Authors: Murray-Clay, R. A. and Loeb, A.
Institution: Harvard-Smithsonian Center for Astrophysics (CfA)
If our solar system lives in suburbia, the center of our galaxy is a sprawling metropolis shining bright for all to see. The center of our Milky Way Galaxy is a crowded, bustling and hectic place. Stars race around like cars on a freeway. Densely-packed hot stars and supernova explosions flood the region with deadly radiation. The supermassive black hole at the center destroys anything that dares to wander too close and test its strength. The galactic center is different than what we’re used to. It’s exciting. It’s dangerous. It’s the kind of place that’s fun to visit, but you wouldn’t want to raise your kids there. The traditional wisdom among astronomers is that stars feel the same way; There is just too much excitement going on in the galactic center for planets to form around stars…Or is there?
Last year, a team of astronomers at the Very Large Telescope (VLT) in Chile discovered a cloud of gas falling towards the black hole at the center, Sagittarius-A* (SgrA*). The team hypothesized that the gas cloud was the result of a collision between two gas clouds streaming from nearby stars. New research from the CfA proposes the seemingly-unlikely explanation the gas cloud is a proto-planetary disk surrounding a star that is too faint to see. A proto-planetary disk is a cloud of gas and dust that orbits a star for millions of years while it slowly coalesces into planets and asteroids and comets; It is where planets are born. Murray-Clay’s team chose to examine how a proto-planetary disk would behave in such an extreme environment with the hope that predictions would match observations. Ultimately, they did, revealing that planetary systems may form in places we never thought they could. Here’s how:
The Stellar Habitat
There is a ring of young stars that orbit SgrA*, the inner edge of which lies approximately 8,400 AU from the black hole. The cloud’s trajectory passed through this region, supporting the idea that a rogue star from the ring is carrying with it the cloud in question. But what are the odds that a runaway star would have a proto-planetary disk? The ring structure itself has been estimated to be somewhere between 4 and 8 million years old based on stellar populations. Low-mass stars can retain their disks as long as 5 million years, so there are likely stars in the ring that do indeed have their proto-planetary disks, assuming the disks are capable of remaining intact in the harsh environment. The star discussed in this paper is an unlucky one that likely had a close encounter with another star, altering its orbit and steering it towards the black hole.
Proto-planetary disk or just a boring cloud of gas?
Although some stars in the ring are of the appropriate age to harbor proto-planetary disks, the question of whether they actually have disks still remains. After all, their environment is extreme. Intense stellar winds from nearby stars and supernovae in addition to gravitational tidal forces from the black hole could have a major impact on the structure and evolution of disks, perhaps to the extent that they prevent its formation altogether.
Murray-Clay and her team determined that a proto-planetary disk would be exposed to a substantially large flux of high-energy photons. The radiation would have the effect of heating the disk up to 10,000 K, causing a substantial amount of the ionized gas to “blow away” through the process of photoevaporation. This would prevent the disk from having a radius larger than 100 AU. At its current distance from the black hole, the disk would be distorted by strong gravitational forces, but still able to retain its structure closer to the host star. As the disk moved closer to the black hole, the distortion would affect a greater portion of the structure until it breaks apart completely.
The observed properties of the gas cloud are consistent with these theoretical predictions, namely the temperature of 10,000 K and the cloud radius of 100 AU; These results are not definitive but Murray-Clay et al have shown that indeed there truly could be planets forming near the black hole, and they could look just like that gas cloud does. Inside of that intense, busy maelstrom that is the galactic center, there are pockets of serenity where, surprisingly, planets can form. After all, there are plenty of people who do successfully raise families in the big city.
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