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Highlights
from the AAS Meeting
FUSE SPACECRAFT OBSERVES INTERSTELLAR LIFEBLOOD
OF GALAXIES The
extended halo of half-million-degree gas that surrounds the Milky Way was generated
by thousands of exploding stars, or supernovae, as our galaxy evolved, according
to new observations by NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft.
The spacecraft has nearly completed its shakedown phase, and its first results
are already providing a wealth of new information to astronomers about the material
that becomes stars, planets and ourselves.
The new findings confirming
the nature of the Milky Way halo are being presented today in Atlanta at the 195
meeting of the American Astronomical Society (AAS).
The roughly football-shaped
hot gas halo which surrounds our galaxy extends about 5,000 -10,000 light years
above and below the galactic plane and thins with distance. One light year is
almost six trillion miles.
"The hot gas halo has been known for
some time, but we weren't sure how it got there or stayed hot," said FUSE
co- investigator Dr. Blair Savage of the University of Wisconsin in Madison. "The
new FUSE observations reveal an extensive amount of oxygen VI (oxygen atoms that
have had five of their eight surrounding electrons stripped away) in the halo.
Some scientists thought that ultraviolet radiation from hot stars could produce
the halo, but the only way to make the observed amount of oxygen VI is through
collision with the blast waves from exploding stars, called supernovae."
"Stars destined to explode don't live long, compared to stars like our
Sun, so star explosions are actually a record of star formation," said Dr.
George Sonneborn, FUSE project scientist at NASA's Goddard Space Flight Center,
Greenbelt, MD. "By comparing supernova generated halos among galaxies, we
may be able to compare their star formation histories."
"FUSE
measures the pulse of the lifeblood of our galaxy, the thin gas between stars,"
said Dr. Warren Moos, FUSE principal investigator at Johns Hopkins University
in Baltimore. "This interstellar gas courses through our veins, because dense
clouds of it collapsed to form new stars and planets, including our solar system."
The FUSE observatory is now "open for business," Moos said. "After
an extended on-orbit checkout and debugging period, common for complex space observatories,
we are now performing observations on a routine basis for both members of the
principal investigator team and the 62 guest investigators from around the world
selected by NASA for the first year of operations.
"We are continuing
to tune the instrument," Moos added. "In the spring we expect to begin
a comprehensive study of the abundance of deuterium, a fossil atom left over from
the Big Bang. As our team becomes more practiced, we need less time to optimize
the instrument, and the amount of time we can spend on scientific observations
will go up. This means higher scientific productivity."
FUSE is
able to detect interstellar gas and determine its composition, velocity and distance
by viewing bright celestial objects further away. The intervening gas selectively
absorbs the light from these objects in a unique pattern of colors, depending
on the composition of the gas. The spectrograph on FUSE separates the light into
its component colors, similar to the way a prism separates white light into a
rainbow. The resulting patterns identify the gas like optical fingerprints. When
the patterns shift to different colors, velocity and distance measurements can
be inferred.
The FUSE spectrograph is at least 100 times more powerful
than previous instruments, helping it reveal a large number of new atomic and
molecular features in interstellar gas that could only be guessed at before. The
ultraviolet light analyzed by FUSE is invisible to the human eye.
FUSE
scientists are also reporting early results at the AAS meeting about investigations
into two other components of the galactic "circulatory system": cold
clouds of molecular hydrogen where new stars are born, presented by Dr. Michael
Shull of the University of Colorado, and hot gas "winds" from stars
so bright they nearly blow themselves apart, presented by Dr. John Hutchings of
the National Research Council of Canada.
Images related to this science,
as well as detailed information about FUSE, is available at: http://fuse.pha.jhu.edu/
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| Image
1 | | IMAGE
CREDIT: NASA, George Sonneborn, U. of Wisconsin, Blair Savage, AlliedSignal MaxQ
Digital Group, Walt Feimer. | This
sequence of images is taken from a computer animation illustrating the process
that produces a halo of 500,000 degree gas around our galaxy. The sequence is
ordered clockwise, starting at the top left. In the first image, the galaxy is
seen long its plane, with its many billions of stars represented by a horizontal
band of white, red and blue specks. The central white flash represents an exploding
star, called a supernova. Gas from the dying star is heated and ejected into interstellar
space by the violent supernova explosion. This is seen in the second image (top
right), with the hot gas represented by a yellow hourglass shape where the supernova
explosion occurred. Many years later, another supernova explosion erupts nearby
(white and red flash on the left). Over periods spanning millions of years, regions
of interstellar gas in the disk are heated to high temperatures by supernovae.
The heated and highly pressurized gas created by these explosions bursts out of
the plane of the galaxy and rises into the halo, illustrated in the third image
(bottom right). Here, the galaxy is seen from above the plane, and the structure
of the galactic disk is visible with the stars in spiral bands of blue, white
and red. Hot gas ejections into the halo are represented by yellowish clouds in
different areas above and below the galactic disk. The bright white area in the
center is the concentration of stars in the middle of the galactic disk, which
is the galaxy's core. The white flash to the upper right of the core represents
hot gas from a relatively recent supernova. The last image (bottom left) shows
the halo as seen from the galactic plane. The
roughly football-shaped hot gas halo which surrounds our galaxy extends about
5,000 -10,000 light years above and below the galactic plane and thins with distance.
One light year is almost six trillion miles. This thin gas is not visible to the
human eye, and is best detected using observatories in space, such as NASA's Far
Ultraviolet Spectroscopic Explorer (FUSE). FUSE is able to detect interstellar
gas and determine its composition, velocity and distance by viewing bright celestial
objects further away. The intervening gas selectively absorbs the light from these
objects in a unique pattern of colors, depending on the composition of the gas.
The spectrograph on FUSE separates the light into its component colors, similar
to the way a prism separates white light into a rainbow. The resulting patterns
identify the gas like optical fingerprints. When the patterns shift to different
colors, velocity and distance measurements can be inferred. The FUSE spectrograph
is at least 100 times more powerful than previous instruments, helping it reveal
a large number of new atomic and molecular features in interstellar gas that could
only be guessed at before. The ultraviolet light analyzed by FUSE is invisible
to the human eye.
Hubble
images of supernovae: http://oposite.stsci.edu/pubinfo/pr/97/03.html http://oposite.stsci.edu/pubinfo/pr/1999/04/pr-photos.html http://oposite.stsci.edu/pubinfo/pr/96/22.html http://oposite.stsci.edu/pubinfo/pr/95/13.html Hubble
image of a doomed star that will explode soon: http://oposite.stsci.edu/pubinfo/pr/96/23.html Hubble
images of star formation: http://oposite.stsci.edu/pubinfo/pr/2000/01/index.html http://oposite.stsci.edu/pubinfo/pr/1999/33/index.html http://oposite.stsci.edu/pubinfo/pr/1998/42/ http://oposite.stsci.edu/pubinfo/pr/1998/21/ http://oposite.stsci.edu/pubinfo/pr/97/34/ Hubble
images of star birth among the clouds: http://oposite.stsci.edu/pubinfo/pr/95/44.html http://oposite.stsci.edu/pubinfo/pr/1999/42/index.html Hubble
images of massive, hot stars: http://oposite.stsci.edu/pubinfo/pr/1998/38/ http://oposite.stsci.edu/pubinfo/pr/97/33.html Back
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HUBBLE
OBSERVATIONS INDICATE MASSIVE PLANETS MAY FORM QUICKLY OUT IN THE COLD A
young star may be forming massive planets much earlier and at greater distances
than current planet formation models predict, according to new observations from
the Hubble Space Telescope. NASA astronomers using the Space Telescope Imaging
Spectrograph (STIS) instrument on board Hubble discovered that the disk of gas
and dust surrounding a young star has a gap, possibly caused by gravitational
influence of a nascent planet.
The gap appears about 30 billion miles
from the star, more than seven times the distance from the Sun to Pluto, the most
remote planet in our solar system. The STIS instrument also revealed that the
star is ejecting jets of gas at hundreds of miles per second from its poles, a
feature usually seen in much younger stars.
"If the clearing in
the disk is due to planet formation, it suggests we have a lot more to learn about
how planets form, but we may be on the right track, which is very exciting,"
said Dr. Carol Grady of NASA's Goddard Space Flight Center, Greenbelt, Md. "The
potential planet is much further away from its star than any known bodies in the
plane of our solar system, and it is forming much faster than most models predict."
"This star is also interesting because of its jets," adds Dr. David
Devine of Goddard. "Protostellar jets are a byproduct of the accretion of
material onto a young star, and are thought to have lifetimes of a few hundred
thousand years or so. It came as a big
surprise to find them associated with
a 4 million year old star. One possible explanation for these 'old' jets is that
the formation of planets in the disk results in periodic 'meteor showers' of material
falling onto the central star, which rejuvenates the jets. In simpler terms, the
star gets the hiccups while eating dessert."
Grady and Devine will
present their results at the winter meeting of the American Astronomical Society
(AAS) in Atlanta, Georgia, Jan. 12, and the research will be published in the
Astrophysical Journal.
The star, designated HD163296, is approximately
400 light years away from Earth in the direction of the constellation Sagittarius
(one light year is almost six trillion miles). It is about twice as massive as
the Sun and has an estimated age of 2-10 million years. This is relatively young,
as stars of its type have lives spanning up to one billion years.
If
the gap in the disk is due to a single planet, the planet has an estimated mass
1.3 times that of Saturn. It is likely that the planet will gain additional mass
as it continues to pull material from the disk.
It would have been very
difficult to detect the jets and the gap in the disk without the use of a coronagraph.
Normally, a young star's bright light prevents astronomers from seeing material
that is close to it (imagine a match next to a spotlight). However STIS has a
coronagraph that blocks the star's light and allows the study of the much fainter
surrounding material. Unfortunately, the inner part of the disk can't be directly
seen, because it is obscured by the coronagraph. However the astronomers were
able to trace the disk to within about 180 AU of the star (1 AU is the distance
from the Earth to the Sun, about 93 million miles (150 million kilometers)).
Structures in the jets provide hints of what is happening in the inner disk
blocked from view by the coronagraph. "We do see dense clumps in the jet,"
said Devine. "They are regularly spaced, and appear to be ejected about once
every five years."
"One exciting, but speculative, possibility
is that there is another planet forming in the inner part of the disk," said
Grady. "As the planet progresses in its orbit, it may periodically
disrupt the disk and toss material on the star, some of which is ejected as denser
clumps of material in the jets."
"The five-year spacing of
the knots in the star's jets may be related to a five-year orbital period for
any potential companion object," said Grady. "This is in the habitable
zone for the star, where liquid water could exist. However, since the system is
young and the object is still forming, there is likely a constant rain of meteorites
on its surface. I wouldn't want to spend a vacation there."
"On the other hand, the knots in the jets may have nothing to do with planet
formation, and may be due to some unobserved feature of the star or disruptions
caused by a low-mass companion like a brown dwarf. A brown dwarf is an object
not quite massive enough to shine by nuclear fusion like a star but instead glows
dimly with heat left over from its formation. If the companion were a low-mass
star, it should produce X-rays, but other observatories sensitive to X-rays have
not seen any nearby. However, we can't yet rule out a brown dwarf," said
Grady.
"We have only recently had observatories powerful enough
to tell us something about planet formation, so the field is very young,"
said Grady. "We don't yet know the full diversity of solar systems
-- if a star with fast, remote planet formation and jets is common or rare. Recent
observations of stellar disks, announced at last January's AAS meeting, indicate
that at least one other star (HD141569) may have planets forming rapidly at great
distances. If this were a rare occurrence, we would not expect to find two examples
so quickly."
"We will use Hubble to take longer exposures of
HD163296 in the summer to see if there is anything else going on in its faint,
outer disk, and we plan similar observations of 8-10 other stars by September.
We are very fortunate to be exploring this new frontier, and are tremendously
excited by future observatories, such as the Space Interferometry Mission and
the Terrestrial Planet Finder, that promise to tell much more," said Grady.
In addition to taking pictures, STIS separates light into its component,colors,
much like a prism separates white light into a rainbow. It turns out that each
element can only emit light of certain colors, so by analyzing the light emitted
by an object astronomers can figure out its composition. Astronomers can also
learn about its motion because light from an object that is moving toward us is
shifted to more energetic, bluer colors, and light from an object moving away
is shifted to less energetic, more red colors. The effect is similar to the way
the pitch of the siren on
a speeding ambulance appears to rise as it approaches
and fall as it rushes away. HD
163296 is an isolated young star, seen against a background of fainter stars in
the plane of the Galaxy. Material close to the star can only be seen during man-made
eclipses with the star blocked from our direct view by a coronagraph. The
figure on the left shows a composite image formed from three coronagraphic observations
of HD 163296 obtained with the Hubble Space Telescope and the Space Telescope
Imaging Spectrograph (STIS), centered on the star and covering 1/2 percent of
the diameter of the full moon on the sky (20"x20"). The image is shown
with north up and east to the left. The dark, irregularly shaped polygons are
regions which were obscured by the STIS coronagraph in all three observations.
The disk surrounding the star is the oval structure resembling Saturn's rings
running from lower left to upper right (SE to NW) and is shown in false color
(white is bright, with yellow to red and finally black corresponding to fainter
signals), with the very high dynamic range image displayed as the human eye perceives
brightness (logarithmic stretch). The disk extends to 450 times the Earth-Sun
distance. All of the material that we can see is at a distance well beyond the
orbit of Pluto and the known size of our Solar System's Kuiper Belt. The
disk exhibits a surprising amount of structure. There is a bright dust ring at
350 times the Earth-Sun distance which can be seen on either side of the star's
location, together with a darker lane just inside it. The dark lane is 0.4"
or 50 times the Earth-Sun distance across. If this lane is cleared by a single
body, the inferred mass of the body is 1.3 Saturn masses. Inside the dark lane
the disk becomes as bright as the outer bright ring, but does not continue to
brighten with decreasing distance from the star. This suggests that there may
be a region between 300 and 180 times the Earth-Sun distance which is partially
cleared. The coronagraphic image also reveals three regions of nebulosity (the
fuzzy, bright features) aligned perpendicular to the disk. These features were
completely unexpected in a star this old, and are more typically detected in association
with really young protostars which are 10 times younger than HD 163296. The
middle figure shows a spectral image of the vicinity of the star in the light
of glowing hydrogen gas. In this image, we sample material along a 0.2" (24
times the Earth-Sun distance) wide slit passing through the nebulosities (vertical
axis of the middle image). Along the horizontal axis we see the light of the star
spread out into its constituent colors. The nebulosities above the star (NE in
the coronagraphic image) are displaced toward the red, indicating that the gas
is moving away from us at velocities of 250 miles/second (400 km/s). Below the
star, the data reveal a jet of material moving toward us at 250 miles/second (400
km/s). This velocity information enables us to determine the three-dimensional
space orientation of the disk and jet system. A cartoon combining the information
from the coronagraphic image and the spectrum is shown on the right. The nebulosities
seen in both the image and the spectrum have been identified as ejected gas features
termed Herbig-Haro objects and collectively are cataloged as HH409. Photo
credits: NASA and C.A. Grady and David Devine (NOAO, NASA Goddard Space Flight
Center), B. Woodgate and R. Kimble (NASA Goddard Space Flight Center), F.C. Bruhweiler
and A. Boggess (Catholic University of America), J.L. Linsky (JILA, University
of Colorado and NIST), P. Plait (Advanced Computer Concepts), M. Clampin and P.
Kalas (Space Telescope Science Institute). SCIENCE
BACKGROUND: HD
163296 is one of the brighter and better studied 2 solar mass young stars, and
has an estimated age of 2-10 million years, with a most probable age estimate
of 4 million years. The HD 163296 system shows features more commonly associated
with older, planetary systems, and with younger protostars. Previous
ground-based millimeter observations had shown that the star had a large gas and
dust disk, while space-based and ground-based infrared observations had indicated
that the disk material extended in to very close to the star, much further in
than we can image. Dark lanes in a dust disk have previously been observed in
a somewhat older (10 Million years) star, HD 141569
(STScI-PR99-03, which
can be viewed at oposite.stsci.edu/pubinfo/pr/1999/03/index.html), and not in
younger systems such as AB Aur (STScI-PR99-21 which can be viewed at oposite.stsci.edu/pubinfo/pr/1999/21/index.html)
at 2-4 million years. The STIS data indicate that features interpreted as the
effects of planet formation occur sooner than predicted by models, and at larger
distances from the star. The
movement of the nebulosities between the time of the coronagraphic images and
the spectral observations suggests that the Herbig-Haro objects are moving in
the plane of the sky with velocities of approximately 190 miles/second (300 km/s),
and that the spacing between the brighter knots corresponds to ejections separated
by approximately 5 years. Such a spacing could be due either to stellar activity
cycles, which have not previously been known to occur in a star of HD 163296's
type, or possibly due to the motion of a companion orbiting the star with a 5
year period. Such a period would place the companion within the habitable zone,
where liquid water can be present on a planetary surface, for a star of HD 163296's
temperature. In
principle, such a companion could be a low-mass star, a brown dwarf, or a planet.
If the companion were a low-mass star, it should produce X-rays, but other observatories
sensitive to X-rays (ROSAT) have not seen any X-ray emission from the vicinity
of the star. This suggests that any companion must be either a brown dwarf, an
object not quite massive enough to shine by nuclear fusion, but which glows dimly
with heat left over from its formation, or a planet. The presence of structure
further out in the disk suggesting the presence of at least one planetary mass
body suggests that the inner companion is more likely to be a planet. If correct,
HD 163296 may be the youngest known example of a multiple-planet system and indicates
that planets form both faster and over a wider zone in their natal circumstellar
disks than expected. Where
else has Hubble seen hints of planet formation? http://oposite.stsci.edu/pubinfo/pr/1999/03/index.html http://oposite.stsci.edu/pubinfo/pr/1999/21/index.html More
about Hubble: http://www.stsci.edu/hst/ More
about the Space Telescope Imaging Spectrograph instrument: http://www.stsci.edu/hst/#stis More
about Hubble pictures: http://oposite.stsci.edu/pubinfo/looklike/index.html
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Highlights
of Chandra from the AAS Meeting Chandra
x-ray background images are online at: http://universe.gsfc.nasa.gov/press/chandra/.
Chandra
Resolves Mysterious X-ray Glow into Millions of Galaxies While
taking a giant leap towards solving the greatest mystery of X-ray astronomy, NASA's
Chandra X-ray Observatory also may have revealed the most distant objects ever
seen in the Universe and discovered two puzzling new types of cosmic objects.
Not
bad for being on the job only five months. Chandra
has resolved most of the X-ray background, a pervasive glow of X-rays throughout
the Universe, first discovered in the early days of space exploration. Before
now, scientists have not been able to discern the background's origin, because
no X-ray telescope until Chandra has had both the angular resolution and sensitivity
to resolve it. "This
is a major discovery," said Dr. Alan Bunner, Director of NASA's Structure
and Evolution of the Universe science theme. "Since it was first observed
thirty-seven years ago, understanding the source of the X-ray background has been
a Holy Grail of X-ray astronomy. Now, it is within reach." The
results of the observation will be presented today at the 195th national meeting
of the American Astronomical Society in Atlanta, GA. An article describing this
work has been submitted to the journal Nature by Dr. Richard Mushotzky, of NASA
Goddard Space Flight Center, Greenbelt, Md., Drs. Lennox Cowie and Amy Barger
at the University of Hawaii, Honolulu, and Dr. Keith Arnaud of the University
of Maryland, College Park. "We
are all very excited by this finding," said Mushotzky. "The resolution
of most of the hard X-ray background during the first few months of the Chandra
mission is a tribute to the power of this observatory and bodes extremely well
for its scientific future," Scientists
have known about the X ray glow, called the X-ray background, since the dawn of
X-ray astronomy in the early 1960s. The German-led ROSAT mission, now completed,
resolved much of the lower-energy X-ray background, showing that it arose in very
faraway galaxies with extremely bright cores, called quasars or Active Galactic
Nuclei (AGN). The
Chandra team sampled a region of the sky about one-fifth the angular area of a
full moon and resolved about 80 percent of the more-energetic X-ray background
into discrete sources. Stretched across the entire sky, this would account for
approximately 70 million sources, most of which would be identified with galaxies.
Their analysis confirms that a significant fraction of the X-ray background cannot
be due to diffuse radiation from hot, intergalactic gas. Combined
X-ray and optical observations showed that nearly one third of the sources are
galaxies whose cores are very bright in X rays yet emit virtually no optical light
from the core. The observation suggests that these "veiled galactic nuclei"
galaxies may number in the tens of millions over the whole sky. They almost certainly
harbor a massive black hole at their core that produces X rays as the gas is pulled
toward it at nearly the speed of light. Their
bright X-ray cores place these galaxies in the AGN family. Because these numerous
AGN are bright in X rays, but optically dim, the Chandra observation implies that
optical surveys of AGN are very incomplete. A
second new class of objects, comprising approximately one-third of the background,
is assumed to be "ultra-faint galaxies." Mushotzky said that these sources
may emit little or no optical light, either because the dust around the galaxy
blocks the light totally or because the optical light is eventually absorbed by
relatively cool gas during its long journey across the Universe. In
the latter scenario, Mushotzky said that these sources would be well over 14 billion
light years away and thus the earliest, most distant objects ever identified. Drs.
Cowie and Barger are searching for the optical counterparts to the newly discovered
X-ray sources with the powerful Keck telescope atop Mauna Kea, Hawaii, in hopes
of determining their distance. However, these sources are very faint in visible
light and show up as a dim blue smudge or not at all. Further observations with
the Hubble Space Telescope or Keck will be extremely difficult, and may have to
wait until the Next Generation Space Telescope and Constellation-X for scientists
to be able to fully understand these sources. Resolution
of the X-ray background relied on a 27.7-hour Chandra observation using the Advanced
CCD Imaging Spectrometer (ACIS) in early December 1999, and also utilized data
from the Japan-U.S. Advanced Satellite for Cosmology and Astrophysics (ASCA).
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