Researchers recently identified four white dwarfs, dead stars, that were hidden by the intense glow of their red dwarf companions.
This is the first time that white dwarfs have been located in double star systems so close to Earth. All of them are approximately 65 light years away, with one of them being among the ten closest white dwarfs to our solar system.
White dwarfs are stellar remnants that emerge after stars similar in size to the Sun exhaust their fuel for nuclear fusion, resulting in the collapse of their cores. Without fusion, these celestial bodies cool and become extremely faint, which allows the intense light from red dwarfs, which are larger and more luminous, to hide them very effectively.
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“Normally, nearby isolated white dwarfs are simple to detect, but we were unable to observe these four directly at visible wavelengths as their red dwarf companions eclipsed their light,” said Mairi O’Brien, leader of the research team. She added that the discovery serves as a reminder that even in our cosmic neighborhood, “we can still find surprises if we look the right way, at the right wavelengths.”
Unexpected oscillations reveal hidden stars
Despite decades of intensive scans of our cosmic neighborhood, white dwarfs are masters of remaining invisible. The only clue that gave them away were peculiar “oscillations” in the movement of the stars behind which they hid, a phenomenon comparable to the movement of a curtain caused by a child at play.
The team investigated these revealing clues using the Hubble Space Telescope. The analysis was carried out in ultraviolet light, employing a special calibration to prevent the brightnesses of the red dwarfs from being confused with signals from the white dwarfs.
In addition to exposing the four hidden white dwarfs, this investigation revealed that one of the systems, G 203-47, located just 25 light-years away, has intriguing characteristics. A gap of 27 years passed between the detection of the initial radial oscillation and the location of this hidden dead star.
The most peculiar fact, however, is that the red dwarf companion of G 203-47 takes about 100 Earth days to complete one rotation, but orbits its dead star in just 15 days. This indicates that gravitational forces were unable to lock the two stars together, a common phenomenon in similar systems.
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“It is fascinating that G 203-47 should not rotate so slowly if its formation had occurred in the same way as similar systems. This suggests that these binaries had distinct evolutionary histories,” commented David Wilson, a member of the research team. He detailed that some systems experienced violent and prolonged interactions at their inception, which locked them gravitationally, while others, such as G 203-47, had gentler and briefer encounters, resulting in this unusual state.
The discovery of these white dwarfs helps researchers better understand the number of these dead stars throughout the Milky Way. Predictions pointed to the existence of four to five pairs of white and red dwarfs in close orbit within 65 light years of our solar system. Thus, confirmation of four of these systems significantly reinforces confidence in current theoretical models.
“Only about 30% of red dwarfs within 20 parsecs [65 light years] have been systematically investigated for hidden white dwarf companions,” said team researcher Pier-Emmanuel Tremblay. He estimated that “we could have as many as nine or ten additional binary systems in our local stellar environment that have not yet been discovered.” Tremblay concluded by suggesting that, with a more focused observation effort on red dwarfs, “perhaps we will find more surprises like this.”