This illustration offers a plausible scenario for how vagabond stars exploded as supernovae outside the cozy confines of galaxies. Panel 1: A pair of black holes comes together during a galaxy merger, dragging with them up to a million stars each. Panel 2: A double-star system wanders too close to the two black holes. Panel 3: The black holes then gravitationally catapult the stars out of the galaxy. At the same time, the stars are brought closer together. Panel 4: After getting booted out of the galaxy, the binary stars move even closer together as orbital energy is carried away from the duo in the form of gravitational waves. Panel 5: Eventually, the stars get close enough that one of them is ripped apart by tidal forces. Panel 6: As material from the dead star is quickly dumped onto the surviving star, a supernova occurs. Credit: NASA, ESA, and P. Jeffries and A. Feild (STScI)
Several exploding stars have been found outside the cozy confines of galaxies, where most stars reside. These wayward supernovae are also weird because they exploded billions of years before their predicted detonations. Astronomers have developed a theory for where these doomed stars come from and how they arrived at their current homes.
Ryan Foley thought the doomed stars had migrated to their final resting spots. He studied data from Lick Observatory, CA and WM Keck Observatory & Subaru Telescope, HI to determine how fast the stars were traveling. To his surprise, the doomed stars were zipping along at about the same speed as stars that have been tossed out of our Milky Way galaxy by its central supermassive black hole, at more than 5M miles/hr. He then looked at aging galaxies in the area of the speeding supernovae. Studying Hubble archival images, he confirmed many are massive elliptical galaxies that were merging or had recently merged with other galaxies. The lanes are the shredded remnants of a cannibalized galaxy.The location of the supernovae in relation to ancient galaxies indicates the original stars must have been old, too. And if the stars were old, then they must have had companions with them that provided enough material to trigger a supernova blast.
How does a double-star system escape the boundaries of a galaxy? Foley hypothesizes a pair of supermassive black holes in the merging galaxies can provide the gravitational slingshot to rocket the binary stars into intergalactic space. Hubble reveals nearly every galaxy has a massive black hole at its center. After 2 galaxies merge, their black holes migrate to the center of the new galaxy, each with a trailing a cluster of stars. As the black holes dance around each other, slowly getting closer, one of the binary stars in the black holes’ entourage may wander too close to the other black hole. Many of these stars will be flung far away, and those ejected stars in surviving binary systems will orbit even closer after the encounter, which speeds up the merger.
“With a single black hole, occasionally a star will wander too close to it and have an extreme interaction,” Foley said. “With two black holes, there are two reservoirs of stars being dragged close to another black hole. This dramatically increases the likelihood that a star is ejected.” While the black hole at the center of the Milky Way may eject about 1 stara century, a binary supermassive black hole may kick out 100 stars a year.
After getting booted out of the galaxy, the binary stars move closer together as their orbits continue to accelerate, which speeds up the binary stars’ aging process. The binary stars are likely both white dwarfs, which are the burned out relics of stars. Eventually, the white dwarfs get close enough that one is ripped apart by tidal forces. As material from the dead star is quickly dumped onto the surviving star, an explosion occurs, causing the supernova. The time it takes for one of these ejected stars to explode is relatively short, about 50 million years. Normally, these kinds of binary stars take a long time to merge, probably much longer than the age of the universe, which is more than 13 billion years.
Some mysteries remain unsolved, such as why they are unusually weak explosions. These supernovae produced more than 5X as much calcium as other stellar explosions. Normally, supernova explosions have enough energy to create much heavier elements, such as iron and nickel, at the expense of producing the lighter calcium. However, for these atypical explosions, the fusion chain stops midway, leaving lots of calcium and very little iron.
http://hubblesite.org/newscenter/archive/releases/2015/28/full/
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