VLT captures young stars

in detail
BY KULVINDER SINGH CHADHA
ASTRONOMY NOW

Posted: October 13, 2008

The European Southern Observatory’s Very Large Telescope array in Chile has used its interferometric mode to capture details of young stars in unprecedented detail, which could hope to end years of debate on the behaviour of matter in young stellar systems. Interferometry is a way of matching up light wavelengths from several telescopes in order to simulate a much larger aperture, thereby allowing high resolutions. The international team of astronomers are from Germany, France, Italy and the UK.

The closest star-forming regions to Earth start out at 500 light years in distance, meaning that they are incredibly small on the sky. In fact the AMBER instrument used by the astronomers for this study has a milliarcsecond resolution. That is, the ability to resolve the full stop at the end of this sentence from a distance of 50 kilometres.

Artist’s impression of a dusty disc surrounding an inner gas disc around a newly forming star. Image: ESO/L. Calçada.

Using this technique the astronomers were able to get a high-resolution view of the behaviour of the disc of matter surrounding the young stars. "So far, interferometry has mostly been used to probe the dust that closely surrounds young stars, but it composes only one percent of the total mass of the discs. Their main component is gas, and its distribution may define the final architecture of planetary systems that are still forming," says Dr Eric Tatulli from France’s Grenoble Observatory.

The different physical processes between the young stars and the gas discs can be investigated from different emission lines in the spectra. Dr Stefan Kraus from Bonn in Germany says, "The origin of gas emissions from these young stars has been under debate until now, because in most earlier investigations the resolution was not high enough to study the distribution of the gas close to the star. Astronomers had very different ideas about the physical processes that have been traced by the gas, and by combining spectroscopy and interferometry, the VLTI has given us the opportunity to distinguish between the physical mechanisms responsible."

For example, the team found evidence for matter infalling into the star in two cases, whereas they detected stellar matter outflow (powerful stellar winds, or disc winds) in four other cases. In the case of one other star, dust is so close to the star that it should evaporate, but it hasn’t. The conclusion is that the gas may shield the dust from the star’s radiation.

Various processes have been proposed as the source of some of the strong emission lines in the Herbig Ae/Be stars observed by the team. One idea is that they may be due to an accreting gaseous inner disc, or, they may be due to magnetospheric accretion processes or a stellar wind. Most of these processes take place close to the star (less than the distance between Earth and the Sun) and are therefore not accessible with direct imaging facilities. In fact the VLT is unique in its capability to take spectra whilst performing milliarcsecond resolution Interferometry.

Finally Kraus says, "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 in fact caused by a jet launched from the disc or by a stellar wind."