Infant Stars Caught Feeding

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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