Rare events may be required to form the Universe’s heaviest metals

Up to a certain point, the elements of the periodic table are largely formed in the hearts of stars. But for elements that are heavy enough (heavier than zinc typically), fusing two lighter nuclei just won’t do it efficiently enough to produce significant quantities. To form those elements, another process is needed: neutron capture.

Neutrons get captured when they collide with an atomic nucleus and get stuck together, creating a heavier nucleus. Neutrons can undergo these collisions at lower energies because they’re electrically neutral, so they won’t be repelled, unlike protons. If the resulting nucleus is unstable, however, one of its neutrons can decay into a proton, creating a heavier element.

A lot of heavy elements are formed by neutron capture, but the details of how it happens haven’t been well worked out. That’s in part because there are two kinds of neutron capture processes: the slow process (s-process) and rapid process (r-process). Each of these accounts for about half of the elements produced by neutron capture in our Universe.

The s-process usually takes place inside a class of stars known as asymptotic giants. These are stars in a late stage of their evolution that have expanded to become red giants, with luminosities often thousands of times that of the Sun. The Sun itself will become one later in its life.

The r-process, though, is more mysterious. We know it’s taking place and that it accounts for much of the heavier elements in the Universe, but we haven’t been able to nail down where it’s happening, despite 60 years of trying.

Some studies looked at old stars in the Milky Way’s halo; these suggested that the r-process takes place constantly, in objects such as core-collapse supernovae. But other evidence has suggested that the r-process takes place only in rare events, such as mergers between pairs of neutron stars.

The element europium, produced by neutron capture, is often taken as evidence of the r-process. Some studies have observed the abundances of europium in dwarf spheroidal galaxies, small galaxies that often orbit larger ones. These studies have argued that the presence of europium there indicates that the r-process took place via rare events like neutron star mergers in the early Universe.

That idea, however, relies on an assumption: that there’s not much material flowing into these galaxies. If there is, then it would favor the model in which the r-process takes place in common events like supernovae. That’s because any incoming gas will mostly be hydrogen, and so it lowers the relative amount of europium in the galaxy. And since we observe a steady fraction of europium, something has to be constantly producing more of it—a common event, such as a supernova.

The study

To find out whether the r-process is rare or common, a research team set its sights on another, more ancient sub-class of dwarf spheroidals: ultrafaint dwarf galaxies. These are smaller and chemically simpler than typical dwarf spheroidals, and they formed shortly after the Universe’s first stars did, within the first three billion years of the Universe’s history. This makes them an ideal place to look for early neutron capture activity. The researchers focused on Reticulum II, one of the most metal-poor dwarfs known.

Metals, (for astronomers, any element heavier than helium), are a sign that stars have already gone supernova in the galaxy. The low levels of them indicate that this hasn’t happened much in Reticulum II.

Using high-resolution spectroscopy, the researchers examined the nine brightest stars in the galaxy, seven of which were extremely rich in europium and other neutron-capture elements. The abundances of those elements matched that of what was expected if they were formed through the r-process. A full 78 percent of the galaxy’s stars are highly enriched in r-process elements in comparison to an estimated five percent of the Milky Way’s halo stars.