Baby star gives up secrets

German astronomers have boosted our understanding of how very young stars grow, thanks to observations made with the Very Large Telescope Interferometer (VLTI) of the European Organisation for
Astronomical Research in the Southern Hemisphere (ESO).

The researchers, from the Max Planck Institute for Radio Astronomy, were interested in studying so-called Herbig Ae/Be objects. These very young stars are larger than our sun and are still
forming, growing larger by swallowing material found in a surrounding disc. However, the morphology of the inner environment of these stars is still a mystery.

The focus of this latest piece of research is a star called MWC-147, which lies around 2,600 light years from Earth in the constellation of Monoceros (‘the Unicorn’) and is 6.6 times larger
than our sun. The distant star is just half a million years old; if our sun, at 4.6 billion years, can be compared to a person in their forties, then MWC-147 is like a one-day-old baby.

The scientists combined the light from different telescopes with the ESO’s MIDI and AMBER instruments to obtain interferometric observations of MWC-147 at different wavelengths. Near-infrared
observations probe the hot material in the innermost disc regions, where temperatures can reach a few thousand degrees. Meanwhile mid-infrared observations provide information on the cooler
dust further out in the disc.

‘Different wavelength regimes trace different temperatures, allowing us to probe the disc’s geometry on the smaller scale, but also to constrain how the temperature changes with distance from
the star,’ explained Stefan Kraus, the lead author of the paper.

Their results, which are published in the Astrophysical Journal, add to our understanding of how stars and their planets form. They reveal that the temperature changes with increasing distance
from the star are much steeper than had been predicted by earlier models. This indicates that most of the near-infrared emissions come from very hot material very close to the star. These
results also imply that the immediate surroundings of the star must be dust-free, as the energy radiated by the star would heat and ultimately destroy any grains of dust.

‘We have performed detailed numerical simulations to understand these observations and reached the conclusion that we observe not only the outer dust disc, but also measure strong emission from
a hot inner gaseous disc,’ said Dr Kraus. ‘This suggests that the disc is not a passive one, simply reprocessing the light from the star. Instead, the disc is active, and we see the material,
which is just transported from the outer disc parts towards the forming star.’

According to the astronomers, the disc likely extends out to 100 astronomical units (one AU is the distance between the Earth and the sun), with the stellar baby growing at a rate of seven
millionths of a solar mass per year.

‘Our study demonstrates the power of ESO’s VLTI to probe the inner structure of discs around young stars and to reveal how stars reach their final mass,’ commented Dr Kraus.

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