Supernovae can be divided into two broad categories: those produced by the deaths of very massive stars, and those that involve the explosions of white dwarfs. Within the first category, supernovae vary greatly depending on a number of details, including the mass of the exploding star. On the other hand, white dwarfs seem to all explode in very similar ways, which is why they have proven useful in measuring distances across the Universe.

Beginning in 2002, astronomers started recognizing a peculiar type of explosion. Since then, they've identified 25 of them; they resemble white dwarf supernovas in many respects, but strongly differ in others. A new paper by Ryan J. Foley and colleagues offered an explanation: these were an entirely new type of white dwarf explosion, one involving less energy and more material from a companion star. So much less energy, in fact, that the authors suspect that the white dwarf may not be fully destroyed in these odd events.

In the early days of supernova research, explosions were classified primarily by how much hydrogen and helium they had in their spectra. Type I supernovas, for example, mostly lack both elements. Since stars are mostly composed of hydrogen and helium, that indicates progenitor systems for type I supernovae are unlikely to be exploding stars.

Over subsequent decades, a subclass of type I supernovae—the type Ia supernovae—were linked with exploding white dwarfs, the remnants of stars like our Sun. These objects are mostly made of carbon and oxygen, after the fusion of nearly all of the hydrogen and helium over the lifespan of a star. When these white dwarfs exceed a certain mass (the Chandrasekhar limit), however, they undergo a thermonuclear explosion: an uncontrolled fusion reaction and gravitational collapse that destroys the object.

Since the Chandrasekhar limit is universal, all type Ia supernovae explode in very similar ways, producing spectra that are shaped the same. By contrast, supernovae involving the explosions of stars mostly have hydrogen in their spectra, explode in different ways, and therefore have a variety of emission spectra.

A peculiar supernova observed in 2002—labeled SN 2002cx—lacked hydrogen and helium emission, like a type Ia, but there were striking differences. The shape of the spectrum and the chemical elements present in the ejected materials were notably inconsistent with the type Ia model. The supernova was less energetic, producing less light and ejecting material into interstellar space with far less velocity. It also contained the signature of hot gas, a feature of stars, but not one found in white dwarf supernovae.

Astronomers identified 24 additional supernovae that seemed peculiar since finding SN 2002cx, which were both like and unlike the prototype. All had strong iron emission, low light output, and relatively low ejection velocities, but two contained helium emission. All of these supernovae resided in spiral galaxies, but not in regions with strong star formation. Since massive stars are short-lived, their supernovae sit in or near star-forming regions, so a supernova located elsewhere would tend to rule out the explosion of massive stars.

The authors of the new paper argue that these factors collectively point to a fundamentally new type of white dwarf supernova. The location of the supernovae, the shape of their spectra, and most aspects of their chemistry hinted that a white dwarf must be involved, so they designated these 25 explosions as type Iax supernovae. The authors also noted a great deal of variation between the examples we've seen; whether these differences are intrinsic or due to environmental factors is still uncertain.

Their type Iax model involves a white dwarf in a binary system with a high-mass ordinary star, which produces strong helium emission lines. Because of the lower energies involved and the much lower amount of mass ejected, the researchers suspected that some of the white dwarfs were not fully destroyed in the explosion. That would help explain the presence of a photosphere: a hot light-emitting region in the supernova remnant.

Because there are only 25 of these explosions known (mainly due to how faint they are), many questions still remain about their frequency and explosion mechanism. The authors suggested that type Iax supernovae may be about 1/3 as common as type Ia supernovae, which means large scale surveys of the sky should identify many more of them. With more data, astronomers should be able to settle details about the progenitor system, including sorting out those pesky variations between the different explosions.

The arXiv. Abstract number: 1212.2209 (About the arXiv). To be published in The Astrophysical Journal.