Posted on by John Williams

New Kind of “Runt” Supernovae Could be Lurking Unseen

Imagine this “Death from the Skies” scenario; a tiny supernova lurks unseen near our Sun. Astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) announced the discovery of just such an object today and while it is not nearby, this new kind of supernova is so faint it has been hiding in the shadows.

Until now, supernovae have come in two main versions. In one scenario, a huge star, 10 to 100 times more massive as our Sun, collapses causing a colossal stellar explosion. Another scenario, known as Type Ia supernovae, occurs when material from a parent star streams onto the surface of a white dwarf. Over time, so much material falls onto the white dwarf that it raises the core temperature igniting carbon and causing a runaway fusion reaction. This event completely disrupts the white dwarf and results in a colossal stellar explosion.

Now astronomers have found a third type that is fainter and less energetic than a Type Ia. Called a Type Iax supernova, it is “essentially a mini supernova,” says lead author of the study Ryan Foley, Clay Fellow at the Harvard-Smithsonian Center for Astrophysics (CfA). “It’s the runt of the supernova litter.”

Being only about one-hundredth as bright as their supernova siblings, Foley calculates that Type Iax supernovae are about as third as common as Type Ia supernovae. The researchers also did not find them in elliptical galaxies, filled with older stars, suggesting that Type Iax supernovae come from young star systems.

So far, Foley and his team identified 25 examples of this new type of supernova. Based on observations, the team found that the new Type Iax supernovae come from binary star systems containing a white dwarf and a companion star that has burned all of its hydrogen, leaving an outer layer that is helium rich.

In a press release, Foley says they are not sure what triggers the Type Iax supernova. One explanation involves the ignition of the outer helium layer from the companion star. The resulting shockwave slams into the white dwarf and disrupts it, causing the explosion. Alternately, the white dwarf might ignite first due to the overlying helium shell it has collected from the companion star.

“Either way, it appears that in many cases the white dwarf survives the explosion unlike in a Type Ia supernova where the white dwarf is completely destroyed,” says Foley. “The star will be battered and bruised but it might live to see another day.”

Supernovae explosions release so much energy as heat and light that they outshine entire galaxies for brief periods of time. The extremely hot conditions naturally create new heavier elements, such as gold, lead, nickel, zinc and copper. The explosion enriches the surrounding area leaving material for new stars to form.

“Type Iax supernovas aren’t rare, they’re just faint,” explains Foley. “For more than a thousand years, humans have been observing supernovas. This whole time, this new class has been hiding in the shadows.”

This research has been accepted for publication in The Astrophysical Journal and is available online.

11 Replies to “New Kind of “Runt” Supernovae Could be Lurking Unseen”

  2. Here is an Astrobites with similar models. Figure 2 is a nice summary.

    “A double white dwarf explosion addresses the problems of the white dwarf/red giant model: double-degenerate pairs are both common and would leave no trace (when one white dwarf explodes, so would the other). Pakmor et al. suggest a new explosion mechanism for the white dwarf/white dwarf pair–one that could lead towards a single model for all Type Ia supernovae.

    In the Pakmor et al. model, a carbon and oxygen white dwarf grabs helium from a second, lower mass white dwarf, which can either be composed of carbon and oxygen, or of pure helium (Figure 2). (Helium white dwarfs come from very low mass stars, in which electron degeneracy pressure stops the core from contracting early on in its evolution. In these stars, the core never reaches the high temperatures needed to fuse helium into carbon.) Clumpiness in the inflowing material or a collision of the two white dwarfs sparks helium fusion in the thin layer around the primary white dwarf. At that point there’s no turning back: the primary heats up and everything that can explode, does explode.

    A unified model?

    This theory nicely explains the variety of Type Ia supernova we see. Carbon and oxygen companions produce brighter supernovae that last for a long time since they have more material to fuse; helium companions produce dimmer supernovae that fade away quickly. In an extreme white dwarf/white dwarf interaction, the companion could be ripped apart by tidal forces and form a disk of material around the primary; this might explain rare, ultraluminous supernovae.”

    1. Sounds like it to me Ken. Especially if the white dwarf survives, unless “they” have changed the novas’ respective definitions.

    2. It is sort of in between the two in a way. However, because this involves a fusion “flash” of helium it fits more within the definition of a supernova.

      LC

  4. Which helium layer does explode? The one on the white dwarf or the outher helium layer of the companion star? The CfA press release suggests it’s the helium layer on the white dwarf, but doesn’t state this litterally. I’s a bit uncleaur what John means by the ‘outher helium layer from the companion star.’ Does this mean that de helium layer on the surface of the white dwarf originated on the companion star? In that case the explosion takes place on the white dwarf. Or does he mean the helium layer of the companion star, in wich case the explosion happens on the companion star. Who can bring light into the darkness?

  5. I wonder why all the “layman” explanations of white dwarfs never mention electron degenerate matter. Surely it isn’t too taxing on the audience to explain that it doesn’t expand when the temperature rises so any increase in temperature causes a runaway reaction.

  6. Seems more like a type of cataclysmic variable – dwarf nova more than a proper supernovae variety to me.

  9. A “runt” super nova, with the equivalent of only 50 MiIl years of solar nucleair power. Maybe meganova would be clearer then mini-supernova??

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