Monday, February 26

Scientists Find Fastest Milky Way Runaway Star 2023

The fastest runaway star in the Milky Way was found after six more were found.

Two of the stars had record-breaking heliocentric radial velocities for runaway stars. J1235 travels 1,694 kilometers (1,053 miles) every second, and J0927 2,285 kilometers (1,420 miles).

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

This has allowed them to calculate the birth rate of these stars, which matches the anticipated rate of Type Ia supernovae. The Open Journal of Astrophysics published their findings on arXiv.

“A significant population of fainter low-mass runaways may still await discovery,” researchers conclude.

When a supernova bursts, the force may launch whatever remains into space at incredible speeds. The dynamically driven double-degenerate double-detonation (D6) supernova is expected to provide hypervelocity stars an extra boost.

This describes a Type Ia supernova.

Start with two binary white dwarf stars. These remnant cores are low-mass stars that have run out of fusion material and collapsed into a dense core with residual heat. Degenerate stars.

The Chandrasekhar limit, 1.4 times the Sun’s mass, applies to white dwarfs. Above that limit, the star becomes unstable and explodes in a Type Ia supernova.

A white dwarf must be in a tight binary system with another star to gravitationally draw stuff from its partner and develop to that critical mass.

Depends on partner star type. Read how a classical nova occurs as the white dwarf pulls hydrogen.

If the partner is a white dwarf with a thick helium layer, the cannibal star will drain that instead.

This forms a more massive helium layer on the donor star, which will swiftly fuse into carbon at tremendous pressure and heat.

Like hydrogen in the classical nova, this causes a thermonuclear explosion.

The helium detonation causes a second detonation in the white dwarf’s core, creating a massive explosion. The double-degenerate double-detonation is supposed to send the donor star, which didn’t explode twice like some enormous overachiever, flying.

Hypervelocity stars travel above 1,000 km/s. Hypervelocity stars must depart the Milky Way at 550 kilometers per second to enter intergalactic space.

We don’t know how many hypervelocity stars Type Ia supernovas create. El-Badry and his colleagues examined data from the Gaia survey, which is mapping the Milky Way with unprecedented precision, including the correct movements of stars.

They discovered 4 D6-originated hypervelocity stars. That doesn’t sound like much, but paired with 10 previously detected hypervelocity stars given a supernova push, it allows for a more accurate estimation of their numbers. More are needed.

We should have some speeding stars from distant galaxies.

“If a significant fraction of Type Ia supernovae produce a D6 star, the Galaxy has likely launched more than 10 million of them into intergalactic space,” researchers wrote.

“An interesting corollary is that there should be large numbers of faint, nearby D6 stars launched from galaxies all throughout the local volume passing through the Solar neighborhood.”

Faster Milky Way stars have distinct settings. The fastest star in the galaxy orbits the supermassive black hole at 24,000 kilometers per second.

Unless a violent three-body interaction kicks them out of their orbits, they won’t exit the galaxy.

The fastest runaway star was a D6 white dwarf pair with a heliocentric radial velocity of 1,200 km/s and a velocity of 2,200 km/s. We detect that velocity. The researchers estimated total velocities of 2,753 and 2,670 km/h for J0927 and J1235.

Faster stars may exist. We only locate the brightest, suggesting we are missing many. The new discovery provides many fresh data points for finding them.

“There is now a sizable population of hypervelocity stars associated with thermonuclear supernovae,” researchers write.

Modeling this population will reveal the rate of thermonuclear runaways and the proportion of Type Ia supernovae created by the double-degenerate channel.

Our estimate of D6 star birth rate is compatible with a scenario in which most Type Ia supernovae spawn a hypervelocity runaway white dwarf, but the observable population is dominated by the most massive and brightest runaways. Better birth rate estimates need D6 star thermal evolution models.”

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