Astronomers have long since concluded that our 4.6 Billion-year-old solar-system was born from the remnants of a previous generation of stars. Thus far researchers have theorised that this involved the explosion of a massive supernova, which may have compressed a pre-existing dust cloud triggering the formation of the Sun. There has been one problem hanging over these theories. The chemical balance of the solar system is not in-line with the universe in general and the expected results of a supernova explosion. Now researchers at the Enrico Fermi Institute in Chicago think they may have come up with a solution to this chemical imbalance, a solution that involves the solar-system emerging from the shell of a dead Wolf-Rayet star.
The problem of the solar-system’s chemical imbalance has emerged from the investigation of extra-solar planetary systems and the study of meteorites. The relative abundances of isotopes in meteorite samples give us a clue as to the original cloud material from which the solar system formed as well as tracing the solar processes involved in that formation and the galactic chemical evolution up to the time of formation.
A chemical element is defined by the number of protons within its atomic nucleus, and an element’s atomic mass is the total number of protons and neutrons in the nucleus. An isotope is a variety of a chemical element with different numbers of neutrons within the atomic nucleus. This gives various isotopes different atomic masses. Different isotopes of an element have the same chemical behaviour. Thus, the main difference between isotopes is that some are stable, whilst others degenerate into other chemical elements known as daughter products. The most commonly referenced example of differing isotopes is Carbon-12 and Carbon-14. Both exist in equal abundances in the Earth’s atmosphere, but Carbon-14 is unstable. This means that living organisms ingest equal amounts of both isotopes up until death. After this point, the amount of Carbon-12 remains stable in the organisms remains, whilst the Carbon-14 degenerates into daughter products. This is the principle behind Carbon dating. The relative amounts of Carbon-14 to Carbon-12 in organic remains allow us to estimate long ago that organism lived.
The difficulty arises from the fact that there is a much higher concentration of Aluminium-26 in samples of meteorites than can be accounted for in models of the chemical evolution of the galaxy. Astronomers have previously suggested that the solution to this problem may well be solved by the injection of such material into the early solar system from a nearby external source. A likely culprit for this has been speculated to be stellar winds from other massive stars or winds from a cool older star known as an AGB star. The latter speculation is difficult to justify as AGB stars are highly evolved and unlikely to exist at the time and place of solar-system formation. The former explanation also has its problems.
One way to checking if Aluminium-26 was distributed into the early solar system by stellar winds from massive stars is to measure the relative abundances of another unstable isotope, iron-60, which is created by neutron capture in the inner part of massive stars. Unfortunately, abundances of the daughter product of iron-60, nickel-60, measured using mass spectrometry, imply a low abundance of iron-60 in the early solar system. In fact, an abundance that is lower than galactic composition at large. This seems to rule out stellar winds from massive stars as a source of aluminium-26 in the early solar system.
How can models of solar system formation add enough aluminium-60 without adding too much iron-60?
The recently published paper ‘Triggered star formation inside the shell of a Wolf-Rayet bubble as the origin of the solar system’ (Dwarkadas, Rosenberg, et al, The Astrophysical Journal, 2017), posits a revolutionary new solution to the above problems. The team at the Enrico Fermi Institute in Chicago supported by the NASA Emerging Worlds program, have suggested that the relative abundances of these isotopes could be accounted for if the solar-system was born within the shell of a dead massive star. More specifically, a Wolf-Rayet (W-R) star.
A W-R star represents the final stage of a star of upwards of twenty times the mass of the Sun evolution. Whilst smaller stars die when their allotment of hydrogen and helium has been exhausted, massive stars have the required mass to fuse heavier and heavier elements. W-R stars of masses 25 times greater than the Sun have been shown to be able to fuse elements denser than iron. Importantly, whilst these stars are known to produce aluminium-26, they do not seem to produce iron-60.
When these stars finally collapse and trigger massive super-nova explosions these shock fronts can trigger the increase in density necessary for gas and dust clouds in its wake to form stars. This shock wave can also form ‘low-density’ bubbles, surrounded by high-density shells. These bubbles are generally considered excellent places for the formation of stars due to they allow the trapping of dust and gas. Winds from the remnant of the W-R carry condensed dust grains of aluminium-26 to these bubbles. The dust grains penetrate the shells into the low-density cavity to varying depths. The density of the shell is therefore increased triggering star-formation. The shell eventually dissipates leaving an exposed, now forming solar-system.
The researchers involved elaborated on this: “These grains have enough momentum to punch through one side of the shell, where they are mostly destroyed — trapping the aluminium 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.”
Though not accepted by the astronomical community at the moment this study represents an exciting step forward in addressing the finer points of the formation of our and other solar systems and should provide an exciting platform for further research.