Brown dwarf pair mystifies astronomers
David
Brown dwarf pair mystifies astronomers
December 21, 2009 Brown dwarf pair mystifies astronomers
(PhysOrg.com) -- Two brown dwarf-sized objects orbiting a giant old
star show that planets may assemble around stars more quickly and
efficiently than anyone thought possible, according to an
international team of astronomers.
"We have found two brown dwarf-sized masses around an ordinary star,
which is very rare," said Alex Wolszczan, Evan Pugh professor of
astronomy and astrophysics, Penn State and lead scientist on the
project.
The star, BD +20 2457, is a K2 giant -- an old bloated star nearing
the end of its life. Seeing a pair of brown dwarfs around a K-type
giant is a first for astronomers and offers a unique window into how
they can be produced. The researchers from the Torun Center for
Astronomy, Poland and the Center for Exoplanets and Habitable Worlds,
Penn State report their findings in the current issue of the
Astrophysical Journal.
Brown dwarfs are dim, elusive objects that straddle the dividing line
between planets and stars. They are too massive to be planets, but not
massive enough to generate the fusion-powered energy of a star. These
stellar cousins represent a kind of "missing link" between planets and
stars, but little is known about how they are made.
"If we find one brown dwarf, we are not sure where it came from,"
Wolszczan explained. "It could be either from the process of planet
formation or it could be a direct product of star formation."
Seeing two of them around a parent star means they must have
originally formed from the enormous supply of raw materials that
surrounded the star when it was young. Astronomers call this thick,
solar system-sized pancake of gas and dust the "circumstellar disk."
"If that is the case," he continued, "then if we add up the minimum
masses of these two objects, we know the disk had to be extremely
massive."
To find these faint companions, the astronomers used the High
Resolution Spectrograph on the Hobby-Eberly Telescope in west Texas to
split up the light of BD +20 2457. This technique is similar to the
way a prism breaks light into a rainbow -- spectrum -- of colors. They
looked for shifts in color of certain features in the spectrum, called
spectral lines, as the dwarfs moved around the star and caused the
star to wobble back and forth from their gravitational tugs.
When the brown dwarfs' gravitational influence causes BD +20 2457 to
move towards Earth slightly, its spectral lines decrease in
wavelength, becoming slightly bluer. As it moves away, the wavelengths
increase, becoming slightly redder. By noting how quickly and strongly
the spectral lines shift, astronomers can infer the objects' masses,
as well as the sizes and shapes of their orbits.
The scientists determined that the two companions are at least 21 and
13 times the mass of Jupiter. Therefore, they are likely to exceed the
minimum mass of a brown dwarf, 13 times the mass of Jupiter. They are
separated from their star by about 1.5 and 2 times the distance
between the Earth and the sun and complete a "year" in 380 and 622
days, respectively.
What is even more unusual is the timescale involved in making these
brown dwarfs.
Several million years ago, BD +20 2457 was on the "main sequence," the
stage in stellar evolution where the star produced light by burning
its hydrogen fuel, much like our sun does now. Except this star, three
times the mass of the sun, was much hotter and more luminous.
"The intense radiation of this star would have heated up and
evaporated anything that was still forming around it," Wolszczan said.
"The fact that these dwarfs are still here means that they had to
accumulate a lot of material very quickly and be fully formed by the
time the star 'switched on.' "
A star like BD +20 2457 takes about 10 million years to form and enter
the main sequence. As a rough estimate, in order keep up with their
parent star, the dwarfs would have to accrue as much mass as the
Earth's moon every year.
"The lesson from this is that a combination of physical mechanisms may
be responsible for making brown dwarfs," Wolszsczan said. "Instead of
just growth by accretion (the steady accumulation of material), the
dwarfs' own gravity may help them gather more mass and speed up their
formation."
Provided by Pennsylvania State University (news : web)