An unusual type of star may be showing us something about the origin of our own Solar System. Wolf-Rayet stars display unusual spectra, prominent in which are heavy elements as well as broad emission lines of ionized helium, nitrogen and carbon. These are massive objects 40 to 50 times the size of our Sun, with surface temperatures ranging up to 200,000 K. Have a look at one of these, showing another Wolf-Rayet trait, the strong stellar winds ejecting material into nearby space. A bubble with a dense shell forms around such stars, trapping gas and dust that could form into new stars.

Image: Here we see the spectacular cosmic pairing of the star Hen 2-427 — more commonly known as WR 124 — and the nebula M1-67 which surrounds it. Both objects, captured here by the NASA/ESA Hubble Space Telescope, are found in the constellation of Sagittarius and lie 15,000 light-years away. The star Hen 2-427 shines brightly at the very centre of this explosive image and around the hot clumps of gas that are ejected into space at over 150,000 kilometres per hour. Hen 2-427 is a Wolf-Rayet star, named after the astronomers Charles Wolf and Georges Rayet. Wolf-Rayet are super-hot stars characterized by a fierce ejection of mass. The nebula M1-67 is estimated to be no more than 10,000 years old — just a baby in astronomical terms — but what a beautiful and magnificent sight it makes. A version of this image was released in 1998, but has now been re-reduced with the latest software. Credit: ESA/Hubble.

Vikram Dwarkadas (University of Chicago) and colleagues believe that Wolf-Rayet stars can unlock the mystery of how our Solar System emerged. The researchers are hoping to update the older view that our system formed in the vicinity of a relatively conventional supernova, noting peculiarities in the proportion of two isotopes in the early Solar System. One of these is aluminium-26, which turns up in relatively high proportion in our system compared with the rest of the galaxy.

The other issue is with iron-60, which earlier work by Nicolas Dauphas, a co-author on the current paper, suggests is found in smaller amounts than we would expect. We couple this with the interesting fact that Wolf-Rayet stars release a good deal of aluminium-26, but are not associated with iron-60. Add into the mix the giant stars’ ability to shed mass through intense stellar winds. We wind up with a bubble structure with a dense shell, a potential star-making factory. Dwarkadas and team estimate that 1 to 16% of Sun-like stars could form in this way.


Image: This simulation shows how bubbles form over the course of 4.7 million years from the intense stellar winds off a massive star. UChicago scientists postulated how our own Solar System could have formed in the dense shell of such a bubble. Credit: V. Dwarkadas & D. Rosenberg.

It’s an interesting explanation because we would expect both isotopes to be produced in the kind of supernova long held to have provided materials for the infant Solar System. Given the proportions we actually find in meteorites from the early system, the question becomes why one isotope is found in the days of system formation while the other was not. Says Dwarkadas:

“The idea is that aluminum-26 flung from the Wolf-Rayet star is carried outwards on grains of dust formed around the star. These grains have enough momentum to punch through one side of the shell, where they are mostly destroyed—trapping the aluminum inside the shell.”

Over time, the shell begins to collapse inward due to gravity, forming our Solar System. The original Wolf-Rayet star is long gone, doubtless through a supernova explosion or, the authors note, through direct collapse into a black hole. The latter would produce little iron-60, while the former could have trapped any iron-60 formed in the supernova within the bubble walls.

Image: Slices of a simulation showing how bubbles around a massive star evolve over the course of millions of years (moving clockwise from top left). Credit: V. Dwarkadas & D. Rosenberg.

The paper is Dwarkadas et al., “Triggered Star Formation inside the Shell of a Wolf-Rayet Bubble as the Origin of the Solar System,” Astrophysical Journal Vol. 851, No. 2 (22 December 2017). Abstract available. The earlier paper on iron-60 is Tang & Dauphas, “Low 60FE Abundance in Semarkona and Sahara 99555,” Astrophysical Journal Vol. 802, No. 1 (17 March 2015). Abstract available.