Astronomers classify a specific kind of supernova as a “cosmic yardstick” because of its steady brightness, which aids in measuring distances across the universe. These Type Ia supernovas are fascinating yet puzzling, as researchers remain unclear on the triggers for their explosions. NASA suggests that they take place merely twice every thousand years in the Milky Way.
A leading hypothesis indicates that in a binary system of white dwarfs—remaining cores of sun-like stars—one undergoes a rapid explosion while the other escapes, liberated from its partner’s gravitational pull. Nonetheless, this does not encompass all Type Ia supernovas, as highlighted by scientists at Technion – Israel Institute of Technology. By investigating potential causes for a stellar remnant to suddenly “go rogue” and escape the galaxy, the international group proposed a novel scenario for triggering a white dwarf explosion.
Utilizing the occurrence rate of Type Ia supernovas, researchers can gauge how many runaway white dwarfs should theoretically be observable, stated Hagai Perets, co-author of new findings featured in Nature Astronomy. “When you perform that calculation, there appear to be insufficient numbers,” Perets explained to Mashable. “If any form of Type Ia supernova results in a hyper-velocity white dwarf, then there should be around 100 times more than we currently observe.”
In 2018, the European Space Agency’s Gaia telescope identified high-velocity white dwarfs. A significant enigma surrounding Type Ia supernovas is that, based on the prevailing theory known as D6, a white dwarf requires a companion star to trigger an explosion. This “dynamically driven double-degenerate double-detonation” framework has yet to uncover such a partner.
Gaia’s identification of high-velocity white dwarfs raised a possible explanation for the absent companions, as noted by Samuel Boos, a researcher at UC Santa Barbara who was not involved in the Technion study. It also reinforced the expectation of extreme velocities for these white dwarfs — the fastest stars in the galaxy. “It seemingly provided the smoking gun,” he remarked to Mashable in an email.
While Gaia confirmed the presence of runaways, scientists examined their origins. The new Technion study tackles this topic. Researchers employed a 3D supercomputer simulation to explore the interactions following the collision of two “hybrid” white dwarfs. These uncommon white dwarfs possess a carbon-oxygen core encased by a thicker helium layer.
The simulation revealed that as the lighter star spirals into the denser one, it becomes “partly consumed,” transferring helium onto its companion. This initiates a two-stage explosion: first the outer helium, followed by the inner carbon. The explosion obliterates the heavier white dwarf and propels the lighter one at approximately 4.5 million mph, sufficient to escape the Milky Way’s gravitational grip.
“The vast velocity indicates they were formed through a very violent process,” suggested Hila Glanz, the study’s co-lead author. “So initially, how do they survive, and why have we not observed the remnant of the explosion?”
The team posits that their hybrid collision model offers a clearer explanation for actual runaway white dwarfs. The explosion is considerably less bright than a typical Type Ia supernova, with debris dispersing thinly in space, which may clarify why astronomers haven’t detected the anticipated bright, dense remnants.
In this case, both stars are smaller, with one partially destroyed prior to the explosion of the other. The closer proximity results in a quicker and more forceful ejection of the surviving star. The reduced size of the surviving white dwarf — following partial destruction — could also account for its appearance as hotter and puffier, in line with three real runaway instances. The conventional theory fails to address these characteristics.
“They sort of sacrificed parts of themselves to ensure their survival,” Perets stated.
Recently, astronomers captured the first photographic documentation of a double-detonation supernova occurring in the Large Magellanic Cloud.
This fresh perspective not only accounts for how dead stars might become fugitives, but also implies a previously unrecognized method for producing a dimmer white dwarf explosion. The researchers underscore the significance of comprehending the diversity and behavior of Type Ia supernovas, as these events serve as the universe’s primary source of iron.
One possible implication is that absent companions in classical Type Ia supernovas might arise if both white dwarfs explode, stated Boos, who focuses on the D6 hypothesis. He recently published a study in The Astrophysical Journal illustrating that a “quadruple detonation” — where one double detonates following the other — could yield the expected brightness and chemical signatures along with a singular explosion.
“It’s quite possible that there are few or no genuinely D6 runaways (since they exploded alongside the primary),” Boos mentioned in an email, “and that all these candidates indeed originate from these hybrid mergers.”
As researchers ponder the implications, they can also speculate on the future of these exceptionally fast runaway white dwarfs.