The ancient binary star system was found to have traveled from the outer halo of the Milky Way

The ancient binary star system was found to have traveled from the outer halo of the Milky Way

By Sophie Jenkins

LONDON, UK (SPX) August 26, 2024

A team of researchers has identified a rare binary star system about 10 billion years old that traveled from the far reaches of the Milky Way’s halo to our local stellar neighborhood.

An international team, including experts from the University of Hertfordshire, England, Spain and China, made the discovery while studying near-Earth stars. They identified a pair of stars – a white dwarf and an ultracool companion that are gravitationally bound together. The pair is believed to originate from the Milky Way’s halo, a distant and sparsely populated region around the galaxy.

The white dwarf, named VVV1256-62A, has about half the mass of our Sun and has cooled over billions of years, placing it at the bottom of the white dwarf cooling sequence. White dwarfs form at the end of a star’s life cycle, and because they lack the nuclear fusion that powers other stars, they cool and become redder over time.

Its companion, VVV1256-62B, is a low-metallicity subdwarf, meaning it contains much fewer elements than hydrogen and helium. Such a low metallicity indicates an advanced age of the star, as heavy elements were scarce during the early formation of the Milky Way. This star is particularly noteworthy because it lies at the boundary of interstellar and substellar material, providing valuable insights into metal-poor ultracool atmospheres.

The binary system has a very eccentric orbit, which leads to significant variations in the distance between the two stars. Normally residing in the Milky Way’s halo, the orbits of stars periodically bring them into the Milky Way’s plane, the central region of the galaxy where most of its mass is concentrated.

The ultracool subdwarf was first identified by University of Hertfordshire PhD student Lee Smith due to its large motion. It was later confirmed as an ultracool subdwarf by Zenghua Zhang, a former PhD student at the University of Hertfordshire. The discovery of its white dwarf companion and the binary nature of the system was co-authored by Xian Baig, a current University of Hertfordshire PhD student and Zenghua Zhang, now a faculty member at Nanjing University.

The findings were made possible with support from the Science Technology and Research Council, funding from the University of Hertfordshire and the European Space Agency’s Gaia mission. Several telescopes, including the Gemini South Telescope, the European Southern Observatory’s Largest Telescope, and the Dark Energy Camera Plane Survey of the Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile, were instrumental in confirming the findings.

„These fascinating discoveries open a window into the far reaches of our galaxy. Understanding the connection between the Milky Way’s halo and plane is a step toward understanding how the galaxy formed. The massive, eccentric orbits of these stars are still missing. To be explained, it could be the presence of an inner halo or the Milky Way associated with a past merger with another galaxy. Could be,” said Professor Hugh Jones, Professor of Astronomy at the University of Hertfordshire. „I’m really proud of all the current and former University of Hertfordshire students involved.”

You can read more about the discovery online via the European Space Agency’s webpage on the star system Here.

Research report:Early very low-mass stars and brown dwarfs

Related links

University of Hertfordshire

Stellar chemistry, the universe and everything in it