Warwick astronomers using the Hubble Space Telescope have found four new 'local' stars, showing us we can still find surprises in the nearby region of space that we thought we knew so well.
Researchers at University of Warwick and University of Colorado Boulder have directly observed, for the first time, four white dwarfs stars orbiting in double star systems in our nearby region of space. These stellar binaries are all located within 65 light-years of Earth, and one of them is the ninth closest white dwarf to our Sun.
The four systems all have red dwarf star companions, larger and brighter stars, making it look like these were single star systems. The new results, published in MNRAS, have identified that each of these nearby red dwarfs stars were hosting a hidden white dwarf companion star.
First author, Dr Mairi O'Brien, Research Fellow, 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 neighbourhood, we can still find surprises if we look in the right way, at the right wavelengths."
Astronomers have performed detailed surveys of the local neighbourhood 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 backwards and forwards, indicating a massive companion object is orbiting.
Using the Hubble Space Telescope's ultraviolet spectrograph data the team then obtained detailed observations from the four systems. White dwarfs usually stand out in ultraviolet observations, but red dwarfs complicate things due to their intense flaring that can often mimic a white dwarf signal. The researchers deployed custom calibration techniques to officially confirm the presence of the four white dwarf stars.
One system, G 203-47, has proven particularly enigmatic. Despite being only 25 light-years away, it has taken 27 years after its initial radial wobble observation to find the companion white dwarf. It is now officially the ninth closest white dwarf to the Sun.
G 203-47 is also unusual because its red dwarf rotates once every 100+ days but orbits its white dwarf every 14.9 days. Normally, gravitational forces would tidally lock them in sync, like the Moon and Earth, where the same face always points toward each other. Instead, the red dwarf rotates far too slowly for that to happen.
Coauthor Dr David Wilson, Research Associate, 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."
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 roughly 4 to 5 closely orbiting white dwarf-red dwarf pairs should exist, and the team found exactly 4, comparable to the theoretical work.
Professor Pier-Emmanuel Tremblay, Astronomy and Astrophysics Group, University of Warwick said: "Only about 30 per cent of red dwarfs within 20 parsecs have been systematically surveyed for hidden white dwarf companions. 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."
