The stellar nursery in near-infrared. (João Alves/ESO VISIONS)

This Eerie Star Nursery Shows How The Solar System Got Radioactive Elements

16 AUGUST 2021

A roiling cloud complex, thick with the turbulence of star formation, is yielding up new clues as to the formation of our Solar System.

Analysis of gamma rays from the Ophiuchus star-forming complex has given us even more evidence that short-lived radioactive elements in the early Solar System were delivered via the supernova explosions of nearby stars when the Sun was being born.

 

This validates an elemental enrichment model suspected for decades, and gives us valuable insight into the breathtaking life-and-death cycle of stars.

"Our Solar System was most likely formed in a giant molecular cloud together with a young stellar cluster, and one or more supernova events from some massive stars in this cluster contaminated the gas which turned into the Sun and its planetary system," said astronomer and astrophysicist Douglas N. C. Lin of the University of California, Santa Cruz.

"Although this scenario has been suggested in the past, the strength of this paper is to use multi-wavelength observations and a sophisticated statistical analysis to deduce a quantitative measurement of the model's likelihood."

Stars are born when a spinning knot of dense gas in a molecular cloud collapses under its own gravity. Material in the cloud flattens out into an accretion disk that feeds into the growing star; once the star has finished forming, the leftover disk forms everything else in the planetary system - so while elemental abundances may vary from body to body, everything in a planetary system is made from the same piece of molecular cloud.

 

These molecular clouds are huge, vast complexes that give birth to many stars. These are called stellar nurseries. Our Sun was probably born this way, although it has long since left its birthplace and siblings behind.

Figuring out how the Solar System was born and came to be the way it is, requires detective work by piecing together clues from within the Solar System, and observing others coming into existence.

The Ophiuchus star-forming complex is just 460 light-years away - that's a pretty short distance on relative cosmic scales. And in this complex, astronomers have detected gamma rays emitted by the short-lived radionuclide aluminum-26.

Aluminum-26 has a half-life of 717,000 years. Therefore, any of this isotope that may have been around in the early Solar System - 4.6 billion years ago - would be long gone by now.

In the 1970s, though, scientists found inclusions in pristine meteorites that they concluded were the decay products of short-lived radionuclides, which raised the question: where did they come from? The answer was from nearby supernovae, or the stellar winds from dying Wolf-Rayet stars, but how many sources, where they are, and the penetration rate of aluminum-26 remained unknown.

 

It's not unusual for stellar nurseries to be bathed by the radiation of supernovae. Such regions produce a variety of stars, including some so massive that they live and die while other stars are still being born.

Using observations across a range of wavelengths, including incredible new infrared images, the researchers noted a stream of aluminum-26 from a nearby star cluster that had hosted such supernovae to a star-forming region of the Ophiuchus complex.

"The enrichment process we're seeing in Ophiuchus is consistent with what happened during the formation of the Solar System 5 billion years ago," said astrophysicist John Forbes of the Flatiron Institute.

"Once we saw this nice example of how the process might happen, we set about trying to model the nearby star cluster that produced the radionuclides we see today in gamma rays."

These models accounted for every massive star that could have existed in the region in the window to produce aluminum-26, the probability of those stars going supernova, and the potential yields of the radionuclides from supernovae as well as stellar winds.

Based on this modelling, the researchers were able to conclude that there is a 59 percent chance that the aluminum-26 is produced by a supernova, and a 68 percent chance that there were multiple sources and more than one supernova.

This suggests that there is a wide range of radionuclide abundances that can be incorporated into a forming planetary system. In turn, this could have implications for the search for habitable systems.

"Many new star systems will be born with aluminum-26 abundances in line with our solar system, but the variation is huge - several orders of magnitude," Forbes said.

"This matters for the early evolution of planetary systems, since aluminum-26 is the main early heating source. More aluminum-26 probably means drier planets."

The research has been published in Nature Astronomy.