A team of astronomers from the University of Chicago and Clemson University has laid out a theory for how our Solar System could have formed in the dense shell of a large cloud of gas and dust around a Wolf-Rayet-type star. Their work appears in the Astrophysical Journal.
This Hubble image shows an enormous, balloon-like bubble being blown into space by a super-hot, massive Wolf-Rayet star. Image credit: NASA / ESA / Hubble Heritage Team / STScI / AURA.
The general prevailing theory is that our Solar System formed billions of years ago near a supernova.
But the new scenario instead begins with a Wolf-Rayet star, which is more than 40 to 50 times the size of the Sun.
Wolf-Rayet stars are incredibly hot and expel their outer layers of gas at extremely high velocities.
As such a star sheds its mass, the stellar wind plows through the material that was around it, forming a bubble structure with a dense shell — a so-called Wolf-Rayet bubble.
“The shell of a Wolf-Rayet bubble is a good place to produce stars, because dust and gas become trapped inside where they can condense into stars,” said co-author Professor Nicolas Dauphas, from the University of Chicago.
Professor Dauphas and colleagues estimate that 1% to 16% of all Sun-like stars could be formed in such stellar nurseries.
“This setup differs from the supernova hypothesis in order to make sense of two isotopes that occur in strange proportions in the early Solar System, compared to the rest of our Milky Way Galaxy,” they said.
“Meteorites left over from the early Solar System tell us there was a lot of aluminium-26. In addition, studies increasingly suggest we had less of the isotope iron-60.”
“This brings astronomers up short, because supernovae produce both isotopes.”
“It begs the question of why one was injected into the Solar System and the other was not,” said lead author Dr. Vikram Dwarkadas, also from the University of Chicago.
This brought the scientists to Wolf-Rayet stars, which release lots of aluminium-26, but no iron-60.
“The idea is that aluminum-26 flung from the Wolf-Rayet star is carried outwards on grains of dust formed around the star,” Dr. Dwarkadas said.
“These grains have enough momentum to punch through one side of the shell, where they are mostly destroyed — trapping the aluminum inside the shell.”
“Eventually, part of the shell collapses inward due to gravity, forming our Solar System.”
“As for the fate of the giant Wolf-Rayet star that sheltered us: its life ended long ago, likely in a supernova explosion or a direct collapse to a black hole,” the researchers said.
“A direct collapse to a black hole would produce little iron-60; if it was a supernova, the iron-60 created in the explosion may not have penetrated the bubble walls, or was distributed unequally.”
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Vikram V. Dwarkadas et al. 2017. Triggered Star Formation inside the Shell of a Wolf–Rayet Bubble as the Origin of the Solar System. ApJ 851, 147; doi: 10.3847/1538-4357/aa992e
