A 2-D snapshot of a pair-instability supernovae as the explosion waves are about to break through the star's surface. The tiny disturbances represent fluid instability - in a region where different elements interact and mix. Image Credit: ASIAA/Ken Chen
The answers to many questions in astronomy are hidden behind the veil of deep time. One of those questions is around the role that supernovae played in the early Universe. It was the job of early supernovae to forge the heavier elements that were not forged in the Big Bang. How did that process play out? How did those early stellar explosions play out?
A trio of researchers turned to a supercomputer simulation to find some answers.
Artist's conception of SN2016aps, a candidate pulsational pair instability supernova. The explosion energy of SN2016aps, fueled by the shedding of a massive shell of gas, was ten times that of a normal-sized supernova, making SN2016aps the most massive supernova ever identified. Credit: M. Weiss
It’s easy to run out of superlatives and adjectives when your puny human language is trying to describe humongously-energetic events in the Universe. So now it’s down to this: a really powerful supernova is a “super-supernova.”
But whatever name we give it, it’s a monster. A monsternova.
SDSS001820.5-093939.2 (seen in white) is a small, second-generation star bearing the chemical imprint of one of the universe's first stars. It shines at an apparent magnitude of 15.8, just south of the celestial equator in the constellation Cetus. Credit: SDSS / NAO
The young universe was composed of a pristine mix of hydrogen, helium, and a tiny trace of lithium. But after hundreds of millions of years, it began to cool and giant clouds of the primordial elements collapsed to form the first stars.
The first “Population III” stars were extremely massive and bright, synthesizing the first batches of heavy elements, and erupting as supernovae after relatively short lifetimes of just a few million years. This cycle of star birth and death has steadily produced and dispersed more heavy elements throughout cosmic history.
Astronomers haven’t spotted any of the first stars still shining today. But now, a team using the 8.2-meter Subaru Telescope has discovered an ancient low-mass star that likely formed from the elements produced in the supernova explosion of a very massive first generation star.
Pop III stars with masses exceeding 100 times that of the Sun would have died in a peculiar explosion that theorists call a pair-instability supernova.
Like its lower-energy comrade, a pair-instability supernova occurs when a massive star no longer produces enough energy to counteract the inward pull of gravity. But with so much mass, the star’s core is squeezed to such a high temperature and pressure that runaway nuclear reactions power a devastating explosion. The whole star is obliterated and no compact remnant, such as a black hole or neutron star, is left behind.
Astronomers have seen hints of these rare events before. But now, Wako Aoiki from the National Astronomical Observatory of Japan and colleagues have approached the search in a different way, by finding a star that bears the chemical fingerprints of these ancient explosions.
The elements we see lacing a star’s surface provide a key to understanding the supernova that preceded the star’s birth. And the star, dubbed SDSS001820.5-093939.2, exhibits a peculiar set of chemical abundance ratios. It has high levels of heavy elements, such as nickel, calcium, and iron, but low levels of light elements, such as carbon, magnesium and cobalt.
Note that the star is still metal poor in the grand scheme of things. Its iron abundance is 1/100 of the solar level. But compared with most metal-poor stars, where the iron abundance can be 1/100,000 or less of the solar level, the star is metal rich.
The chemical abundance ratios (with respect to iron) of SDSS J0018-0939 (red circles) compared with model prediction for explosions of very-massive stars. The black line indicates the model of a pair-instability supernova by a star with 300 solar masses, whereas the blue line shows the model of an explosion caused by a core-collapse of a star with 1000 solar masses. The abundance ratios of sodium (Na) and aluminum (Al), which are not well-reproduced by these models, might be produced during the evolution of stars before the explosion, but that is not included in the current model. (Credit: NAOJ)
These odd fingerprints suggest the star formed from material seeded by the death of a very massive Pop III star. In fact, the chemical composition of the star matches the elements that pair-instability supernovae are predicted to create.
The team notes that this is the only star of about 500 in the same low-metallicity range that has this peculiar makeup. It is — at the moment — our only window into the early universe and the first generation of stars.
