Ancient stardust in meteorites helps explain evolution
of galaxy, Maui conference will learn:
Meteorites contain microscopic grains of dust from stars
that existed long before our Solar System formed. This
ancient stardust holds information not only about the
processes that operate inside stars, but about the dynamics
and evolution of the Milky Way Galaxy as well. That is the
conclusion Donald Clayton of Clemson University's Department
of Physics and Astronomy will report Monday afternoon, July
21, at the 60th meeting of the Meteoritical Society.
The July 21-25 conference of meteorite researchers takes
place at the Maui Prince Hotel in Makena, Maui, Hawai'i, and
will be hosted by the Hawai'i Institute of Geophysics and
Planetology. Clayton's study is also published in the July
20 issue of Astrophysical Journal Letters, and a version for
the general public appears in the popular web science
magazine, Planetary Science Research Discoveries,
www.soest.hawaii.edu/PSRdiscoveries.
The research that led to these dramatic conclusions
builds on a decade of painstaking work by meteoriticists.
One of the pioneers in the study of "presolar grains" is
Ernst Zinner of Washington University in St. Louis. Zinner
will receive the Meteoritical Society's prestigious Leonard
Medal on Wednesday evening, July 23.
Scientists isolate the presolar grains in a meteorite by
using a mixture of acids to dissolve away 99.9% of the
meteorite. One meteoriticist, Edward Anders (formerly of the
University of Chicago, now living in Bern, Switzerland),
likens this process to 'burning down the haystack to find
the needle.' The grains are identified as "presolar" by
their highly unusual abundances of isotopes of several
elements, including silicon, carbon, nitrogen, and
titanium&emdash;compositions that are drastically different
from normal material from the Solar System.
One class of presolar grains, silicon carbide (SiC), was
the focus of Clayton's work. Meteorite scientists discovered
a decade ago that these presolar grains have a higher
proportion of silicon-29 and silicon-30 than common Solar
System silicon. (Silicon-28, the most abundant isotope of
silicon, forms the "comparison standard." That is, for equal
numbers of silicon-28 atoms, the presolar silicon has more
silicon-29 and silicon-30 atoms than does average Solar
System silicon.) This discovery, primarily by researchers at
Chicago, Caltech, Washington University (St. Louis) and Bern
University in Switzerland, has astonished, delighted and
challenged scientists for several years.
How, they wondered, can presolar stars have more
silicon-29 and silicon-30 than the sun has when those stars,
by definition, formed and lived their lives prior to the
birth of the sun? Theoretical considerations led scientists
to expect that presolar stars should have less silicon-29
and silicon-30 than the sun, not more.
Clayton's new work presents a solution that couples his
earlier ideas to growing astronomical evidence that the
galactic orbits of stars change over time. Because the
element abundances are higher in more central regions of the
galaxy, Clayton suggests that the carbon stars were born
there, rather than at the location of the sun's birth.
During their approximately 2-billion-year lifetimes, their
orbits enlarged, taking them far from the galaxy's center.
Many, Clayton suggests, ended their lives near the cloud in
which our Solar System formed, and deposited their dust
there.
This explanation meets astronomical knowledge and
expectations except in one respect: Why would the carbon
stars move to larger orbits? Clayton's explanation involves
their "scattering" from molecular clouds in the inner
galaxy. Because of their great masses (about 100,000 times
that of the sun), these molecular clouds provide a strong
gravitational force. When the carbon stars approach, the
clouds' gravity field causes the stars to accelerate; the
stars' higher speeds then place them on orbits that reach
further out. This same scattering phenomenon, Clayton notes,
is used in current human efforts to visit the outer solar
system. For example, the Voyager spacecraft was allowed to
approach Jupiter's strong gravity as it flew past the giant
planet and its moons. Jupiter's gravity caused Voyager to
pick up speed and it was scattered past that planet to a new
orbit farther out. NASA has used this type of "gravity
assist" on other missions as well. Clayton now suggests that
it was also used by our own Milky Way galaxy to fling stars
into orbits far from their origins.
Astronomers as well as meteorite scientists are excited
by this possibility. There are not many ways to learn about
stars that existed before the sun in the more central
portion of our galaxy. Clayton's work suggests that their
stardust carries the message of their history. This means
that carefully measuring the relative frequencies of these
particles will allow astronomers to map the dynamic history
of our Milky Way.
"What an astronomical story to read within a stone!"
Clayton says.
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