From European Southern Observatory: “First identification of a heavy element born from neutron star collision”

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From European Southern Observatory

23 October 2019

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

Newly created strontium, an element used in fireworks, detected in space for the first time following observations with ESO telescope.

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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.

ESO X-shooter on VLT on UT2 at Cerro Paranal, Chile


ESO X-shooter on VLT on UT2 at Cerro Paranal, Chile

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.

MIT /Caltech Advanced aLigo


VIRGO Gravitational Wave interferometer, near Pisa, 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.

Notes

[1] The LIGO–Virgo detection localised the source to an area on the sky of about 35 square degrees.

Caltech/MIT Advanced aLigo Hanford, WA, USA installation

Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

VIRGO Gravitational Wave interferometer, near Pisa, Italy

[2 The galaxy was only observable in the evening in August and then was too close to the Sun in the sky to be observed by September.

[3] On the VLT, observations were taken with: the X-shooter spectrograph located on Unit Telescope 2 (UT2); the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) and Nasmyth Adaptive Optics System (NAOS) – Near-Infrared Imager and Spectrograph (CONICA) (NACO) on Unit Telescope 1 (UT1); VIsible Multi-Object Spectrograph (VIMOS) and VLT Imager and Spectrometer for mid-Infrared (VISIR) located on Unit Telescope 3 (UT3); and the Multi Unit Spectroscopic Explorer (MUSE) and High Acuity Wide-field K-band Imager (HAWK-I) on Unit Telescope 4 (UT4). The VST observed using the OmegaCAM and VISTA observed with the VISTA InfraRed CAMera (VIRCAM). Through the ePESSTO programme, the collected visible spectra with the ESO Faint Object Spectrograph and Camera 2 (EFOSC2) spectrograph and infrared spectra with the Son of ISAAC (SOFI) spectrograph. The MPG/ESO 2.2-metre telescope observed using the Gamma-Ray burst Optical/Near-infrared Detector (GROND) instrument.

ESO FORS2 VLT mounted on Unit Telescope 1 (Antu)

ESO/NACO on Unit Telescope 1 (UT1); VIsible Multi-Object Spectrograph (VIMOS) and VLT Imager and Spectrometer for mid-Infrared (VISIR) located on Unit Telescope 3 (UT3

ESO/VISIR on UT3 of the VLT

ESO MUSE on the VLT on Yepun (UT4)

ESO HAWK-I on the ESO VLT on Unit Telescope 4 (UT4)

ESO OmegaCAM on VST at ESO’s Cerro Paranal observatory,with an elevation of 2,635 metres (8,645 ft) above sea level

VIRCAM on the VISTA telescope

ESO Faint Object Spectrograph and Camera 2 (EFOSC2) on the NTT

ESO SofI Instrument is the infrared imaging camera on the VST

ESO GROND imager on 2.2 meter MPG/ESO telescope at LaSilla

[4] The comparatively small distance between Earth and the neutron star merger, 130 million light-years, made the observations possible, since merging neutron stars create weaker gravitational waves than merging black holes, which were the likely case of the first four gravitational wave detections.

[5] When neutron stars orbit one another in a binary system, they lose energy by emitting gravitational waves. They get closer together until, when they finally meet, some of the mass of the stellar remnants is converted into energy in a violent burst of gravitational waves, as described by Einstein’s famous equation E=mc2.

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).

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ESO La Silla HELIOS (HARPS Experiment for Light Integrated Over the Sun)

ESO/HARPS at La Silla

ESO 3.6m telescope & HARPS at Cerro LaSilla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres.

MPG/ESO 2.2 meter telescope at Cerro La Silla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres

ESO/Cerro LaSilla, 600 km north of Santiago de Chile at an altitude of 2400 metres.

ESO VLT at Cerro Paranal in the Atacama Desert, •ANTU (UT1; The Sun ),
•KUEYEN (UT2; The Moon ),
•MELIPAL (UT3; The Southern Cross ), and
•YEPUN (UT4; Venus – as evening star).
elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo,

2009 ESO VLTI Interferometer image, Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level, •ANTU (UT1; The Sun ),
•KUEYEN (UT2; The Moon ),
•MELIPAL (UT3; The Southern Cross ), and
•YEPUN (UT4; Venus – as evening star).

ESO VLT 4 lasers on Yepun

Glistening against the awesome backdrop of the night sky above ESO_s Paranal Observatory, four laser beams project out into the darkness from Unit Telescope 4 UT4 of the VLT.

ESO/NTT at Cerro La Silla, Chile, at an altitude of 2400 metres

ESO VLT Survey telescope

Part of ESO’s Paranal Observatory, the VISTA Telescope observes the brilliantly clear skies above the Atacama Desert of Chile. Credit: ESO/Y. Beletsky, with an elevation of 2,635 metres (8,645 ft) above sea level

ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

ESO/E-ELT,to be on top of Cerro Armazones in the Atacama Desert of northern Chile. located at the summit of the mountain at an altitude of 3,060 metres (10,040 ft).

ESO/APEX high on the Chajnantor plateau in Chile’s Atacama region, at an altitude of over 4,800 m (15,700 ft)

Leiden MASCARA instrument, La Silla, located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)

Leiden MASCARA cabinet at ESO Cerro la Silla located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)

ESO Next Generation Transit Survey at Cerro Paranel, 2,635 metres (8,645 ft) above sea level

ESO Speculoos telescopes four 1m-diameter robotic telescopes at ESO Paranal Observatory 2635 metres 8645 ft above sea level

ESO TAROT telescope at Paranal, 2,635 metres (8,645 ft) above sea level

ESO ExTrA telescopes at Cerro LaSilla at an altitude of 2400 metres

A novel gamma ray telescope under construction on Mount Hopkins, Arizona. a large project known as the Cherenkov Telescope Array, composed of hundreds of similar telescopes to be situated in the Canary Islands and Chile. The telescope on Mount Hopkins will be fitted with a prototype high-speed camera, assembled at the University of Wisconsin–Madison, and capable of taking pictures at a billion frames per second. Credit: Vladimir Vassiliev