November 19, 2018

19.06.10: A New Breed of Supernova?

 

 

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”?

08.05.10: Does Type Ia Supernova Formation Need Revision?

A key measurement device used by modern astrophysicists may also hold an elusive mystery. It has been long known that a Type Ia supernova occurs when a white dwarf accretes in-falling material from a binary companion, grows past the Chandrasekhar limit of 1.4 solar masses, and promptly blows itself up in a thermonuclear chain reaction that can be seen across the universe. These brilliant displays rise in brightness and then fade in a predictable fashion, allowing them to serve as “standard candles” marking the intergalactic distances to their host galaxies. These accreting white dwarfs should give off copious amounts of X-rays leading up to their eventual ignition. If this is the case, where are these accreting white dwarf SN Ia’s in waiting? An interesting study was released earlier this year by Akos Bogdan and Marat Gilfanov of the Max Planck Institute. Analyzing five elliptical galaxies and the nearby Andromeda with NASA’s Chandra X-Ray observatory, they found X-Ray output to be up to 50 times less than expected if the seeds for Type Ia’s were indeed being sown. Several factors may account for this discrepancy;

  1. Perhaps energetic accretion is not a constant state in these binary systems;
  2. The types of galaxies surveyed (with the exception of Andromeda) are not known for their energetic star formation;
  3. Type Ia’s may be more prevalent during certain epochs of star formation in the universe;
  4. Other mechanisms, such as merging white dwarf binaries, may produce Type Ia supernovae without accretion. But these populations would surely be lower throughout the universe than mixed systems; it isn’t even clear if a merging white dwarf pair would explode, or simply collapse into a neutron star. And white dwarfs are just plain tough to spot at galactic distances!

Whatever is the case, there still isn’t a consensus in the astronomical community as to where the Type Ia’s-in-waiting are hiding. It should be noted that this controversy does not center on the luminosity relationship;   naysayers look elsewhere for your chink in the frame-work of the Big Bang Theory! Instead, we suspect that “sub-breeds” Of Type Ia (Type IAa?) supernovae will come to light as new platforms such as James Webb Space Telescope come on line in the next decade.

06.11.09:A New Type of Supernova?

Astronomers at  the University of California at Berkley may have added a new type of supernova to the list. Typical type I supernovae consist of a carbon-oxygen white dwarf accreting matter from a companion star until a runaway reaction occurs, while type II supernovae involve a collapse of a star perhaps nine times as massive as our Sun. Recently, astronomers uncovered evidence that an extragalactic supernova previously classified as a type II may in fact deserve a class of its own. Named SN 2002bj, this exploding star exhibited the characteristics of a garden variety nova, such as the brief flare up of in-falling hydrogen, but created an explosion 1,000 times more massive. In the case of SN 2002bj, however, the flash also had a conspicuous absence of hydrogen, with instead a strong helium flash and the presence of vanadium in its spectra, a first for a supernova. Theoretical models suggest that this may have been a binary white dwarf pair, with one feeding the other a steady flow of helium until it reached the collapse limit and burst. Also, unlike typical type Ia supernovae, the white dwarf involved survived the explosion. Another unusual signature to this supernova was the way it rapidly faded from sight in about 20 days, about four times faster than usual. SN 2002bj is located in the galaxy NGC 1821 and was spotted in February 2002. Does the classification of supernovae need tweaking?