Hubble confirms four nearby white dwarfs masked by red dwarf partners

Astronomers have confirmed four white dwarf stars in nearby binary systems after their light was obscured by brighter red dwarf companions. The findings matter because they sharpen the census of stars close to the Sun and suggest more compact stellar remnants may be missing from current surveys, according to the University of Warwick.

The work was carried out by researchers from the University of Warwick and the University of Colorado Boulder and published in the Monthly Notices of the Royal Astronomical Society, according to Warwick. All four systems are within 20 parsecs, or about 65 light-years, of Earth, the university said.

Red dwarfs hid the fainter stars

White dwarfs are dense remnants left behind after stars exhaust their fuel, while red dwarfs are cool, common stars that can dominate a system’s visible-light appearance. Warwick said each newly confirmed white dwarf shares a binary system with a red dwarf that made the pair look like a single visible star.

Dr. Mairi O’Brien, a University of Warwick research fellow and the study’s first author, said nearby isolated white dwarfs are usually easier to identify, but these four were missed in visible wavelengths because their red dwarf partners overwhelmed them. She said the result shows that close stellar systems can still evade detection unless astronomers observe them at suitable wavelengths.

The systems had drawn attention because the visible red dwarfs showed a radial wobble, Warwick said. That motion, seen as a slight shift toward and away from Earth, can indicate that a star is being tugged by an unseen orbiting companion.

Ultraviolet observations made the detections

The researchers used ultraviolet spectrograph data from the Hubble Space Telescope to study the suspected binaries, according to Warwick. White dwarfs stand out more readily in ultraviolet light, but the team also had to account for red dwarf flares that can mimic a white dwarf signal, the university said.

Warwick said the researchers developed calibration methods to distinguish flare activity from the signatures of the compact stars. Their analysis confirmed white dwarf companions in all four systems.

One system, G 203-47, is about 25 light-years away and contains what Warwick described as the ninth-nearest known white dwarf to the Sun. The university said astronomers first detected the system’s radial wobble 27 years before the white dwarf was confirmed.

G 203-47 also does not rotate as expected for its tight orbit, according to the researchers. Warwick said the red dwarf takes more than 100 days to spin once, even though it orbits the white dwarf every 14.9 days.

Dr. David Wilson, a University of Colorado Boulder research associate and study co-author, said the slow rotation suggests systems like these did not all evolve in the same way. Warwick said some may have gone through stronger, longer early interactions that tidally locked the stars, while G 203-47 may have had a shorter or gentler interaction.

Survey leaves room for more finds

The four confirmed systems match earlier population estimates, Warwick said. Models had predicted roughly four to five close white dwarf-red dwarf binaries within 20 parsecs, and the study identified four.

Professor Pier-Emmanuel Tremblay of Warwick’s Astronomy and Astrophysics Group said only about 30% of red dwarfs within that distance have been systematically checked for hidden white dwarf companions. He said there may be as many as nine or 10 more such binary systems nearby if astronomers carry out more targeted observations.

This story draws on original reporting from ScienceDaily.