Artist’s impression of strontium emerging from a neutron star merger
X-shooter spectra montage of kilonova in NGC 4993
The galaxy NGC 4993 in the constellation of Hydra
The sky around the galaxy NGC 4993
Videos
ESOcast 210 Light: First identification of a heavy element born from neutron star collision
Neutron star merger animation and elements formed in these events
Animation of spectra of kilonova in NGC 4993
For the first time, a freshly made heavy
element, strontium, has been detected in space, in the aftermath of a
merger of two neutron stars. This finding was observed by ESO’s
X-shooter spectrograph on the Very Large Telescope (VLT) and is
published today in Nature. The detection confirms that the heavier
elements in the Universe can form in neutron star mergers, providing a
missing piece of the puzzle of chemical element formation.
In 2017, following the detection of gravitational waves passing the Earth, ESO pointed its telescopes in Chile, including the VLT, to the source:
a neutron star merger named GW170817. Astronomers suspected that, if
heavier elements did form in neutron star collisions, signatures of
those elements could be detected in kilonovae, the explosive aftermaths
of these mergers. This is what a team of European researchers has now
done, using data from the X-shooter instrument on ESO’s VLT.
Following the GW170817 merger, ESO’s fleet of telescopes began monitoring the emerging kilonova explosion over a wide range of wavelengths. X-shooter in particular took a series of spectra
from the ultraviolet to the near infrared. Initial analysis of these
spectra suggested the presence of heavy elements in the kilonova, but
astronomers could not pinpoint individual elements until now.
“By reanalysing the 2017 data from the merger, we have
now identified the signature of one heavy element in this fireball,
strontium, proving that the collision of neutron stars creates this
element in the Universe,” says the study’s lead author Darach
Watson from the University of Copenhagen in Denmark. On Earth, strontium
is found naturally in the soil and is concentrated in certain minerals.
Its salts are used to give fireworks a brilliant red colour.
Astronomers have known the physical processes that create
the elements since the 1950s. Over the following decades they have
uncovered the cosmic sites of each of these major nuclear forges, except
one. “This is the final stage of a decades-long chase to pin down the origin of the elements,” says Watson. “We
know now that the processes that created the elements happened mostly
in ordinary stars, in supernova explosions, or in the outer layers of
old stars. But, until now, we did not know the location of the final,
undiscovered process, known as rapid neutron capture, that created the
heavier elements in the periodic table.”
Rapid neutron capture is a process in which an atomic
nucleus captures neutrons quickly enough to allow very heavy elements to
be created. Although many elements are produced in the cores of stars,
creating elements heavier than iron, such as strontium, requires even
hotter environments with lots of free neutrons. Rapid neutron capture
only occurs naturally in extreme environments where atoms are bombarded
by vast numbers of neutrons.
“This is the first time that we can directly associate
newly created material formed via neutron capture with a neutron star
merger, confirming that neutron stars are made of neutrons and tying the
long-debated rapid neutron capture process to such mergers,” says Camilla Juul Hansen from the Max Planck Institute for Astronomy in Heidelberg, who played a major role in the study.
Scientists are only now starting to better understand
neutron star mergers and kilonovae. Because of the limited understanding
of these new phenomena and other complexities in the spectra that the
VLT’s X-shooter took of the explosion, astronomers had not been able to
identify individual elements until now.
“We actually came up with the idea that we might be
seeing strontium quite quickly after the event. However, showing that
this was demonstrably the case turned out to be very difficult. This
difficulty was due to our highly incomplete knowledge of the spectral
appearance of the heavier elements in the periodic table,” says University of Copenhagen researcher Jonatan Selsing, who was a key author on the paper.
The GW170817 merger was the fifth detection of gravitational waves, made possible thanks to the NSF's Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US and the Virgo Interferometer in Italy. Located in the galaxy NGC 4993, the merger was the first, and so far the only, gravitational wave source to have its visible counterpart detected by telescopes on Earth.
With the combined efforts of LIGO, Virgo and the VLT, we
have the clearest understanding yet of the inner workings of neutron
stars and their explosive mergers.
More Information
This research was presented in a paper to appear in Nature on 24 October 2019.
The team is composed of D. Watson (Niels Bohr Institute
& Cosmic Dawn Center, University of Copenhagen, Denmark), C. J.
Hansen (Max Planck Institute for Astronomy, Heidelberg, Germany), J.
Selsing (Niels Bohr Institute & Cosmic Dawn Center, University of
Copenhagen, Denmark), A. Koch (Center for Astronomy of Heidelberg
University, Germany), D. B. Malesani (DTU Space, National Space
Institute, Technical University of Denmark, & Niels Bohr Institute
& Cosmic Dawn Center, University of Copenhagen, Denmark), A. C.
Andersen (Niels Bohr Institute, University of Copenhagen, Denmark), J.
P. U. Fynbo (Niels Bohr Institute & Cosmic Dawn Center, University
of Copenhagen, Denmark), A. Arcones (Institute of Nuclear Physics,
Technical University of Darmstadt, Germany & GSI Helmholtzzentrum
für Schwerionenforschung, Darmstadt, Germany), A. Bauswein (GSI
Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany &
Heidelberg Institute for Theoretical Studies, Germany), S. Covino
(Astronomical Observatory of Brera, INAF, Milan, Italy), A. Grado
(Capodimonte Astronomical Observatory, INAF, Naples, Italy), K. E.
Heintz (Centre for Astrophysics and Cosmology, Science Institute,
University of Iceland, Reykjavík, Iceland & Niels Bohr Institute
& Cosmic Dawn Center, University of Copenhagen, Denmark), L. Hunt
(Arcetri Astrophysical Observatory, INAF, Florence, Italy), C.
Kouveliotou (George Washington University, Physics Department,
Washington DC, USA & Astronomy, Physics and Statistics Institute of
Sciences), G. Leloudas (DTU Space, National Space Institute, Technical
University of Denmark, & Niels Bohr Institute, University of
Copenhagen, Denmark), A. Levan (Department of Physics, University of
Warwick, UK), P. Mazzali (Astrophysics Research Institute, Liverpool
John Moores University, UK & Max Planck Institute for Astrophysics,
Garching, Germany), E. Pian (Astrophysics and Space Science Observatory
of Bologna, INAF, Bologna, Italy).
ESO is the foremost intergovernmental astronomy
organisation in Europe and the world’s most productive ground-based
astronomical observatory by far. It has 16 Member States: Austria,
Belgium, the Czech Republic, Denmark, France, Finland, Germany, Ireland,
Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland
and the United Kingdom, along with the host state of Chile and with
Australia as a Strategic Partner. ESO carries out an ambitious programme
focused on the design, construction and operation of powerful
ground-based observing facilities enabling astronomers to make important
scientific discoveries. ESO also plays a leading role in promoting and
organising cooperation in astronomical research. ESO operates three
unique world-class observing sites in Chile: La Silla, Paranal and
Chajnantor. At Paranal, ESO operates the Very Large Telescope and its
world-leading Very Large Telescope Interferometer as well as two survey
telescopes, VISTA working in the infrared and the visible-light VLT
Survey Telescope. Also at Paranal ESO will host and operate the
Cherenkov Telescope Array South, the world’s largest and most sensitive
gamma-ray observatory. ESO is also a major partner in two facilities on
Chajnantor, APEX and ALMA, the largest astronomical project in
existence. And on Cerro Armazones, close to Paranal, ESO is building the
39-metre Extremely Large Telescope, the ELT, which will become “the
world’s biggest eye on the sky”.
Links
Contacts
Darach Watson
Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen
Copenhagen, Denmark
Cell: +45 24 80 38 25
Email: darach@nbi.ku.dk
Camilla J. Hansen
Max Planck Institute for Astronomy
Heidelberg, Germany
Tel: +49 6221 528-358
Email: hansen@mpia.de
Jonatan Selsing
Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen
Copenhagen, Denmark
Cell: +45 61 71 43 46
Email: jselsing@nbi.ku.dk
Bárbara Ferreira
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6670
Email: pio@eso.org
