Finding Ways to Catch Collapsars Making Heavy Metals

Researchers are still working out where heavy metals are made in the universe. A recent publication explores ways to tell if elements heavier than iron can be created when extremely massive stars collapse to form black holes.

Making Heavy Metals

In the cores of stars, nuclear fusion combines light elements into heavier ones, with the largest stars generating elements up to iron. But elements bulkier than iron must arise elsewhere, since a star that attempts to create anything heavier is doomed to collapse in a supernova explosion.

illustration of two neutron stars approaching a merger.

An illustration of two neutron stars approaching a merger. [ESO/L. Calçada]

About half of the elements beyond iron on the periodic table are thought to form through something called the r-process, in which atoms rapidly capture multiple neutrons in a dense, hot environment. Core-collapse supernovae were early contenders for r-process production, but simultaneous observations of light and gravitational waves from colliding neutron stars cemented mergers as an important source of heavy elements. Now, researchers are searching for ways to determine if certain supernovae could be sites of r-process element creation after all.

Collapsars as Candidates

Collapsars are rapidly rotating massive stars that explode as supernovae when they can no longer sustain nuclear fusion, ultimately creating a black hole. As the star’s core collapses, material in the outer layers forms an accretion disk, in which conditions for r-process element formation may exist. To probe the possible role that collapsars play in generating r-process elements, Jennifer Barnes (University of California, Santa Barbara) and Brian Metzger (Columbia University and Flatiron Institute) modeled the effects of r-process nucleosythesis on the light curves of collapsars exploding as supernovae.

illustration of the authors' model

An illustration of the authors’ model, in which r-process-enriched material is surrounded by an r-process-poor shell. [Barnes & Metzger 2022]

Barnes and Metzger first used an analytical model to predict when the presence of r-process products might be observable as the supernova’s emission rises and falls, as well as how best to observe these effects. The team found that it may be possible to discern whether a collapsar explosion contains r-process material by making long-wavelength observations several months after the explosion, depending on how the material is distributed, but early in the explosion might offer a better chance of identifying these events.

Light Curve Modeling

As a follow-on to their initial investigation, the team modeled the evolution of light curves from collapsar explosions that produce varying amounts of r-process material. These models explore how supernova light curves change as a function of the mass ejected in the explosion, the velocity of the ejected mass, the amount of nickel-56 (a radioactive form of nickel that decays into cobalt-56, creating the characteristic shape of many supernova light curves), and the amount and distribution of r-process material.

modeled light curves showing the effect of changing the degree of mixing.

Demonstration of how the degree of mixing (ψmix) affects the resultant light curve. As the degree of mixing increases (higher ψmix), the emission shifts toward the near-infrared. Click to enlarge. [Barnes & Metzger 2022]

In general, the presence of r-process material causes supernova light curves to shift toward redder frequencies, though the distribution of the material plays a large role in how visible this effect is; material concentrated at the center of the explosion will have little effect, while material mixed throughout will have a larger effect. Ultimately, the authors concluded that monitoring supernovae for ~75 days after they explode could be a viable way to identify collapsars that produce r-process elements, paving the way for near-infrared follow-up observations with JWST.


“Signatures of r-process Enrichment in Supernovae from Collapsars,” Jennifer Barnes and Brian D. Metzger 2022 ApJL 939 L29. doi:10.3847/2041-8213/ac9b41