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.
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