Everyone, say hello to Type Iax, a new classification of supernova that astronomers are calling “the runt of the supernova litter.” But these celestial blasts are hardly subtle; they're a newly documented and surprisingly frequent subcategory of supernova in which helium-sucking white dwarfs survive the ensuing explosion.
Above image: Material from a hot, blue helium star (right) is funneling toward a carbon/oxygen white dwarf (left) bathed in an accretion disk. Eventually, it all goes boom, but the white dwarf lives to see another day. Credit: Christine Pulliam.
To date, astronomers have catalogued only two types of supernovas: Type I (explosions involving white dwarfs and with very little hydrogen found in the spectrum) and Type II (when massive stars implode in on themselves, and with large amounts of hydrogen and helium in the light spectrum).
Divided further, core-collapse supernovas, or Type Ib and Ic, can result in massive stellar explosions that radiate as much energy as our Sun does over its entire lifetime.
Thermal runaway supernovas, or Type Ia, involve two stars, one of them being a white dwarf (faint stars in their final evolutionary state and with masses insufficient to become a normal neutron star). In these configurations, a white dwarf has sucked up so much material from its companion that its core temperature is hot enough to ignite carbon fusion. At this point, the white dwarf undergoes runaway nuclear fusion, which completely destroys it.
The new classification of supernova is closely related to the Type Ia. According to Ryan Foley, Clay Fellow at the Harvard-Smithsonian Center for Astrophysics, these supernovas arise in young binary systems containing a white dwarf and a companion star that has lost its outer hydrogen, leaving it extremely rich in helium. This helium is pulled in by the white dwarf.
Eventually, an explosion happens. But Foley says the white dwarf is able to withstand and survive the shock.
"The star will be battered and bruised, but it might live to see another day," noted Foley through a statement.
Interestingly, the researchers aren’t entirely sure what ignites the explosion. One theory is that the outer helium layer ignites first, sending a shock wave into the white dwarf. The other possibility is that the white dwarf ignites first, the result of a helium-heavy outer shell.
To date, events like these have eluded astronomers on account of the extreme faintness of white dwarfs. But they’re anything but rare; Foley’s team was able to document 25 Iax supernovas. Based on these observations, the astronomers say that, for any given volume of space, there are about 31 Iax for every 100 Ia (making them about a third as common).
Foley's study has been accepted for publication in The Astrophysical Journal and is available online.