The
European Southern Observatory's Very Large Telescope has given astronomers
their most detailed look at how infant stars feed off the disks of gas and dust
that swirl around them as they grow.
Galaxies
have regions where large amounts of gas and dust are concentrated. If the gas
is cold and dense enough, clouds of it will collapse and begin to form stars.
The stars continue to feed off the surrounding
gas as they grow. The disk of dust and gas can also give rise to a planetary
system, as it did in our own solar system.
Observing
young stars can help astronomers better understand the processes that control star
formation, but the closest star-forming regions to us are about 500
light-years away. Their great distance makes these gas disks appear very small
in Earth's sky, so specialized techniques must be used to look for fine details
in the structures.
An
international group of astronomers used two such techniques to probe the inner
gaseous environments of six young stars belonging to the family of Herbig
Ae/Be objects, which are a few times the size of our sun and still growing.
The findings are detailed in the October issue of the journal Astronomy
& Astrophysics.
Two
techniques
One of the
best ways to resolve far-away features is interferometry, a technique that
combines the light of two or more telescopes so that the level of detail
corresponds to what would be seen by a telescope with a diameter equal to the
separation between the two telescopes, which can be hundreds of feet apart.
The Very
Large Telescope Interferometer (VLTI) has allowed astronomers to achieve a
resolution of about a milli-arcsecond, an angle equivalent to the size of the
period at the end of this sentence seen from a distance of 31 miles (50
kilometers).
The
astronomers combined interferometry with spectroscopy, or splitting light into
its constituent colors, to examine the gas emission processes around the young
stars, which can be linked to the physical processes involved in the stars'
formation. Astronomers have long debated what exactly causes these gas
emissions.
"The
origin of gas emissions from these young stars has been under debate until now,
because in most earlier investigations of the gas component, the spatial
resolution was not high enough to study the distribution of the gas close to
the star," said study co-leader Stefan Kraus from the Max Planck Institute
for Radio Astronomy in Germany. "By combining spectroscopy and
interferometry, the VLTI has given us the opportunity to distinguish between
the physical mechanisms responsible for the observed gas emission."
Gas
emissions
Most interferometry
so far has studied only the dust that closely surrounds the young stars, but
astronomers wanted to get a closer look at the gas in the same region because
"dust is only one percent of the total mass of the disks. Their main
component is gas, and its distribution may define the final architecture of
planetary systems that are still forming," said Eric Tatulli of the
Observatoire de Grenoble in France and also a co-leader of the study.
Two
processes were suspected to be behind the gas emissions observed coming from
the disk: material falling onto the star and gas being ejected as a wind from
the disk. The study found evidence of both.
Of the six
stars examined, two showed evidence of infalling material. Four other stars
showed evidence of mass outflow, either as an extended stellar wind or as wind
coming from the disks surrounding the stars.
For one of
the stars, it seemed that dust might be present closer to the star than was
expected. The dust is so close that the temperature should be high enough for
it to evaporate, but since that was not seen to be the case, astronomers
suspect that the gas in the area shields the dust from the star's light.
The new
observations have shown that it is possible to study gas in the disks around
young stars, and future observations could yield even more valuable information
about how
stars form.
"Future
observations using VLTI spectro-interferometry will allow us to determine both
the spatial distribution and motion of the gas, and might reveal whether the
observed line emission is caused by a jet launched from the disk or by a
stellar wind," Kraus said.
The study
was partly funded by a grant from the Italian National Institute for
Astrophysics, as well as the Agence Nationale de la Recherche (ANR) of France and by the Programme National de Physique Stellaire (PNPS).
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