Dead stars in our cosmic backyard: Astronomers spot four white dwarfs hiding under our noses

Researchers at the University of Warwick and the University of Colorado Boulder have directly observed, for the first time, four white dwarfs in binary star systems in our nearby region of space. These stellar binaries are all within 65 light-years of Earth, and one contains the ninth-closest white dwarf to the sun.

The four systems each include a red dwarf companion—a larger, brighter star—making the systems appear to be single-star systems. The new results, published in MNRAS, show that each of these nearby red dwarfs hosts a hidden white dwarf companion.

First author Dr. Mairi O’Brien, a research fellow at the University of Warwick, said, “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. It’s a reminder that even in our own cosmic neighborhood, we can still find surprises if we look in the right way, at the right wavelengths.”

How the hidden stars emerged

Astronomers have conducted detailed surveys of the local neighborhood of stars for decades, but white dwarfs like these have been notoriously hard to find. These four nearby systems were of interest because they showed a substantial radial wobble, a phenomenon in which a star subtly wobbles back and forth, indicating that a massive companion object is orbiting.

Using data from the Hubble Space Telescope’s ultraviolet spectrograph, the team obtained detailed observations of the four systems. White dwarfs usually stand out in ultraviolet observations, but red dwarfs complicate matters because their intense flaring can often mimic a white dwarf signal. The researchers deployed custom calibration techniques to confirm the presence of the four white dwarfs.

A nearby system breaks the pattern

One system, G 203-47, has proven particularly enigmatic. Despite being only 25 light-years away, it took 27 years after its initial radial wobble observation to find the companion white dwarf. It is now the ninth-closest white dwarf to the sun.

G 203-47 is also unusual because its red dwarf rotates once every more than 100 days but orbits its white dwarf every 14.9 days. Normally, gravitational forces would tidally lock them in sync, like the moon, whose same face always points toward Earth. Instead, the red dwarf rotates far too slowly for that to happen.

Co-author Dr. David Wilson, a research associate at the University of Colorado Boulder, said, “What’s fascinating is that G 203-47 shouldn’t be rotating this slowly if it formed the same way as similar systems. This suggests that these binaries have had very different evolutionary histories. Some underwent violent, prolonged interactions early on that locked them tidally. Others, like G 203-47, experienced gentler, briefer encounters that left them in this unusual state.”

The census may grow further

These four new white dwarfs have allowed researchers to update the local white dwarf census within 20 parsecs (65 light-years). Crucially, population models had previously predicted that roughly four to five closely orbiting white dwarf-red dwarf pairs should exist, and the team found exactly four, comparable to the theoretical work.

Professor Pier-Emmanuel Tremblay of the Astronomy and Astrophysics Group at the University of Warwick said, “Only about 30% of red dwarfs within 20 parsecs have been systematically surveyed for hidden white dwarf companions. We think there could be as many as nine or 10 additional binary systems in our local stellar environment that we haven’t found yet. If we put more targeted effort into observing red dwarfs, perhaps we will find more surprises like this.”

Who’s behind this story?

Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.

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Andrew Zinin

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