DTU helps to solve a big question about heavy elements’ origin

Researchers from DTU Space are part of an international team who have discovered that some of the heavy elements – in particular strontium – are formed in neutron star collisions.

The first unequivocal evidence of where some of the heavy elements in our universe are forged has now been found by an international research group led by the University of Copenhagen and with contributions from DTU Space.

For the first time, an element heavier than iron has been clearly detected in connection with collision of two neutron stars, resolving one of the fundamental questions about the composition of our universe.

After thorough analysis of data collected in connection with the observation of the famous neutron star collision in 2017 the researchers have found direct evidence that the heavy element strontium is created in the ensuing explosion. The new results are based on observations of the event with the Very Large Telescope (VLT) at the European Southern Observatory (ESO) in Chile and have just been published in the reputable scientific journal Nature.

“We have now detected strontium formed in this explosion in connection with a neutron star collision and hence directly found where some of the heavy elements in our universe are created”

Daniele Bjørn Malesani, astrophysicist at DTU Space

“It is a satisfying discovery based on a very comprehensive and detailed analysis of our data. We have now detected strontium formed in this explosion in connection with a neutron star collision and hence directly found where some of the heavy elements in our universe are created,” says astrophysicist Daniele Bjørn Malesani at DTU Space at the Technical University of Denmark, who is one of the scientists behind the work now published in Nature.

140 million light years away from Earth

The collision took place some 140 million light years from Earth and was discovered in August 2017 through its gravitational wave signature by the LIGO and VIRGO detectors, and from its gamma-ray emission by satellite based detectors such as ESA’s INTEGRAL and NASA’s Fermi.

This event marked the first time that gravitational waves were detected originating in a neutron star collision, and one of the first times they were detected at all.

Since the 1950s it has been known that hydrogen and helium were formed shortly after the Big Bang, and that heavier elements up to iron are created by nuclear fusion in the core of stars, or when stars explode as supernovae. But iron is only number 26 out of about 90 naturally occurring elements in the periodic table. Where the elements heavier than iron came from has long been a mystery.

“Before this we were unable to identify any specific element created in a neutron star merger. There were theoretical motivation and good circumstantial evidence that heavy elements were created in these events, but the unequivocal proof was missing until now,” explains astrophysicist Darach Watson at the Niels Bohr Institute at the University of Copenhagen, and leader of the international team behind the new discovery.

A long standing question has now been answered

One of the most fundamental questions about the universe has always been: where do the elements of the periodic table come from? For some time researchers have known that some of them form in the envelopes of low-mass stars called AGB stars. But only half of the elements heavier than iron are created this way.

With the new results based on the unique neutron star collision in 2017, the scientists have disclosed the origin of at least one of these. The discovery is based on analysis of the light spectra from the event, that is the decomposition of the detected radiation in its single elements, splitting each wavelength separately.

When a neutron star collision occurs it triggers a phenomenon called a kilonova. Here a fraction of the neutron stars’ combined mass is released and spread into the surrounding space in a giant explosion. Darach Watson and colleagues succeeded in explaining the early light spectra of that kilonova in which the element strontium is prominent.

The researchers next step is to try to identify more elements in the spectra of the kilonova.

“The data have yet more to reveal. A more sophisticated analysis may indicate the presence of other elements, including some heavier than strontium, such as barium and lanthanum,” said Giorgos Leloudas, another astrophysicist at DTU Space and co-author of the new study.

Detecting presence of strontium by analyzing spectres of a kilonova resulting from a neutron star collision

• A neutron star is an extremely compact star consisting mainly of neutrons. It is typically only about 20 km in diameter, but can weigh up to two times more than the Sun.

• When two neutron stars collide, a phenomenon known as a kilonova occurs. Darach Watson and his colleagues realized that the kilonova spectrum could be overall approximated using a so-called ‘blackbody spectrum’, which is the simplest radiation emitted by any hot material. However there were deviations from a pure blackbody which the researchers could then identify as being due to the presence of the heavy element strontium.

• When neutron stars merge, a huge mass of heavy elements is produced. A minimum of 10 Earth masses of strontium is required to explain the observed features, and the amount of strontium produced was likely larger.

• The element strontium is a metal, number 38 in the periodic table. It appears in solid form at room temperature. Its main practical application is in the production of cathode ray tubes for television screens.

Four spectre of light from a kilonova indicating the presense of strontium. (Figure: Niels Bohr Institute/DTU Space)

The figure shows four spectra of a kilonova indicationg the presence of strontium. Here the light is decomposed into its spectral components (split wavelength by wavelength).

From left to right is shown (in angstroms) how much light there is in the blue, in the visible, and in the near-infrared spectre. At all epochs, the spectra show a departure from the simple, thermal kilonova spectrum, due to the presence of the element strontium, which both absorbs and re-emits some of the light.

These deviations are indicated by the red shaded areas. These areas highlight the difference between the observed data (the grey line) and the kilonova model without strontium (the dotted blue line). The kilonova model including the effect of strontium is indicated by the red line.

(Figure: Niels Bohr Institute/DTU Space)

In addition to scientists from the University of Copenhagen (Niels Bohr Institute) and DTU Space, researchers from institutions in Iceland, Italy, England, The Netherlands, Germany and USA have participated in the discovery.

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