The discovery might be historic: an international team of astronomers has identified an ancient pair of stars that have made their way from the farthest reaches of the Milky Way into our local stellar neighborhood. The researchers have reported that this binary star system, consisting of a white dwarf and an ultracool subdwarf, is around 10 billion years old, right from the early days of our galaxy’s existence.
Scientists from the University of Hertfordshire were observing stars relatively near to Earth when they came across this incredible pair. Star VVV1256-62A and VVV1256-62B have traveled from the Milky Way halo, a distant and thinly populated region, onto the plane of the Milky Way where most of the galaxy’s mass is situated.
VVV1256–62A is only about half as massive as our Sun and has cooled considerably over billions of years, putting it close to the end of the white dwarf cooling sequence. White dwarfs represent the remnants left over from ordinary stars; since they do not generate energy from active fusion, they gradually cool and redden with time.
Its companion, VVV1256-62B, is a low-metallicity subdwarf and hence formed during an epoch when heavier elements were just being created. This subdwarf is located near the boundary between stellar and substellar objects and hence serves as an important benchmark for the study of metal-poor ultracool atmospheres.
The most interesting thing, perhaps, is that the binary system is in an extremely high eccentricity orbit, suggesting that the stars spent most of their time far away from the visible stars in our galaxy. According to Professor Hugh Jones, of the University of Hertfordshire, this could have resulted from an inner halo or even a past merger of the Milky Way with another galaxy.
“These are exciting discoveries that open a window onto what’s happening in the outer reaches of our galaxy,” Jones said. That’s because realizing the relation between that halo and the plane of the Milky Way is crucial to understanding the way the galaxy formed.
The discovery was enabled by the Science Technology and Research Council, the European Space Agency’s Gaia mission, and by telescopes around the world, including the Gemini South telescope and the Very Large Telescope of the European Southern Observatory.
The work has been published in the Monthly Notices of the Royal Astronomical Society as a series entitled “Primeval very low-mass stars and brown dwarfs.” This series deals with the discovery and characterization of ultracool subdwarfs that have enlightened us about the early phases of our galaxy’s history.
The halo of the Milky Way is still a mysterious place that holds many secrets to the history of the galaxy. VVV1256-62AB is among those systems currently being studied by scientists in an attempt to unravel the serious and complex history of our cosmic neighborhood and the forces that have been shaping it for billions of years.