Periodic Reporting for period 1 - HEAVYMETAL (How Neutron Star Mergers make Heavy Elements)
Período documentado: 2023-09-01 hasta 2025-02-28
The origin of many of the chemical elements that make up the periodic table has been mapped by cosmologists. We know that only the very lightest elements were made in the Big Bang, while supernovae and stars make the common elements. But where the universe’s heavy elements, such as gold, platinum, and uranium were formed is still not known. The goal of HEAVYMETAL is to understand the processes that forged these heaviest of elements and complete the puzzle of where the elements that make up the earth and the stars originated.
To create these elements requires enormous numbers of free neutrons so that rapid neutron capture can take place to build these elements to their high masses. Theoretically, this process is responsible for forming half of the elements in the periodic table heavier than iron. To date the collision and merger of neutron stars is the only confirmed site where the rapid neutron capture process takes place. We can investigate neutron star mergers and their nucleosynthesis by observing the explosion that accompanies the collision and its aftermath. This explosion and aftermath is called a kilonova, a sudden, short-lived astrophysical transient event whose light is powered by the radioactive, freshly synthesized elements, giving us insight into the elements and the incredible physical conditions of ultra-high density, temperature, and gravitational and electromagnetic fields that they formed in.
But kilonovae are challenging: they are extremely rare and the phenomenon is short-lived, requiring rapid follow-up with large telescopes, the outflow is heavy-element dominated making it extremely demanding to model, and the merger itself covers a huge dynamic range and involves complex nuclear physics. To interpret the spectra we require new atomic data, which does not yet exist for most of the heavy elements. To tackle these challenges, HEAVYMETAL assembles experts in astrophysical observations, hydrodynamical merger simulation, numerical radiative transfer, and laboratory heavy element spectroscopy and atomic structure calculation. With this team we will be able to determine the structures and overall geometries of the merger outflow, the elemental abundances, and their stratification within the ejecta. By the full exploitation of kilonovae we will trace the nucleosynthesis pathways in NS mergers, and provide important insights on heavy nuclei, neutrino interactions, and the nature of high-density matter, and we will chart the role of compact object mergers as the cosmic forge of the heaviest elements.
To create these elements requires enormous numbers of free neutrons so that rapid neutron capture can take place to build these elements to their high masses. Theoretically, this process is responsible for forming half of the elements in the periodic table heavier than iron. To date the collision and merger of neutron stars is the only confirmed site where the rapid neutron capture process takes place. We can investigate neutron star mergers and their nucleosynthesis by observing the explosion that accompanies the collision and its aftermath. This explosion and aftermath is called a kilonova, a sudden, short-lived astrophysical transient event whose light is powered by the radioactive, freshly synthesized elements, giving us insight into the elements and the incredible physical conditions of ultra-high density, temperature, and gravitational and electromagnetic fields that they formed in.
But kilonovae are challenging: they are extremely rare and the phenomenon is short-lived, requiring rapid follow-up with large telescopes, the outflow is heavy-element dominated making it extremely demanding to model, and the merger itself covers a huge dynamic range and involves complex nuclear physics. To interpret the spectra we require new atomic data, which does not yet exist for most of the heavy elements. To tackle these challenges, HEAVYMETAL assembles experts in astrophysical observations, hydrodynamical merger simulation, numerical radiative transfer, and laboratory heavy element spectroscopy and atomic structure calculation. With this team we will be able to determine the structures and overall geometries of the merger outflow, the elemental abundances, and their stratification within the ejecta. By the full exploitation of kilonovae we will trace the nucleosynthesis pathways in NS mergers, and provide important insights on heavy nuclei, neutrino interactions, and the nature of high-density matter, and we will chart the role of compact object mergers as the cosmic forge of the heaviest elements.
We have computed atomic structure and both radiative and collision rate data for a range of ions, as motivated by proposed observational identifications (Sr, Y, Z, Te, W). These new data have been incorporated into atomic models used for our radiative transfer to quantify their impact on early phase spectra. Using these new atomic data we have also been able to quantify requirements for ejected masses of particular ions that would be required, if observed features are correctly attributed to these species.
Using the high quality data on the kilonova AT2017gfo, features are shown to evolve rapidly, even hour-by-hour, due to rapid recombination transitions. Our work also indicates the prominent 1µm P Cygni line in AT2017gfo, which appeared suddenly at approximately 1.17 days, is inconsistent with a He I identification based on its temporal evolution, and reinforces its interpretation as Sr II because the emergence time and early spectral shape of this feature are shown to be consistent with Sr II, providing a direct measurement of the ejecta's ionisation temperature from the Sr III to Sr II recombination under LTE conditions. This derived ionisation temperature was found to be highly consistent with the emitted blackbody radiation temperature, and AT2017gfo appeared isotropic in temperature during the initial days post-merger. Our work also reveals evidence for a fast kilonova ejecta component reaching velocities of 0.40–0.45c and highlights that temporal modelling and high-cadence observations are crucial for constraining ejecta properties and physics.
We performed a large number of neutron star merger simulations modeling all channels of mass ejection and analyzed their nucleosynthesis outcome. Those data were further processed by radiative transfer tools to obtain synthetic spectra for comparison to observational data.
We designed and completed procurement for three new experimental photoabsorption setups, and are making progress on the 4th setup. The first three experiments cover the extreme ultraviolet, visible, near and mid-infrared spectral regions, and the fourth setup, which is under design and final procurement, covers the vacuum ultraviolet to blue spectral region. The visible-NIR experiment has delivered new spectral data, between 700 nm and 1100 nm, on neutral and singly-ionised Y, Zr, Nb, Lu, Hf and Ta. We have designed and are in the process of implementing an integrated experimental control and data reporting system, to support access to the large volumes of experimental spectral data we expect to generate.
Using the high quality data on the kilonova AT2017gfo, features are shown to evolve rapidly, even hour-by-hour, due to rapid recombination transitions. Our work also indicates the prominent 1µm P Cygni line in AT2017gfo, which appeared suddenly at approximately 1.17 days, is inconsistent with a He I identification based on its temporal evolution, and reinforces its interpretation as Sr II because the emergence time and early spectral shape of this feature are shown to be consistent with Sr II, providing a direct measurement of the ejecta's ionisation temperature from the Sr III to Sr II recombination under LTE conditions. This derived ionisation temperature was found to be highly consistent with the emitted blackbody radiation temperature, and AT2017gfo appeared isotropic in temperature during the initial days post-merger. Our work also reveals evidence for a fast kilonova ejecta component reaching velocities of 0.40–0.45c and highlights that temporal modelling and high-cadence observations are crucial for constraining ejecta properties and physics.
We performed a large number of neutron star merger simulations modeling all channels of mass ejection and analyzed their nucleosynthesis outcome. Those data were further processed by radiative transfer tools to obtain synthetic spectra for comparison to observational data.
We designed and completed procurement for three new experimental photoabsorption setups, and are making progress on the 4th setup. The first three experiments cover the extreme ultraviolet, visible, near and mid-infrared spectral regions, and the fourth setup, which is under design and final procurement, covers the vacuum ultraviolet to blue spectral region. The visible-NIR experiment has delivered new spectral data, between 700 nm and 1100 nm, on neutral and singly-ionised Y, Zr, Nb, Lu, Hf and Ta. We have designed and are in the process of implementing an integrated experimental control and data reporting system, to support access to the large volumes of experimental spectral data we expect to generate.
Combining our analyses of the helium abundance in AT2017gfo with merger models / nucleosynthesis, we derived new insights on the merger dynamics, gamma-ray burst central engine and properties of high density matter (Sneppen et al. 2025).
Focussing on tungsten, we showed how combining expertise in new atomic theory with spectral modelling and merger simulations can allow both the strength and shape of observed emission lines to constrain fundamental properties of the kilonova ejecta (McCann et al. 2025).
Probing laser-produced plasmas with supercontinuum laser pulses is a novel approach to atomic photoabsorption. The first report of this has been submitted for publication in Experimental Astronomy. Initial experimental data on Zr and Y plasmas show 100s of spectral lines not currently identified. The UCD & QUB groups are working to identify these using GRASP and Cowan atomic structure codes.
Focussing on tungsten, we showed how combining expertise in new atomic theory with spectral modelling and merger simulations can allow both the strength and shape of observed emission lines to constrain fundamental properties of the kilonova ejecta (McCann et al. 2025).
Probing laser-produced plasmas with supercontinuum laser pulses is a novel approach to atomic photoabsorption. The first report of this has been submitted for publication in Experimental Astronomy. Initial experimental data on Zr and Y plasmas show 100s of spectral lines not currently identified. The UCD & QUB groups are working to identify these using GRASP and Cowan atomic structure codes.