From École Polytechnique Fédérale de Lausanne and IAC: “A rare star opens a window on the beginning of time”

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From École Polytechnique Fédérale de Lausanne

08.10.18
Sarah Perrin, with AIP

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Dedicated to the search of the most ancient stars in the Milky Way, the Pristine survey recently identified an extremely rare object. ©iStock

EPFL astrophysicists actively participated in the discovery of a very rare star, which is particularly old and metal-poor. As a messenger from the distant past, it will allow the scientists to learn more about the young Universe, right after the Big Bang.

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“We made a major discovery, which questions our understanding of the formation of the first generations of stars in the universe”. Researcher at EPFL Laboratory of astrophysics (LASTRO), Pascale Jablonka is a founding member of the Pristine survey, an international project dedicated to the search of the most ancient and metal-poor stars. It allowed the recent identification of an extremely rare object. Called Pristine 221, it is among the 10 most metal-poor stars known to date in the halo of our Galaxy. Moreover, it is one out of two outstanding stars that are almost totally carbon free. This breakthrough has recently been published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS).

For the study of the early universe, astronomers have different methods at their disposal: One is to look far into the Universe and back in time, to see the first stars and galaxies growing. Another option is to examine the oldest surviving stars of our home galaxy, the Milky Way, for information from the early universe. The Pristine survey, led by the Leibniz Institute for Astrophysics Potsdam (AIP) and the University of Strasbourg, is looking for exactly these pristine stars.

To find these oldest messengers among the overwhelming population of younger stars is no easy task. Just after the Big Bang, the Universe was filled with hydrogen and helium and a bit of lithium. No heavier elements were around, as these are only synthesized in the hot interior of stars – and those did not exist yet. Our Sun has about 2 % of heavier elements in its atmosphere, as can be seen when we make a spectrum of its light. Because of this fact, astrophysicists can conclude that the sun has emerged as part of a later generation of stars – and has “recycled” in its atmosphere the products of the stars that lived long before it and have since died out. In searching for the oldest stars, scientists look for stars with much more pristine atmospheres than our Sun. The more pristine the atmosphere, the earlier the generation in which this star was born. Studying stars of different generations allows us to understand the formation history of the Galaxy – an area of research that therefore is also called near-field cosmology.

Chemical abundance

The Pristine team used a special narrow band filter on the Canada-France-Hawaii Telescope to preselect candidate stars with pristine atmospheres.


CFHT Telescope, Maunakea, Hawaii, USA, at Maunakea, Hawaii, USA,4,207 m (13,802 ft) above sea level

This step was then followed by a detailed spectroscopic campaign with the telescopes of the Isaac Newton Group in Spain and the European Southern Observatory in Chile.

Isaac Newton Group telescopes, at Roque de los Muchachos Observatory on La Palma in the Canary Islands, Spain, at an altitude of 2400m

(IAC telescopes identified below.)

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

EPFL’s researchers Pascale Jablonka and Carmela Lardo formed one out of three teams, with the Paris Observatory and the Instituto de Astrofísica de Canarias, in charge of the spectroscopic analysis and the chemical abundance measurements, that led to the identification of this very special star. They could demonstrate that the star had indeed very few heavy elements in its atmosphere.

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“Pristine 221 spectrum shows a lot of hydrogen lines and very few other elements except a small amount of calcium. This tells us that the star is ultra metal-poor. This unusual lack of heavier elements in its atmosphere means that it probably belongs to an early generation of stars formed in the Galaxy.”

Most elements are depleted by factors 10.000 to 100.000 compared to the Sun. Additionally, its detailed pattern of different elements stands out. Whereas usually extremely metal-poor stars show a very large enhancement in carbon, this star does not. This makes this star the second of its kind and an important messenger from the early Universe“, says Else Starkenburg, researcher at AIP and first author of the study.

“Scientists used to think that carbon was a necessary cooling agent, enabling small fragmentation of the gas cloud from which stars form, and leading to the subsequent formation of low-mass stars in the high-redshift universe, Pascale Jablonka explains. With now two examples of such old and carbon-poor stars, models need to be revised.”

From IAC article:

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Artistic image of the first supernovas in the Milky Way. Pristine 221.8781 + 9.7844 star formed from the material ejected by these first supernovae. Credit: Gabriel Pérez, SMM (IAC).

An international team of researchers, with doctors at the Institute of Astrophysics of the Canary Islands David Aguado, Jonay González and Carlos Allende part, has discovered the existence of a very metal-poor star, one of the oldest in the Milky Way and, therefore, an excellent messenger of the early universe.

Star Pristine 221.8781 + 9.7844 is one of the oldest in the Milky Way. This is known by the chemical composition of the atmosphere. Just after the Big Bang, the universe was filled with hydrogen, helium and some lithium. If there were no heavier elements because they are synthesized inside stars. In the words of researcher David Aguado, “because the atmosphere of the analyzed star is very poor in metals, we can say that is one of the oldest objects in the Milky Way and, of course, before the birth of the sun.” And he adds: “This star is going to help us better understand certain aspects of the origin of the Milky Way and how the first stars were formed.”

To reach these conclusions, have made detailed follow-up studies with ISIS instrument, the William Herschel Telescope, and the IDS instrument on the telescope Isaac Newton, both of the Isaac Newton Group of Telescopes (ING), installed in the Roque de los Muchachos Observatory (Garafía, La Palma). “The spectroscopic images of intermediate resolution, obtained in INT and WHT telescopes on La Palma have enabled us to check the low carbon content, which in this type of stars is usually very abundant,” says Carlos Allende, one of the researchers this project.


ING 4 meter William Herschel Telescope at Roque de los Muchachos Observatory on La Palma in the Canary Islands, 2,396 m (7,861 ft)


NG Isaac Newton 2.5m telescope at Roque de los Muchachos Observatory on La Palma in the Canary Islands, Spain, Altitude 2,344 m (7,690 ft)

This study also used high-resolution spectroscopy obtained with the UVES spectrograph on the VLT (Paranal, ESO).

UVES spectrograph mounted on the VLT at the Nasmyth B focus of UT2

“The spectroscopic data of high resolution UVES on the VLT have led to measure the abundance of lithium in the atmosphere of this star, which provides additional information about the origin of the Universe,” says Jonay González, researcher Ramon y Cajal IAC and collaborator Pristine in the project.

Team on the paper:

Else Starkenburg David S. Aguado, Piercarlo Bonifacio, Elisabetta Caffau, Pascale Jablonka, Carmela Lardo, Nicolas Martin, Ruben Sanchez-Janssen, Federico Sestito, Kim A.Venn, Kris Youakim, Carlos Allende Prieto, Anke Arentsen, Marc Gentile, Jonay I. Gonzalez Hernandez, Collin Kielty, Helmer H. Koppelman, Nicolas Longeard, Eline Tolstoy, Raymond G. Carlberg, Patrick Côté, Morgan Fouesneau, Vanessa Hill, Alan W. McConnachie, Julio F. Navarro.

See the full EPFL article here .
See the full IAC article here.

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