One of the most satisfying things that an observational astronomer can do is capture the light of an object that theorists had predicted should exist, but no one has previously seen. And I am pleased to say a new group of observers – working with theorists – has just published a paper detailing the first-ever observations of two extremely close binary stars that are in the process of exchanging mass and changing identities.

This work appears in the journal Nature and is led by Kevin Burdge.

This team had a straightforward goal — find pairs of stars that very closely orbit one another, including elder stars in dangerous pairs, where one star is actively consuming its neighbor. Candidates were identified using the Zwicky Transient Facility (ZTF), which looks for things that flicker and flare in the night. One of these stars, cataloged as ZTF J1813+4251, was determined to have a 51-minute orbital period that was consistent with the larger star being deformed into something closer to an ellipsoid than a sphere.

This system appeared to be a pair of stars with the shortest seen orbit. The only way two stars can get this close – and still glow brightly – is if one object is a compact object like a white dwarf, while the other is a regular star that is having its atmosphere pulled off.

These kinds of vampiric systems have been seen many times, but in the past, we’ve only caught white dwarfs in the process of eating hydrogen from the outer atmospheres of their companion stars. According to theory, the white dwarf should keep eating its companion until it has consumed all the star’s hydrogen and even some of the helium that surrounds the stellar core.

But observers had never seen the helium-eating stage. Until now.

This team was able to get observing time on the Keck Telescope to acquire spectra of the system, along with high-quality images with the 10.4-meter Gran Telescopio Canarias. According to Burdge: This is a rare case where we caught one of these systems in the act of switching from hydrogen to helium accretion. People predicted these objects should transition to ultrashort orbits, and it was debated for a long time whether they could get short enough to emit detectable gravitational waves. This discovery puts that to rest.

In the coming millennia, the vampiric white dwarf will consume all accessible material from its companion, and as a result, nuclear reactions in the companion with shut down, and the helium-rich core will be left to shine as a new, very small, white dwarf.

And the vampiric white dwarf may light up as a new star if the mass it has consumed is sufficiently high.

This kind of identity switch is one of the cooler things binary stars can do, and it’s really awesome that we’re now seeing what theory had been predicting.

This is the perfect, Halloween season bit of science. I now want to see a binary star costume at my door, where the little dense object pipes up to say, “I’m a vampire.”

More Information

MIT press release

“A dense 0.1-solar-mass star in a 51-minute-orbital-period eclipsing binary,” Kevin B. Burdge et al., 2022 October 5, Nature