Testing Collapsars as the Key to Heavy Elements

How do we get the heavy elements — elements with atomic mass above iron, like gold, platinum, or uranium — in our universe? A new study suggests that one theorized source, collapsing massive stars, may not be the best option.

Enriching the Universe

The Big Bang produced a universe filled almost exclusively with hydrogen and helium; almost all of the heavier elements in our universe have formed since that time. How and when they formed, however, are still questions we’re working to solve.

element origins

Periodic table showing the origin of each chemical element. Those produced by the r-process are shaded orange and attributed to supernovae in this image; though supernovae are one proposed source of r-process elements, collapsars have been proposed as another. [Cmglee]

We know that the dense, hot cores of stars fuse atoms, producing elements up to iron in mass. But we need more extreme conditions for r-process nucleosynthesis — a set of rapid neutron-capture reactions that we think are responsible for producing about half the atomic nuclei heavier than iron.

Recent research has renewed interest in one potential source of r-process elements: collapsars. Collapsars are massive (>30 solar masses), rapidly rotating stars that suffer catastrophic core-collapses into black holes. In this sudden process, a spinning disk of material accretes onto the core — and conditions in the disk are just right for the r-process. But could collapsars really account for much of the r-process elements in our universe?

Clues from Collapse

abundance ratios

Abundance ratios found by the authors in a sample of low-metallicity stars. The top plot shows good agreement between the collapsar model (red) and observations (black) for the ratio of Mg (not an r-process element) to Fe. But r-process elements Ba, Eu, and Sr show much higher abundances in collapsar models than in the observations. [Macias & Ramirez-Ruiz 2019]

Collapsar r-process nucleosynthesis should leave a visible imprint on the surrounding environment, UC Santa Cruz scientists Phillip Macias and Enrico Ramirez-Ruiz (also University of Copenhagen, Denmark) point out.

In the collapsar model, r-process elements produced in the accreting disk are flung out into the star’s surroundings via disk winds. But collapsars don’t only produce r-process elements — they also create lighter elements like iron, which are spewed from the collapsars via jets. These elements should all then mix, producing a soup of enriched material with a particular ratio of abundances — which will then seed the next generation of stars.

Macias and Ramirez-Ruiz look for signs of this soup imprinted on a sample of 186 very low-metallicity stars that haven’t already been polluted by many additional generations of star formation. If collapsars are the source of most of the r-process material in the universe, then these unpolluted canvases should show the same ratio of r-process elements to iron as the authors calculate from collapsar models.

A Mismatch with the Evidence

Macias and Ramirez-Ruiz find that their stellar sample’s abundance ratios do not match those predicted by the collapsar model — the relative amount of r-process elements would need to be much higher in the observed stars for collapsars to be a good explanation.

Instead, the authors argue that the majority of r-process nucleosynthesis must occur in sources that don’t simultaneously produce iron. One possible source that satisfies this condition is neutron-star mergers, like that observed in the recent gravitational-wave event GW170817. There are challenges to this model as well — but we can hope that future observations will help us to better understand where our universe’s heavy elements come from.


“Constraining Collapsar r-process Models through Stellar Abundances,” Phillip Macias and Enrico Ramirez-Ruiz 2019 ApJL 877 L24. doi:10.3847/2041-8213/ab2049