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Title: Radio Constraints on r-process Nucleosynthesis by Collapsars
Authors: K.H. Lee et al.
First Author’s Institution: University of Florida
Status: Published in ApJL
Most elements in the periodic table originate in stars. Elements lighter than iron are stable and can be formed by nuclear fusion in stellar cores. Heavier elements have unstable nuclei and require additional sources of energy to form. These elements are believed to originate in supernovae: the explosive deaths of stars with masses more than ten times that of the Sun. However, even supernovae cannot produce the heaviest elements in the periodic table — the lanthanides and the actinides. The origin of these “heavy” elements (which include other metals like gold and platinum) is a long-standing mystery. The authors of today’s article use radio observations of collapsars — special types of supernovae — to constrain whether these elusive heavy elements can be formed in them.
r-process Nucleosynthesis
The reason why lanthanides and actinides are so difficult to produce is because they are formed by a rare process involving the rapid capture of neutrons that is known as the r-process. In this process, a free neutron is captured onto the nucleus of an atom, producing a nucleus with a higher atomic mass number. The problem is that free neutrons are inherently unstable particles that undergo the beta-decay process to form a proton, an electron, and an antineutrino on a timescale of a few minutes. To efficiently activate the r-process, neutrons need to be captured faster than they decay. Therefore, a neutron-rich environment is needed. One such environment is formed during the explosive mergers of two neutron stars. So far, only one of these events has ever been observed: GW170817, which was detected in both gravitational and electromagnetic waves. However, other astrophysical explosions have also been proposed to produce neutron-rich ejecta, creating the conditions for r-process nucleosynthesis. One such explosion is known as a collapsar.
CollapsaRs
When massive stars (heavier than about 10 solar masses) die, they can form neutron stars or black holes. If a neutron star is formed, most of the star’s outer layers are ejected at large velocities (~10,000 km/s!), producing an energetic explosion that we all know as a supernova. However, if a black hole is formed instead of a neutron star, most of the star can be gobbled up by the black hole and very little material will be ejected. This changes if the star that formed the black hole had a mass of 20–30 solar masses and was spinning really fast. Owing to the large angular momentum of this star, a larger fraction of its total mass can now be ejected. In fact, some of the ejected material can be accelerated to relativistic speeds (comparable to the speed of light), producing a beam of high-energy photons called a gamma-ray burst. In addition to the gamma-ray burst, the remaining ejected material — which is not relativistic but is still moving really fast (~20,000 km/s) — can produce a regular supernova-like explosion. This explosion of a rapidly spinning massive star is known as a collapsar.
The ejected material from collapsars can power gamma-ray bursts and supernovae. The material that falls into the black hole can also do interesting stuff. Because this material has large angular momentum, it can form a disk around the newly formed black hole. Most of this disk will be accreted by the black hole within a few seconds, destroying the disk material, but the accretion process itself can eject a fraction of the disk in the form of winds. It turns out that these disk winds are extremely rich in neutrons and are thus viable sites for r-process nucleosynthesis (i.e., formation of lanthanides and actinides). However, the signatures of this r-process are extremely difficult to observe as they can be masked by other features arising from the supernova explosion. To date, there has been no direct evidence of r-process nucleosynthesis in collapsars.
Radio
Radio observations provide one way to observe the signatures of this elusive r-process in collapsars. Several months after the supernova explosion, disk-wind ejecta that is rich in lanthanides and actinides should interact with the interstellar medium surrounding the black hole. This will produce a radio flare powered by a mechanism known as synchrotron emission, in which electrons spiral around magnetic field lines. This flare should peak several months after the explosion, evolve slowly, and last for several years (Figure 1). Observations of such a radio flare in collapsars several years post-explosion can provide constraints on the amount of r-process elements synthesized in them.
Figure 1: Expected radio emission from the collapsar GRB060505 that occurred in 2006. The theoretical models assume an r-process ejecta mass of 0.1 solar mass (solid lines) and 0.01 solar mass (dotted lines). Different colors represent different model parameters, specifically interstellar medium density profiles and electron distributions. The black arrow marks actual upper limits from Very Large Array observations that can be used to constrain the r-process ejecta mass. [Lee et al. 2022]
Putting It All Together!
The authors of today’s article searched for the late-time radio emission in collapsars. First, they selected 11 collapsars that exploded in the last decade based on the gamma-ray bursts detected by the Swift space satellite. They then looked for possible radio emission at the locations of these supernovae using data from the Karl G. Jansky Very Large Array radio telescopes. Unfortunately, they did not detect any late-time radio flares from these collapsars. However, based on the sensitivity of the data, the team was able to place upper limits on the observed late-time radio emission. They then used theoretical models to derive upper limits on the total r-process material ejected by the collapsar. They found that the collapsars could not have ejected more than 0.2 solar mass of r-process material. For reference, the neutron star merger GW170817 produced about 0.05 solar mass of r-process material. This means that collapsars do not cause significantly more r-process nucleosynthesis than the one neutron star merger we know of.
The authors note that the derived upper limits depend on assumptions of their models about the interstellar medium density profiles, the energy distribution of electrons in the interstellar medium, and the velocity of the disk-wind ejecta. Despite these caveats, their observations place meaningful constraints on the amount of r-process material synthesized in collapsars. Future, more sensitive radio observations can help confirm whether collapsars can synthesize the heaviest lanthanides or actinides or not. This will be important to identify whether collapsars and neutron star mergers can account for the observed r-process elements in the universe, or whether we need to look for more r-process factories.
Original astrobite edited by Sasha Warren.
About the author, Viraj Karambelkar:
I am a second-year graduate student at Caltech. My research focuses on infrared time-domain astronomy. I study dusty explosions and dust enshrouded variable stars using optical and infrared telescopes. I mainly work with data from the Zwicky Transient Facility and the Palomar Gattini-IR telescopes. I love watching movies and plays, playing badminton and am trying hard to improve my chess and crossword skills.