A study led by the UPC and the IEEC has determined the mass and radius of one of the oldest stars in our galaxy for the first time

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Artist’s impression of the eclipsing binary system composed of the cool subdwarf (yellow) and the white dwarf (white). Credit: Mark Garlick.

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Portrait of Alberto Rebassa-Mansergas

Precision instrumentation in the Canary Islands and Chile

Using the HiPERCAM instrument mounted on the 10.4-meter Gran Telescopio Canarias (GTC) in La Palma (Spain) and the X-Shooter instrument on the 8.2-meter Unit Telescope 2 of the European Southern Observatory’s Very Large Telescope (VLT) in Chile, the researchers were able to accurately determine the masses and radii of the binary stellar components. With these values, together with the temperature and the luminosity of the cool subdwarf star also obtained from the observations, the authors were able to validate the theoretical mass-radius-effective temperature-luminosity relations for one of the oldest stars in our galaxy for the first time.

A study led by the researcher Alberto Rebassa-Mansergas, from the UPC and the IEEC, has determined the mass and radius of one of the oldest stars in our galaxy for the first time and validated the theoretical mass-radius relations for such stars. The results of this study are published in the journal Nature Astronomy.

Apr 24, 2019

An international team led by the researcher from the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) and the Institute of Space Studies of Catalonia (IEEC) Alberto Rebassa-Mansergas has, for the first time, measured the stellar parameters of a very old kind of star, known as cool subdwarf stars, in our galaxy, the Milky Way. Cool subdwarfs are stars like our sun but of smaller mass and radius that formed during the early stages of the Milky Way; therefore, they carry important information about its structure and chemical evolution. The work has been done in collaboration with researchers from the University of Sheffield and the National Astronomical Observatories of the Chinese Academy of Sciences, and its results are published in the journal Nature Astronomy.

When the Milky Way formed, the first stars were mainly composed of hydrogen. Elements heavier than hydrogen and helium are referred to as ‘metals’ in astronomy and their presence determines the metallicity of a star. As time passed and stars died, the content of such metals in the Milky Way and in the new stars born increased. Therefore old stars have lower metallicities than younger ones.

“Since old stars can reveal important information about the structure and the chemical evolution of the Milky Way, it is essential for astronomers to determine their most basic stellar parameters such as masses and radii”, explains Rebassa-Mansergas, who is an assistant professor and a researcher at the Department of Physics and a member of the Astronomy and Astrophysics Group at the UPC.

Because old stars are faint and relatively rare in the vicinity of the Sun, few cool subdwarfs are known in the solar neighbourhood. Only the radii of eighty-eight cool subdwarfs and the masses of six cool subdwarfs had been estimated to date. However, no mass and radius values for a single cool subdwarf had been measured accurately, leaving theoretical studies of such stars untested until now.

In their work, the researchers found the first cool subdwarf in an eclipsing binary, a system in which two stars orbit one another; in this case, a cool subdwarf and a white dwarf, the remnant of a star like our sun. When one of the stars passes in front of the other from our perspective on Earth, astronomers call this system an eclipsing binary.

“Eclipsing binaries offer the opportunity to measure the masses and radii of the two components directly, with unprecedented precision”, argues Rebassa-Mansergas.

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