Hubble Uncovers Four Hidden White Dwarfs Lurking Beside Nearby Red Dwarf Stars
Ultraviolet spectroscopy from Hubble confirmed four white dwarfs hiding behind red dwarf companions within 65 light-years of Earth, one first suspected 27 years ago.
Overview
Astronomers have confirmed that four white dwarf stars near the Sun have been hiding in plain sight, their faint light drowned out for years by brighter red dwarf companions sharing the same systems, according to ScienceDaily. Using ultraviolet spectroscopy from the Hubble Space Telescope, a team led by researchers at the University of Warwick and the University of Colorado Boulder directly detected the white dwarfs in four star systems — G 203-47, GJ 207.1, LHS 1817, and Wolf 1130 — all within about 65 light-years of Earth, according to the arXiv preprint of the team’s paper.
What We Know
The findings, titled “Direct detections of white dwarfs in four WD+dM post-common envelope binaries within 20 pc,” were published in the Monthly Notices of the Royal Astronomical Society and mark the first direct spectroscopic confirmation of the white dwarf components in these four systems, according to the arXiv preprint. The paper’s authors are Mairi W. O’Brien, David J. Wilson, Pier-Emmanuel Tremblay, Boris T. Gänsicke, Conor M. Byrne, Felipe Lagos-Vilches, J. Sebastian Pineda, and Joaquín A. Barraza-Jorquera, according to the MNRAS journal page.
One of the four, G 203-47, sits about 25 light-years from Earth and is the ninth-closest known white dwarf to the Sun, according to Phys.org. Confirming its white dwarf companion took nearly three decades: the University of Warwick says 27 years passed between the initial detection of a telltale wobble in the system and confirmation of the hidden star.
That wobble was the key clue. “Nearby isolated white dwarfs are usually easy to find, but we couldn’t see these four stars directly in visible wavelengths because their red dwarf companions were drowning out their light,” said Dr. Mairi O’Brien, a research fellow at the University of Warwick, according to ScienceDaily. Astronomers instead tracked subtle back-and-forth motion in the red dwarfs caused by the gravitational pull of their unseen companions, then used Hubble’s ultraviolet spectrograph, STIS, to separate the faint white dwarf light from the much brighter red dwarf glow, according to the arXiv preprint. Researchers also applied custom calibration techniques to rule out red dwarf flaring, which can mimic a white dwarf’s signal, according to Phys.org.
The paper reports effective temperatures for each white dwarf: about 5,286 Kelvin for G 203-47, roughly 6,100 Kelvin for GJ 207.1, about 6,300 Kelvin for LHS 1817, and roughly 6,100 Kelvin for Wolf 1130, according to the arXiv preprint. The same paper lists orbital periods ranging from about 14.9 days for G 203-47 down to roughly 0.3 days for LHS 1817, and places the four systems between roughly 7.6 and 16.6 parsecs from Earth using Gaia DR3 distance measurements, according to the MNRAS journal page.
G 203-47 stood out for another reason: its red dwarf orbits the hidden white dwarf every 14.9 days but takes more than 100 days to complete a single rotation, a mismatch that surprised the team, according to ScienceDaily. “What’s fascinating is that G 203-47 shouldn’t be rotating this slowly if it formed the same way as similar systems,” said Dr. David Wilson, a research associate at the University of Colorado Boulder, according to ScienceDaily. Follow-up X-ray observations added another layer to the puzzle: “The unexpectedly weak X-ray signal suggests that the red dwarf companion is rotating much more slowly than the binary orbital period,” Wilson said, according to CU Boulder’s Laboratory for Atmospheric and Space Physics, which noted the team supplemented the Hubble data with observations from the Swift X-ray Observatory.
The discovery also lines up with existing theory. Astronomers had predicted roughly four to five close white dwarf–red dwarf binaries should exist within 65 light-years of the Sun, and finding exactly four validates those estimates, according to ScienceDaily. Even so, the survey has covered only a fraction of the nearby sky: “Only about 30 per cent of red dwarfs within 20 parsecs have been systematically surveyed for hidden white dwarf companions,” said Professor Pier-Emmanuel Tremblay of the University of Warwick’s Astronomy and Astrophysics Group, according to the University of Warwick.
What We Don’t Know
Researchers estimate that as many as nine or ten additional white dwarf–red dwarf binaries may still be hiding undetected in the Sun’s immediate neighborhood, according to ScienceDaily. Why G 203-47’s red dwarf rotates so much more slowly than its orbital period, rather than being tidally locked as is common in such tight binaries, remains unresolved in the published findings, according to the arXiv preprint.