Universe Today
Astronomers have found four new white dwarfs in our solar neighbourhood. Each of them were hiding behind their binary partners, larger and brighter red dwarfs. This made them appear as single stars.
There have been numerous sky surveys over the past decades and none of them found these four white dwarfs. That's nobody's fault: these stars can be very difficult to see. In fact, they were only detected through the wobbling they induce in their binary partners, which was detected spectroscopically.
The discovery is in new research in the Monthly Notices of the Royal Astronomical Society titled "Direct detections of white dwarfs in four WD+dM post-common envelope binaries within 20 pc." The lead author is Professor Mairi O'Brien, a researcher in the Department of Physics at the University of Warwick in the UK.
"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," lead author O'Brien said in a press release. "It's a reminder that even in our own cosmic neighbourhood, we can still find surprises if we look in the right way, at the right wavelengths."
The four are called post-common envelope binaries (PCEBs). In these systems, the pair were binary partners that shared a common envelope when the white dwarf was in its red giant phase.
"Characterizing post-common envelope binaries (PCEBs) containing a white dwarf and a main-sequence companion is essential for improving theories of binary evolution," the authors write. The paper presents the spectroscopic confirmations of four PCEBs within about 65 light years. The spectroscopy was acquired with the Hubble's Space Telescope Imaging Spectrograph (STIS).
White dwarfs are very difficult to detect when close to a red dwarf. Red dwarfs are known for their flaring, which can mimic a white dwarf's light signal. But the white dwarfs induce a slight wobble in their red dwarf partners, and the wobble affects the red dwarfs' rotations. Since the red dwarfs are rotating, the side rotating away from us is red-shifted, while the side facing us is blue-shifted. This shifts slightly due to the wobble, and Hubble's sensitive STIS can detect the shift because of the stars' proximity to each other.
*This artist's illustration shows a red dwarf emitting a powerful flare. Red dwarfs are known for their flaring activity, which can make their binary white dwarf partners very difficult to detect. Image Credit: NASA*
Researchers think there are two paths that create PCEBs and these findings can shed light on them. One is Roche Lobe overflow (RLOF). In this scenario, the white dwarf went through its giant phase and swelled up. Material overflowed from its Roche Lobe and some fell onto its red dwarf companion. But no all material went to its companion. Some formed a common envelope, and that envelop was eventually ejected by the pair, leaving behind the giant's core—the white dwarf—and the red dwarf partner, in a tight binary.
The second way a PCEB is created is through tidal instability, which involves no RLOF. It happens when the primary star expands into its giant phase and the tidal forces aren't powerful enough to keep the two stars tidally locked. The companion red dwarf directly spirals into the primary star's envelope before the primary star can fill its Roche Lobe. Physical forces eject the envelope leaving behind a PCEB.
One of the binary systems is named G 203-47. In this system, the red dwarf rotates once every 100+ days, but orbits the white dwarf every 14.9 days. Normally, these stars would be tidally locked to each other, but they're not. The red dwarf is rotating too slowly for that to happen.
"What's fascinating is that G 203-47 shouldn't be rotating this slowly if it formed the same way as similar systems," said co-author Dr David Wilson, Research Associate at the University of Colorado, Boulder. "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."
Researchers have modelled the local population of white dwarf-red dwarf close in binaries and determined that there should be 4 or 5 of them within 65 light years (20 parsecs). By finding four of them, the team of researchers has validated the theoretical work.
But some think there should be even more of them and that these four are just a small sample of them.
"Only about 30 per cent of red dwarfs within 20 parsecs have been systematically surveyed for hidden white dwarf companions" said co-author Professor Pier-Emmanuel Tremblay, from the Astronomy and Astrophysics Group at the University of Warwick. "We think there could be as many as 9 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."
"We highlighted the need for more M dwarfs to be surveyed for RV measurements, in order to identify more local PCEB systems, and estimated that up to 9–10 new PCEBs could be identified within 20 pc if all M dwarfs were surveyed," the authors write.
