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Amazing Old Stars Give Birth Again

January 21, 2008

Two old
stars appear to be gearing up for a second generation of planet formation, a
phenomenon astronomers say they have never seen before.

“This
is a new class of stars, ones that display conditions now ripe for formation of
a second generation of planets, long, long after the stars themselves
formed,” said UCLA astronomy graduate student Carl Melis, who reported the
findings at a recent meeting of the American Astronomical Society in Austin,
Texas.

The stars
are BP Piscium in the constellation Pisces and TYCHO 4144 329 2, which resides
in the constellation Ursa Major. The exact ages of the stars are unknown, but
it is estimated they are at least hundreds of millions or possibly billions of
years old, and might have already given birth to planets long ago.

“Most
astronomers now believe that most stars are accompanied by first-generation
planets of some sort, even if the planets are not massive enough to be picked
up by the radial velocity [detection] technique,” Melis said.

Second
generation of planets

The unusual
thing about these stars is that they appear to be giving birth to planets
again.

“We
currently understand planet formation to occur around stars when they are very
young and enshrouded in dusty and gaseous disks, the material necessary to form
planetary bodies,” Melis told SPACE.com. “This material is
completely used up after a couple to ten million years after the star is born
and is not replenished during the star’s life. As such, we would never expect a
star to undergo planet formation late in its life as the necessary conditions
are not present.”

How they
can do this is still unclear, but the stars seem to have kept many of their
youthful qualities. For instance, the researchers found orbiting disks of gas
and dust extended around the stars, and, in the case of BP Piscium, jets of gas
being ejected into space. These gas-and-dust rings provide the fodder for the
making of planetesimals, such as comets
and asteroids that can merge to form larger bodies, along with planets.

“With
all these characteristics that match so closely with young stars, we would
expect that our two stars would also be young,” Melis said. “As we
gathered more data, however, things just did not add up.”

Aging
stars

The lack of
lithium gave away the true stellar ages. Since stars
burn lithium as they get older, younger stars should pack large stores of the
chemical element. The astronomers found, however, that BP Piscium contained
much less lithium than would be expected for a young star of its mass.

“There
is no known way to account for this small amount of lithium if BP Piscium is a
young star,” Melis said. “Rather, lithium has been heavily processed,
as appropriate for old stars. Other spectral measurements also indicate it is a
much older star.”

The
researchers speculate that the senior stars might be borrowing material from
their neighbors to construct new worlds.

“Our
team believes that these stars, as they aged and began to expand into giant
stars, engulfed very short-period companion stars orbiting around them,”
Melis said. “Interactions with these companions caused matter to be flung
into disks surrounding the two stars.”


Source: imaginova



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