"New Supernova Rewrites Supernova Rulebook" - Energetic eruptions leading to a peculiar hydrogen-rich explosion of a massive star.
By Avery Thompson
Nov 15, 2017
The universe is filled with gigantic, world-shattering explosions. Entire stars can explode in spectacular supernovas, black holes can output huge amounts of energy in the form of gamma ray bursts, and black holes and neutron stars can collide with enough force to shake the very fabric of spacetime millions of light-years away.
Given the sheer number of different explosions, it shouldn’t be surprising that there are still some we’re just discovering. Recently, a group of astronomers did just that, identifying a new type of explosion in a distant galaxy.
The explosion, called PS1-10adi, took place in a galaxy 2.4 billion light-years away and was picked up by telescopes in Hawaii and La Palma. This explosion was measured to be bigger than a supernova, which means there are two possible explanations: either it’s a supernova that outclasses all others, or it’s a star getting eaten by a supermassive black hole.
Both of these possibilities are similar to explosions we’ve already seen before, but the scale of this particular one leads scientists to believe something unique is happening. In either case, it’s likely that the extreme size of the explosion is the result of being located close to the center of its galaxy.
“If they are supernova explosions then their properties are more extreme than we have ever observed before, and are likely connected to the central environments of the host galaxies,” says lead study author Erkki Kankare. “If these explosions are tidal disruption events–where a star gets sufficiently close to a supermassive black hole's event horizon and is shredded by the strong gravitational forces – then its properties are such that it would be a brand new type of tidal disruption event.”
Studying this particular explosion could tell us more about similar explosions that astronomers have been picking up all over the universe. Until now, these explosions have been mysteries, but with this new discovery we’ve finally narrowed down what’s causing them. The one thing they all have in common is a location in the center of large galaxies, and understanding these explosions could tell us more about galaxy centers.
“[These explosions’] existence provides us with important information about the extreme environment in the central, hidden, part of galaxies,” says study author Cosimo Inserra. Maybe someday we'll figure them all the way out.
Astronomy: The star that would not die.
An event that initially resembled an ordinary supernova explosion continued to erupt brightly for more than 600 days. Standard theoretical models cannot explain the event's properties.
(Abstract) -
Every supernova so far observed has been considered to be the terminal explosion of a star. Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower-moving material that was previously hidden. In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining1. Here we report observations of iPTF14hls, an event that has spectra identical to a hydrogen-rich core-collapse supernova, but characteristics that differ extensively from those of known supernovae. The light curve has at least five peaks and remains bright for more than 600 days; the absorption lines show little to no decrease in velocity; and the radius of the line-forming region is more than an order of magnitude bigger than the radius of the photosphere derived from the continuum emission. These characteristics are consistent with a shell of several tens of solar masses ejected by the progenitor star at supernova-level energies a few hundred days before a terminal explosion. Another possible eruption was recorded at the same position in 1954. Multiple energetic pre-supernova eruptions are expected to occur in stars of 95 to 130 solar masses, which experience the pulsational pair instability. That model, however, does not account for the continued presence of hydrogen, or the energetics observed here. Another mechanism for the violent ejection of mass in massive stars may be required.
Editorial Summary
A very unusual supernova
Thousands of 'core-collapse' supernovae have been observed over the past 15 years, with common observational elements such as hydrogen absorption lines that slow over time and a single light-curve peak or luminosity that plateaus for around 100 days before declining. Iair Arcavi and colleagues report observations of the supernova iPTF14hls, which does not display the usual elements. Its light curve has multiple peaks and extends over 600 days. They conclude that the properties could be explained by ejection of several tens of solar masses of gas a few hundred days before the explosion, but there is no viable explanation for how this occurred. Although multiple pre-supernova eruptions are predicted by the pulsational pair instability, that model is inconsistent with the energetics involved here and the continued presence of hydrogen absorption lines with no decrease in velocity.
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