Every student of Astrophysics 101 soon learns that there are two main types of supernovae; Type 1a, which occur when a white dwarf star accretes matter from a bloated companion, passes the Chandrasekhar Limit and explodes, and Type II, when a star 8 times the mass of our Sun or larger reaches the end of its fusion burning life and promptly explodes… but are these snapshots of the final phases of stellar evolution really that neat and tidy? Recently, evidence has been mounting that there may be other sub-branches to the supernova tale, and not just the two flavors and the sub-categories that we learned in school. The first round of evidence comes from a team at the Harvard-Smithsonian Center for Astrophysics and their study of supernova 2005E. This blast occurred in the galactic halo of the galaxy NGC 1032 in the constellation Cetus, not your typical supernova breeding territory. Supernovae are usually seen in rich star forming regions, not in metal poor outer galactic suburbs. This event was a fizzle, ejecting only 300 times the mass of Jupiter into its nearby environs.
The mystery deepened as a team from Hiroshima University released their results of a study of another supernova, 2005cz. Located in the elliptical galaxy NGC 4589, this eruption was also only 20% as bright as models predict, showing that while the initial mass may have been just above what was required for a Type II supernova, it beared none of the classic hallmarks of either species of events. Both of these supernovae, along with 6 others recorded, show a high concentration of calcium in their spectra, a hint that they may not be related to either of the previously known types.
So, what’s going on? Do we need to re-write all those old astrophysical texts? It’s unlikely that a progenitor star migrated all the way to a galactic halo region in its short life span simply to explode. A possible scenario could be a pair of binary white dwarfs (or do you say dwarves?) in a tight orbit, with one stealing the helium shell of another and bursting. Spectra taken of both events seem to support this scenario… this mystery may have a tie-in with the seeming lack of “Type 1A’s in waiting” mentioned in this space in an article on a recent survey of nearby galaxies… will this hybrid style of supernova become known as “Type III” or “Type 2.5”?