Fastest Orbiting Stars Circle Each Other in Mere Minutes (SPACE.com)

After

a decade of mystery, astronomers have now shown that a pair of white dwarf

stars spin around each other in just 5.4 minutes, making them the

fastest-orbiting and tightest binary star system ever found, the researchers

claim.

The

record-setting stellar duo, known as HM Cancri or RX J0806.3+1527, offer challenges in explaining

how such a system might form. The super-quick stars may also present a great

future test-bed for detecting gravitational

waves, which are elusive ripples

in space-time.

Researchers

say the stars in HM Cancri are so near to one another – about a quarter of the

distance between the Earth and the moon – that they could not get much closer

without smushing together. 

"A

five-minute orbit is about as close as two white dwarf stars can get without

merging," said Danny Steeghs, an astrophysicist at the University of

Warwick, and co-author of a new paper describing the results.

Dying

stars

White

dwarfs are the white-hot cores left over from stars that have sloughed off their

outer, cooler layers while aging several billion years longer than our sun.

Paradoxically,

smaller white

dwarfs are more massive than bigger, puffier white dwarfs, given these

stars' extreme densities.

In

the case of HM Cancri, this phenomenon has led to the smaller, though heavier

star gravitationally stealing matter from its nearby companion. A ring of gas

has likely formed around the smaller star, while altering the appearance of the

donor dwarf. 

"The

more massive star is distorting the shape of the lighter one into that of a

pear, or a raindrop structure," said paper co-author Arne Rau, a post-doctoral

researcher in astrophysics at the Max-Planck Institute for Extra-Terrestrial

Physics in Germany.

Matter

striking the heavier star unleashes bursts of light and energy, which is what

first drew astronomers' attention to HM Cancri more than 10 years ago. This

periodic brightening as the stars revolve about each other implied the

astounding 5.4 minute orbital period, which astronomers initially were at a

loss to explain.

Several

alternative scenarios about HM Cancri, such as magnetic field interactions of

more typically spaced-out, slower dwarfs, had been proposed over the years. Bad

weather had scuttled many previous attempts to get a sharper look at HM Cancri.

Upon

Closer Inspection

To

finally nail down the orbital period, an international team of astronomers made

fresh, clear observations of the stellar duo last year with the Keck I

telescope atop Mauna Kea, Hawaii.

The

new, detailed spectroscopic measurements showed a shift in HM

Cancri's light that fits with the model of two stars whirling around each other

nearly every five minutes. (By the time one finishes reading this article, the

pair will have made a furious revolution.) The next fastest-orbiting pair comes

in at a comparatively slow-poke 9.6 minutes, almost twice as long. (For

comparison, Earth takes a year to go around the sun.)

Simon

Albrecht, a postdoctoral researcher at MIT who studies binary star systems but

was not involved in this research, says he finds the data analysis "thorough,"

though researchers have had to make a few assumptions about the nature of the

HM Cancri system.

Exactly

how HM Cancri got to look the way the paper's authors think it does now remains

rather puzzling, said Steeghs. The stars that smoldered down into HM Cancri's

white dwarfs must have initially been much farther apart, given how little

space remains between them now.

Two

stars similar to our sun, yet with one initially about twice as massive as the

other must have ended up having their life cycles timed so that this evolution

could occur, Steeghs said. Further study of HM Cancri may inform models of how

binaries interact with each other over lifetimes that last billions of years.

Gravitational

Wave Maker

In

any case, as they have wound closer and continue along at their current

breakneck pace, HM Cancri's stars should shed energy in the form of

gravitational waves. These ripples in space-time were first predicted by Albert

Einstein as part of his general

theory of relativity in 1915, but have yet to be directly detected.

Given

HM Cancri's properties and relative closeness within the Milky

Way galaxy, Rau thinks the two quick stars' gravitational waves will be

right in the range of frequencies detectable by the space-based Laser

Interferometer Space Antenna (LISA), set to launch later this decade.

"HM

Cancri has the shortest orbital period we know, and is therefore the most

interesting system to find gravitational waves," Rau told SPACE.com.

The

research was published in the March 10 issue of Astrophysical Journal Letters.

Taken from here