Speedy Stars Weigh Milky Way’s Dark Matter Halo

It’s a bird! It’s a plane! It’s a . . . pair of hypervelocity stars? The surprising stellar duo may place constraints on the mass of our galaxy’s unseen dark matter halo.

A hypervelocity binary system is zooming through the outskirts of the Milky Way galaxy. This image shows its current location as well as that of our Sun.
Thorsten Brand

Here in galactic suburbia, the Sun circles the center at a placid 240 kilometers per second (540,000 mph). But stars out in the wild west of the Milky Way halo disregard order. Rather than following collective circular orbits, they run headlong toward or away from the galactic plane — some outlaws even make a break for it, flying out of the galaxy altogether.

It’s out there, in our galaxy’s halo, that astronomers found two stars racing along at twice the Sun’s speed, at about 500 km/s (1.3 million mph). These aren’t the first so-called hypervelocity stars discovered in the Milky Way, and they’re not the first hypervelocity binary system either. But they’re the first case whose existence has proven so difficult to explain.

A Runaway Star?

The story began in 2011 when astronomers spotted the brighter of the pair, dubbed SDSS J121150.27+143716.2. Those first measurements showed this star to be fleeing the galaxy at 700 km/s.

But new measurements threw that picture into confusion. Péter Németh (Friedrich-Alexander University Erlangen-Nuremberg, Germany) and colleagues split starlight into a sharp visual spectrum. The newer measurements lowered the runaway’s speed down to 500 km/s. And much to the team’s surprise, they also revealed the unmistakable signature of a cooler, dimmer companion star.

The higher-resolution spectra enabled the team to collect better information on the racing pair, now called PB 3877. The brighter star is an older, evolved star known as a hot subdwarf. These unique stars lose their outer envelope of hydrogen while waiting for the core to compress enough to fuse helium — usually a binary companion is involved. In this case, the companion is a dimmer K-type star, an orange dwarf slightly less massive than the Sun. The pair lies between 16,000 and 20,000 light-years from the Sun, and the stars probably orbit each other every few hundred days.

Mysterious Origins

Usually, hypervelocity stars are relatively easy to explain. The supermassive black hole at the center of our galaxy is like a kid with a slingshot — every now and then, it can’t resist flinging a star out into intergalactic space. But for the black hole’s slingshot to work, it must disrupt a binary system, so only one star is ejected. And even if it weren’t for SDSS J1211 companion star, the stars’ trajectory simply doesn’t match an encounter with the galaxy’s central black hole.

So, on to option #2: an asymmetric supernova could give a companion star a kick. But even if the system initially contained three stars, the kick from one stellar explosion would disrupt its companions’ mutual orbit.

There are even more options to consider, but simply put: there is no way this system is a runaway outlaw from the galactic disk.

Instead, the authors conclude, the binary system must be a halo denizen. But that still leaves an important question: was the binary born in the halo eons ago, already set on its current (bound) orbit? Or was it born into a dwarf galaxy that was then swallowed into the larger Milky Way? In that case it may yet leave our galaxy behind.

Watch simultaneous illustrations of the two scenarios in the video below:

Bound to See the Milky Way Halo

Yet Warren Brown (Harvard-Smithsonian Center for Astrophysics), who discovered the first hypervelocity star in 2005, says the authors’ calculations leave little wiggle room for an unbound orbit. “This binary is a very interesting object but is very likely bound to the Milky Way,” Brown concludes.

What makes the binary nevertheless so interesting is the gravitational ties that bind it to the larger whole. The stars’ orbit around the Milky Way provides a way to measure our galaxy’s total mass, including the mass we cannot see.

We already know that roughly a trillion Suns’ worth of mass is locked in the Milky Way’s dark matter halo. But large uncertainties remain, because we can’t see the halo, and we have to guess at its shape and proportions.

The stars’ orbit puts a limit on the halo’s total mass without assuming anything about the dark matter distribution. If the stars are to remain bound to our galaxy, Németh says, the dark matter halo within their orbit must contain at least 3 trillion Suns’ worth of mass — an estimate that’s larger than what previous studies have measured.

The team is following up with additional observations to confirm the system’s orbital properties. And they’re still on the lookout for more of these systems. “Our quest for similar strangers will continue,” Németh says.