Scientists detect the fastest runaway star ever seen in the Milky Way

The new discovery of six more stars fleeing the Milky Way has landed the fastest object of its type ever detected in the galaxy.

In fact, two of the stars hold records, with heliocentric radial velocities higher than ever seen for runaway stars. The star J1235 travels at 1,694 kilometers (1,053 miles) per second; and J0927 at a staggering 2,285 kilometers (1,420 miles) per second.

But four of the newly measured objects are so-called hypervelocity stars, traveling at speeds exceeding the escape velocity of the Milky Way; and all four, according to a team led by astrophysicist Kareem El-Badry of the Harvard-Smithsonian Center for Astrophysics, are likely the result of spectacular type Ia supernovae, the “standard candles” by which we measure the Universe.

This, they say, allowed a new calculation of the rate at which these stars are born, and found that it is consistent with the estimated rate of Type Ia supernovae. Their findings were detailed in a paper presented at Open Journal of Astrophysicsand available on the arXiv prepress server.

‘A significant population of low-mass fugitives may still await discovery,’ the researchers write.

Whenever a star explodes, the force of the detonation can throw what’s left into space at high speed. Hypervelocity stars are thought to be the product of a special type of supernova that gives the star an even bigger kick than usual, what is known as the dynamic double degeneration double detonation, or D⁶supernova.

This is a scenario to explain what happens during a type Ia supernova.

You have to start with a pair of white dwarf stars in a binary system. These are the residual cores of low-mass stars, up to about eight times the mass of the Sun, that have exhausted their fusion material, expelled most of their mass, and collapsed into a dense core that glows intensely with residual heat . Such objects are known as degenerate stars.

A white dwarf has a mass limit, known as the Chandrasekhar limit, of about 1.4 times that of the Sun. Above that limit, the star becomes unstable, exploding in a Type Ia supernova.

To reach that critical mass, a white dwarf must be in a binary system close enough with another star to gravitationally pull matter away from its companion, becoming more and more massive over time.

What happens depends on the type of companion star. If the white dwarf is mining for hydrogen, it results in a classic nova; you can read how it happens here.

However, if the companion is a white dwarf with a significant surface layer of helium, the cannibal star will absorb it instead.

This creates a more massive layer of helium on the donor star’s surface, which, when it reaches high enough pressure and heat, will begin to rapidly fuse into carbon.

This triggers a thermonuclear explosion, similar to what happens with the hydrogen in the classic nova.

But the detonation of helium goes further: its shock wave triggers a second detonation in the core of the white dwarf, producing a colossal kaboom. This is the double degenerate double detonation, and is thought to positively fly the donor star that hasn’t simply exploded twice as a huge overachiever.

The speeds of these hypervelocity stars are dizzying, exceeding 1,000 kilometers per second. Since something has to travel at 550 kilometers per second to leave the Milky Way, hypervelocity stars are destined for intergalactic space.

But we don’t know how many of them are out there or how often a Type Ia supernova produces a hypervelocity star. Then, El-Badry and his colleagues dug into data from the Gaia Survey, an ongoing project to map the Milky Way with the greatest precision ever, including the own motions of stars as they move through the galaxy.

They found 4 previously unknown hypervelocity stars with a D⁶ origin. That doesn’t sound like a lot, but combined with 10 previously identified hypervelocity stars that have been given a supernova kick, it allows for a much more precise calculation of the actual number of these things that are out there. And there should be more than a few.

In fact, our galaxy should have some speeding stars from other galaxies.

“If a significant fraction of type Ia supernovae produce a D⁶ stars, the Galaxy has probably launched more than 10 million into intergalactic space,” the researchers write.

“An interesting corollary is that there should be large numbers of faint, close D⁶ stars thrown from galaxies throughout the local volume passing through the solar neighborhood”.

There are faster stars in the Milky Way, but their contexts are slightly different. Stars orbiting the supermassive black hole at the center of the galaxy can reach incredible speeds; the fastest moving at an astonishing 24,000 kilometers per second as it approaches the black hole on its long, elliptical orbit.

However, they are gravitationally bound in their orbits and won’t be leaving the galaxy any time soon, unless a wild three-body interaction appears to kick them out.

Previously, the fastest known runaway star was a D⁶ white dwarf binary with a speed of about 2,200 kilometers per second; its heliocentric radial velocity has been measured at 1,200 kilometers per second. This is the velocity as it appears to us observers. J0927 and J1235 could have total speeds of 2,753 and 2,670 kilometers per hour, respectively, according to the researchers.

There may be even faster stars out there. We tend to only find the brightest ones, suggesting that we are missing many. What the new discovery offers us is a significant number of new data points to understand where they are and how to find them.

‘There is now a sizable population of hypervelocity stars associated with thermonuclear supernovae,’ the researchers write.

“Modeling this population will ultimately make it possible to infer the rate of thermonuclear escape formation and, ultimately, the fraction of Type Ia supernovae formed through the double degeneracy channel.

“Our estimate of the birth rate of D⁶ stars is consistent with a scenario in which most type Ia supernovae produce a hypervelocity fleeing white dwarf, but the observed population is dominated by the most massive and luminous escapes. Models for the thermal evolution of D⁶ the stars are needed for more robust estimates of their birth rate.”

The research was submitted to Open Journal of Astrophysicsand is available on arXiv.