An international team of astronomers has recreated the orbital history of an enormous star that is currently barreling away from our galaxy at a phenomenal rate, having been ejected from one of the Milky Way's spiral arms. The discovery challenges the popular theory that escaping stars originate from the galactic center, and could hint at the presence of an undiscovered black hole.
Our Milky Way shines with the light of billions of stellar bodies of all shapes and sizes. Most of these stars travel around the center of our galaxy in sweeping spiral arms, bound together by gravitational force.
Back in 2005, astronomers discovered a number of stars that seemed to be moving fast enough to overcome the Milky Way's gravity, and shoot off into the void between galaxies. In order to break free, these stars had to be travelling at roughly 310 miles per second (500 km per second) - roughly twice the speed of a normal star orbiting within the Milky Way.
The stellar speedsters, which were fittingly named hypervelocity stars, are thought to be relatively rare, with only 30 discovered to date. It would take an extreme gravitational event to fling a star outward with enough momentum to break free of the Milky Way. Enter the cosmic heavyweight lurking at the heart of our galaxy – the supermassive black hole Sagittarius A* (Sgr A*).
It had previously been thought that the majority of hypervelocity stars once orbited near the galactic center as part of a binary system comprised of two stellar bodies. If such a system passed too close to Sgr A*, it is possible that the titanic gravitational influence of the singularity would capture one star, and give the other a powerful gravity boost, effectively catapulting it out of the galaxy. Theoretical models also show that stars could be ejected from the galaxy through arguably less dramatic interactions with multiple massive stellar bodies.
A newly-published study has provided evidence to show that runaway stars may not originate solely from the galactic center, but also from the spiral arms surrounding it, and that their presence could hint at the presence of hidden black holes.
The international team behind the new discovery focused their attention on LAMOST-HVS1 – the closest hypervelocity star to our solar system. The stellar giant, which has a mass the equivalent to 8.3 Suns, was initially observed by one of the Magellan telescopes located at the Las Campanas Observatory in Chile, to discern its distance and speed.
These observations were supplemented with information collected by ESA's Gaia telescope, which is currently engaged in creating the most accurate three-dimensional map of the Milky Way to date. With this information the team was able to reconstruct LAMOST-HVS1's orbital trajectory, and in so doing divine its point of origin.
It was discovered that LAMOST-HVS1 was created outside of the galactic center, in the Milky Way's Norma spiral arm. Since such an environment does not play host to a supermassive black hole, the team had to consider other extreme gravitational events that could have led to so massive a star being ejected from our galaxy.
It is possible that the star could have come into contact with a smaller, intermediate black hole embedded in a stellar cluster, or that its trajectory was influenced by the gravity of several huge stars.
Models that account for a hypervelocity ejection event due to the gravity of multiple massive stars struggle to account for the speed of LAMOST-HVS1. It is more likely that the speeding heavenly body was cast out of the Milky Way by the gravity of a black hole lurking within a star cluster.
The team note that the Norma spiral arm is not currently known to host any massive star clusters in which an intermediate black hole might dwell. However, the cluster could simply be shielded from sight behind a dense cloud of cosmic dust.
The discovery of such a cluster could represent the first opportunity to discover an intermediate black hole embedded in the stellar disk populating the Milky Way's spiral arms. It would also provide valuable insight regarding the formation of these singularities within the stellar beehives, and the dynamic interactions that follow thereafter.
The research paper has been published in the Astrophysical Journal.
Source: The University of Michigan
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