If your
idea of fun is whirling around on a dizzying carnival ride, astronomers have
found a stellar adventure that would stop you in your tracks. A sizzling-hot
star is spinning around at near break-up velocity, according to a new study.
Astronomers
wonder if material will be ejected from the star, called Alpha Arae.
"Alpha Arae
is very close to its breakup velocity, and the matter may freely escape the
equatorial regions, 'launched' by the centrifugal force, as if you were on a
crazy merry-go-round," lead researcher Philippe Stee of the Observatory of Côte
d'Azur in France, told SPACE.com.
Located
about 300 light-years from Earth, Alpha Arae is the nearest "Be
star"--a class of rapidly rotating stars that are very luminous, massive
and hotter than the Sun.
On Aug. 23, 1866, Italian astronomer Father Angelo Secchi
discovered the first Be star, Gamma Cassiopeiae. Since then, the dizzying stars have continued to baffle
astronomers.
Just a
couple 140-year-old puzzlers: How does the ring of gas that surrounds Be stars
form? And what keeps the gas disk in motion?
The new
study results, which will be detailed in an upcoming issue of the journal Astronomy
and Astrophysics, bring astronomers closer to answering these questions.
The need
for speed
Keeping gas
particles swirling together in a disk
requires top speed.
"For
material to be in orbit in a disk, it requires a great deal of angular
momentum," said Ken Gayley, a researcher at the University of Iowa who was not involved in the study.
A source of this rotational velocity would be from the central, rotating star.
Stee and colleagues examined the Be star and its ring of gas with
the European Southern Observatory's Very Large Telescope atop a mountain in Paranal,
Chile. They looked into Alpha Arae's ring with a level of detail equivalent to
distinguishing, from Earth, the headlights of a car on the moon, the astronomers
said.
Prior
observations of Alpha Arae indicated the star, although rapidly rotating, was
not spinning fast enough to supply the required angular
momentum needed to maintain a disk. One limitation had been determining the
star's tilt, or inclination angle, a key component to finding an accurate
rotating speed.
"It is very
difficult to know if the star is rotating slowly or if it is a fast rotator but
seen nearly pole-on," Stee explained. Using their observational data and a
complicated computer model, the team calculated a true rotational velocity.
At its
equator, the star is spinning at 1 million mph (470 kilometers per
second)--near its break-up velocity--and speedy enough to supply the needed
angular momentum to the disk.
This rapid
rotation could actually fling off some of the star's matter.
Still
perplexed
The
astronomers also isolated very small regions within the disk and studied each
region's velocity using the Doppler effect, a compression and expansion of
radiation waves which, with sound, causes a siren to change pitch when an
ambulance races toward you compared to racing away.
"For
instance if the matter is flowing in your direction the emitted light is
shifted to smaller wavelengths [such as blue] whereas if it is flowing away it
is shifted to larger wavelengths [such as red]," Stee explained. The velocity
of the disk material decreases with the square root of the distance from the
star, they found.
Still,
astronomers are perplexed as to what physical processes create the disk
surrounding Be stars.
Many stars
and even some planets flaunt a disk
of gas. Astronomers have found these disks form as a result of the celestial
body tugging interstellar material into orbit around itself. Depending on the
density of surrounding matter, the resulting rings of gas can be either
jam-packed or sparse. For Be stars, the disks are quite dense.
"Such dense
disks generally only appear around stars
that are forming from a region that is already a high density gas, so they draw
from their environment to form disks. But Be stars have cleared out their environment
so it is expected that the disk must come from the star itself, which is
unusual," Gayley said.
The high
spin rate at the equator may cause material to be ejected from the star and
into a swirling disk.
"Critical
rotation may be the clue to the 'Be phenomenon' because we were underestimating
the true stellar rotation measured by the spectroscopy," Stee said.
Boosting
the theory
Many
astronomers have supported a theory that Be stars supply disk-making material
but couldn't prove the spin speed was high enough to fling off that matter.
"But this requires that the underlying star be rotating 'critically'"
which is the crucial finding in the Stee observation. Thus we are optimistic
that this paper is a first step in a process of confirmation of this overall
theoretical view," Gayley said.
The icing
on this stellar cake would be to figure out some of the finer details of this
disk-forming process.
"We still
don't know it the matter around this star has been flinging off by the nearly
critical rotation, or if it will do it again very soon," Stee said. And whether
the star ejects material in a massive outburst or gradually with persistent
loss of matter is still unknown, he said.
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