed in thetheory, that 4 – 8 million years ago the red giants would haveneeded to pass through the disk dozens of times, with each passtaking anywhere between a few days to a number of weeks.
As they passed throughthe disk, the stars would have created a bow wave, and left in theirwake a gaseous tail of discarded matter. The end result of theinteraction would be a population of under-luminous red giant stars,which, having lost 20 – 30 percent of their kinetic energy thanksto the drag of carving through the dense accretion disk, inhabit arelatively close orbit with Sgr A*.
It is estimated thatthe disk must have boasted an overall mass between 100 – 1,000times the mass of the young stars currently inhabiting the galacticcentre. It is possible that a small population of the red giants arestill just above the detection threshold of present day telescopes,providing the scope for direct observation, and corroboration of thetheory.
Source: Georgia Institute of Technology