Recently, an exciting discovery has been made using data from ESA's Gaia survey in June. Previous to this study, only ten stars were known to be on trajectories that would enable them to escape the gravitational pull of our Milky Way galaxy. These stars were propelled away by the immense power of supernova explosions. However, the new research has unveiled an additional six runaway stars, two of which have shattered the previous record for the fastest radial velocity ever observed in a runaway star. These stars are hurtling through space at mind-boggling speeds of 1,694 km/s and 2,285 km/s.
The supernovas responsible for launching these stars are of a special kind known as Type 1a supernovas. These explosions are highly significant in astronomy due to their consistent brightness, which makes them valuable for determining distances in the vastness of space. Type 1a supernovas occur in binary star systems, where one white dwarf star gradually consumes its companion by stripping away stellar material as they orbit each other. As the feasting star accumulates enough matter from its companion, it eventually reaches a critical mass known as the “Chandrasekhar Mass,” named after the renowned Indian-American theoretical physicist Subrahmanyan Chandrasekhar.
Once the white dwarf reaches this critical mass, it can no longer withstand the gravitational forces acting upon it. As a result, it undergoes a catastrophic collapse, leading to an enormous explosion—an event we observe as a Type 1a supernova.
However, there are still some puzzling aspects surrounding Type 1a supernovas that continue to elude scientists. Theoretically, white dwarf binaries reaching the Chandrasekhar Mass should be less common than they actually are. This discrepancy has prompted astronomers to explore alternative mechanisms that could trigger similar supernova events, such as a process known as a “double detonation.”
In this particular scenario, an intriguing phenomenon occurs where a white dwarf star engages in a process of helium theft from its neighboring star's outer shell. The stolen helium ignites first, triggering a shockwave that sets off a second detonation within the carbon core of the star. Interestingly, in this case, the white dwarf can undergo a supernova explosion without reaching the Chandrasekhar limit, as long as the feasting star possesses a sufficiently large carbon core.
As a result of this double detonation scenario, the remnants of the neighboring star are propelled into space at velocities similar to those at which it was orbiting its now-deceased companion. This mechanism enables the runaway star to achieve extraordinary speeds as it traverses through and eventually exits the Milky Way galaxy.
The authors of the study, available on the arXiv pre-print server, describe these runaway white dwarfs as “smoking guns” of double-degenerate detonations, shedding light on their distinct nature and origins.
It is worth noting that runaway stars can also arise from single detonation supernovae. In such cases, it is the remains of the exploding star itself that attains tremendous velocities, rather than the companion star. These events are classified as type 1ax supernovae, wherein the explosion does not entirely obliterate the star, leaving behind the swiftly moving remnants of the white dwarf's core.
Researchers can discern the origins of runaway stars and classify them accordingly by analyzing differences in their velocities and spectroscopic signatures. As the population of observed runaway stars continues to expand, it will eventually provide valuable insights into the occurrence rates of each type of supernova.
Currently, the fastest known star in our galaxy is J0927, followed closely by J1235, both of which were recently discovered. J0927 boasts a radial velocity nearly twice that of any other star in the Milky Way, except for a few exceptions. For instance, stars orbiting closest to the supermassive black hole at the center of our galaxy exhibit incredibly rapid speeds as they whirl around it. However, these objects remain trapped in their orbits and do not follow a runaway trajectory like J0927 and J1235.
Source: Universe Today