Evidence presented for new supernova explosion model
BY JEFF FOUST
Posted: January 9, 2000
New research, based on observations of a brilliant supernova, is challenging existing models of how one type of the powerful explosions take place in the Universe.
At a conference in Texas last month, a group of astronomers from the University of Texas presented new studies of supernova SN 1987a that show evidence of a jet-like structure, emanating from a new neutron star at the heart of the explosion, that may have provided the energy for the star's initial explosion fourteen years ago.
The direct observation of the jets adds to indirect evidence for their existence. Past observations showed chemical inhomogeneities in the ejecta, polarization of the light from the ejecta, and hints of a secondary source of light located off-center from the bulk of the ejecta: all evidence consistent with, but not necessarily proof of, jets in the supernova ejecta. The new Hubble images build a stronger case for the jets' existence.
The existence of the jets may challenge existing models for supernovas like SN 1987a. Astronomers believe that many supernovae form as the cores of supermassive stars collapse as they exhaust their supply of nuclear fuel. As the core collapses, protons and electrons are squeezed together, forming neutrons. Much of the energy of the collapse is carried away in nearly massless neutrinos, byproducts of the formation of neutrons; these neutrinos, it had been believed, triggered the supernova explosion.
However, those models created remnants that were essentially spherical, as opposed to the asymmetric ejecta seen from SN 1987a. This leads astronomers to rethink those models, noting that jets from the neutron star formed at the core of the collapsing supergiant have enough energy themselves to create the supernova and explain the asymmetric aftermath.
"The classic models based on neutrinos could give some short lived or fine scale irregularity, but we do not believe those models can account for the asymmetry we see," said Craig Wheeler, a University of Texas astronomer. "What we have shown is that jets from the newborn neutron star can both blow the star up without any boost from the neutrinos and provide the asymmetries we see."
What causes the jets themselves to form remains unclear, however. "The origin of the jets, presumably from some magnetic or rotational processes, still needs to be understood," said Wheeler.
SN 1987a is the first supernova remnant where such asymmetries have been directly observed, although this is primarily because the supernova, located in the Large Magellanic Cloud 168,000 light-years from the Earth, is the closest supernova to the Earth in several hundred years and thus the easiest to observe. Wheeler noted that observations of about a dozen other similar supernovae have shown polarizations consistent with asymmetries "at about the same level we deduce by more direct observations in SN 1987a."
"We see no evidence," he concluded, "that any collapse-type supernovae work by any other than the 'jet' mechanism."