Thursday, December 24, 2009
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 Toruń Center for
Astronomy, Poland and the Center for Exoplanets and Habitable
Worlds, Penn State report their findings in the current issue of
theAstrophysical 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."
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Penn State: http://live.psu.edu
Thanks to Penn State for this article.