The Growing-up of a Star (Forwarded)

ESO Education and Public Relations Dept.

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Science Contact:
Stefan Kraus
Max-Planck-Institute for Radio Astronomy, Bonn, Germany
Phone: +49 (0)228 525-395

For Immediate Release: 29 January 2008

ESO Science Release 03/08

The Growing-up of a Star

VLT decodes the innermost surroundings of a star in the maturing

Using ESO's Very Large Telescope Interferometer, astronomers have probed
the inner parts of the disc of material surrounding a young stellar
object, witnessing how it gains its mass before becoming an adult.

The astronomers had a close look at the object known as MWC 147, lying
about 2,600 light years away towards the constellation of Monoceros ('the
Unicorn'). MWC 147 belongs to the family of Herbig Ae/Be objects. These
have a few times the mass of our Sun and are still forming, increasing in
mass by swallowing material present in a surrounding disc.

MWC 147 is less than half a million years old. If one associated the
middle-aged, 4.6 billion year old Sun with a person in his early forties,
MWC 147 would be a 1-day-old baby [1].

The morphology of the inner environment of these young stars is however a
matter of debate and knowledge of it is important to better understand how
stars and their cortège of planets form.

The astronomers Stefan Kraus, Thomas Preibisch, and Keiichi Ohnaka have
used the four 8.2-m Unit Telescopes of ESO's Very Large Telescope to this
purpose, combining the light from two or three telescopes with the MIDI
and AMBER instruments.

"With our VLTI/MIDI and VLTI/AMBER observations of MWC147, we combine, for
the first time, near- and mid-infrared interferometric observations of a
Herbig Ae/Be star, providing a measurement of the disc size over a wide
wavelength range [2]," said Stefan Kraus, lead-author of the paper
reporting the results. "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 the distance
from the star."

The near-infrared observations probe hot material with temperatures of up
to a few thousand degrees in the innermost disc regions, while the
mid-infrared observations trace cooler dust further out in the disc.

The observations show that the temperature changes with radius are much
steeper than predicted by the currently favoured models, indicating that
most of the near-infrared emission emerges from hot material located very
close to the star, that is, within one or two times the Earth-Sun distance
(1-2 AU). This also implies that dust cannot exist so close to the star,
since the strong energy radiated by the star heats and ultimately destroys
the dust grains.

"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.
This suggests that the disc is not a passive one, simply reprocessing the
light from the star," explained Kraus. "Instead, the disc is active, and
we see the material, which is just transported from the outer disc parts
towards the forming star."

The best-fit model is that of a disc extending out to 100 AU, with the
star increasing in mass 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," said Stefan Kraus.

More Information

The authors report their results in a paper in the Astrophysical Journal
("Detection of an inner gaseous component in a Herbig Be star accretion
disk: Near- and mid-infrared spectro-interferometry and radiative transfer
modeling of MWC 147", by Stefan Kraus, Thomas Preibisch, Keichii Ohnaka").

Notes

[1]: Being 6.6 times more massive than the Sun, however, MWC 147 will only
live for about 35 million years, or to draw again the comparison with a
person, about 100 days, instead of the 80 year equivalent of our Sun.

[2]: MIDI is the mid-infrared instrument of the VLT interferometer. It
operates between 8 and 13 microns. AMBER observes in the near-infrared,
from 1 to 2.4 microns.

National contacts for the media:

Belgium: Dr. Rodrigo Alvarez, +32-2-474 70 50
Czech Republic: Pavel Suchan, +420 267 103 040
Finland: Ms. Tiina Raivo, +358 9 7748 8369
Denmark: Dr. Michael Linden-Vørnle, +45-33-18 19 97
France: Dr. Daniel Kunth, +33-1-44 32 80 85
Germany: Dr. Jakob Staude, +49-6221-528229
Italy: Dr. Leopoldo Benacchio, +39-347-230 26 51
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Spain: Dr. Miguel Mas-Hesse, +34918131196
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25
USA: Dr. Paola Rebusco, +1-617-308-2397

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