Astronomers have discovered a new supernova class where the star might survive the explosion.Supernovae are typically thought to herald the transformation, if not utter destruction, of a star. But scientists have recently discovered a new, milder type of supernova, one where the star can survive the explosion.
Ryan Foley (Harvard-Smithsonian Center for Astrophysics) and his team collected observations old and new of 25 supernovae that look almost — but not quite — like their Type Ia brethren, explosions in which a white dwarf accretes too much material from a companion star. The added material pushes the white dwarf over its limit of 1.4 solar masses (the so-called Chandrasekhar limit), setting off a chain nuclear reaction that releases light 5 billion times more luminous than the Sun.
Because Type Ia white dwarfs explode when they surpass their mass limit, astronomers can estimate the explosions’ intrinsic luminosities to use the objects as cosmic yardsticks. Observe a Type Ia supernova in a distant galaxy and you know that galaxy’s distance — observe many Type Ia supernovae throughout the universe and you can measure how quickly space is expanding.
But not all Type Ia supernovae look alike. The 25 supernovae gathered by Foley’s team share a dozen or so properties that distinguish them from their normal Type Ia siblings, including lower peak brightnesses and lower ejecta velocities. So Foley and his colleagues place these supernovae in their own class: Type Iax.
Type Iax supernovae also result from thermonuclear fusion deep within a white dwarf that rises to its surface. But the reaction fails to demolish the entire star: instead, only half-a-Sun’s worth of material (on average) is ejected, including ash from the thermonuclear burning. In many cases, some of the white dwarf will survive the deflagration.
“The proposed explanation . . . fits many of the observed properties,” says Craig Wheeler (University of Texas at Austin), who was not involved in the study. “Modeling of these events is nevertheless a new art and probably deserves maturing before firm conclusions are reached,” he cautions.
Foley and his team estimate that the new class is about one-third as common as regular Type Ia supernovae. But are these explosions worthy of the supernova title if they don’t destroy the star? As Wheeler puts it, “I'm not at all sure whether we should call these supernovae or ‘super novae.’”
“My take,” Foley says, “is that (1) some of these objects may in fact completely disrupt their star, and (2) in every way except for the possibility of the remaining star, SNe Iax are more like supernovae than novae. Maybe we need a new word. ‘Pretty good’ novae?”