Galactic Dust Bunnies Found to Contain Carbon After All

Feature March 12, 2009


Galactic Dust Bunnies Found to Contain Carbon After All

Using NASA's Spitzer Space Telescope, researchers have found evidence suggesting that stars
rich in carbon complex molecules may form at the center of our Milky Way galaxy.

This discovery is significant because it adds to our knowledge of how stars form heavy elements
-- like oxygen, carbon and iron -- and then blow them out across the universe, making it possible
for life to develop.

Astronomers have long been baffled by a strange phenomenon: Why have their telescopes never
detected carbon-rich stars at the center of our galaxy even though they have found these stars in
other places? Now, by using Spitzer's powerful infrared detectors, a research team has found the
elusive carbon stars in the galactic center.

"The dust surrounding the stars emits very strongly at infrared wavelengths," says Pedro García-
Lario, a research team member who is on the faculty of the European Space Astronomy Center,
the European Space Agency's center for space science. He co-authored a paper on this subject in
the February 2009 issue of the journal Astronomy & Astrophysics.

"With the help of Spitzer spectra, we can easily determine whether the material returned by the
stars to the interstellar medium is oxygen-rich or carbon-rich."

The team of scientists analyzed the light emitted from 40 planetary nebulae – blobs of dust and
gas surrounding stars -- using Spitzer's infrared spectrograph. They analyzed 26 nebulae toward
the center of the Milky Way -- a region called the "Galactic Bulge" -- and 14 nebulae in other
parts of the galaxy. The scientists found a large amount of crystalline silicates and polycyclic
aromatic hydrocarbons, two substances that indicate the presence of oxygen and carbon.

This combination is unusual. In the Milky Way, dust that combines both oxygen and carbon is
rare and is usually only found surrounding a binary system of stars. The research team, however,
found that the presence of the carbon-oxygen dust in the Galactic Bulge seems to be suggestive
of a recent change of chemistry experienced by the star.

The scientists hypothesize that as the central star of a planetary nebula ages and dies, its heavier
elements do not make their way to the star's outer layers, as they do in other stars. Only in the
last moments of the central star's life, when it expands and then violently expels almost all of its
remaining outer gasses, does the carbon become detectable. That's when astronomers see it in the
nebula surrounding the star.

"The carbon produced through these recurrent 'thermal pulses' is very inefficiently dredged up to
the surface of the star, contrary to what is observed in low-metallicity, galactic disk stars," said
García-Lario. "It only becomes visible when the star is about to die."

This study supports a hypothesis about why the carbon in some stars does not make its way to
the stars' surfaces. Scientists believe that small stars -- those with masses up to one-and-a-half
times that of our sun -- that contain lots of metal do not bring carbon to their surfaces as they
age. Stars in the Galactic Bulge tend to have more metals than other stars, so the Spitzer data
support this commonly held hypothesis. Before the Spitzer study, this hypothesis had never been
supported by observation.

This aging and expelling process is typical of all stars. As stars age and die, they burn
progressively heavier and heavier elements, beginning with hydrogen and ending with iron.
Towards the end of their lives, some stars become what are called "red giants." These dying stars
swell so large that if one of them were placed in our solar system, where the sun is now, its
outermost border would touch Earth's orbit. As these stars pulsate – losing mass in the process –
and then contract, they spew out almost all of their heavier elements. These elements are the
building blocks of all planets, including our own Earth (as well as of human beings and any other
life forms that may exist in the universe).

The paper is co-authored by José Vicente Perea-Calderón of the European Space Astronomy
Center in Villanueva de la Cañada, Spain; Domingo Anibal García-Hernández of the Instituto de
Astrofísica de Canarias, on Spain's Tenerife island; Ryszard Szczerba of the Nicolaus Copernicus
Astronomical Center in Torun, Poland; and Matt Bobrowsky of the University of Maryland,
College Park.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope
mission for NASA's Science Mission Directorate, Washington. Science operations are conducted
at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech
manages JPL for NASA.

For more information about Spitzer, visit http://www.spitzer.caltech.edu/spitzer and
http://www.nasa.gov/spitzer .

-end-

To remove yourself from this mailing, please go to http://www.kintera.org/TR.asp?a=dnJGLHMnFbJII2K&s=jtJ0J4NJLfKMI3MPItE&m=dgIOK5MIKkLYF

To remove yourself from all mailings from NASA Jet Propulsion Laboratory, please go to http://www.kintera.org/TR.asp?a=gqLMIQNzHeIPKcJ&s=jtJ0J4NJLfKMI3MPItE&m=dgIOK5MIKkLYF