University of Pennsylvania Physics & Astronomy Colloquium

About half of the elements heavier than iron are created in extreme, neutron-rich cosmic environments. In these settings, atomic nuclei rapidly absorb neutrons before they can decay, building up very heavy elements in what scientists call the “r-process.” The first direct evidence for this process came in 2017, when a glow known as a kilonova was observed following the gravitational wave event GW170817, confirming that merging neutron stars are an important source of heavy elements. However, observations of old stars in our Galaxy suggest that neutron star mergers alone may not explain all of the heavy elements in the universe.

Heavy elements form in mergers when neutron-rich matter is ejected into space, typically from a dense disk of material surrounding the newly formed black hole. Similar disks can also arise when massive, rapidly rotating stars collapse—so-called “collapsars.” In especially massive cases, these disks may become unstable and fragment into multiple low-mass neutron stars, which could then merge with one another inside the same environment. This process would generate repeated gravitational wave and light signals—a kind of “multi-messenger symphony.” Recent reports of possible unusually low-mass neutron star mergers associated with supernovae may offer support for this idea.

Another possible source of heavy elements comes from giant eruptions of highly magnetized neutron stars, known as magnetars. During these rare flares, material from the star’s crust can be blasted into space. I will present evidence that the famous Galactic magnetar flare of December 2004 produced about one-millionth of the Sun’s mass in heavy elements. The radioactive decay of this material would also power a very brief flash of ultraviolet and visible light—lasting only minutes—a “mini-kilonova” that future missions such as ULTRASAT and UVEX could detect in other galaxies.