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             Information from the European Southern Observatory

ESO Press Release 15/03

16 June 2003                                               [ESO Logo]

For immediate release
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Curtain-Lifting Winds Allow Rare Glimpse into Massive Star Factory

Part 2 of 3

These hot stars have a significant impact on the surrounding gas and
dust.  They deliver a huge amount of energetic photons that ionize
the interstellar gas in this area. Moreover, fast stellar winds with
speeds up to several hundreds of km/sec impact on, compress and/or
disperse adjacent dense clouds, referred to by astronomers as
"molecular clumps" because of their content of complex molecules,
many of these "organic" (with carbon atoms).

IRS 9: a "hidden" association of nascent massive stars

One of these molecular clumps, designated "NGC 3603 MM 2" is located
about 8.5 light-years south of the NGC 3603 cluster, cf. PR Photo
16a/03. Located on the cluster-facing side of this clump are some
highly obscured objects, known collectively as "NGC 3603 IRS 9". The
present, very detailed investigation has allowed to characterise them
as an association of extremely young, high-mass stellar objects.

They represent the only currently known examples of high-mass
counterparts to low-mass protostars which are detected at infrared
wavelengths. It took quite an effort [2] to unravel their properties
with a powerful arsenal of state-of-the-art instruments working at
different wavelengths, from the infrared to the millimeter spectral
region.

Multi-spectral observations of IRS 9

To begin with, near-infrared imaging was performed with the ISAAC
multi-mode instrument at the 8.2-m VLT ANTU telescope, cf. PR Photo
16b/03. This allowed to distinguish between stars which are bona-fide
cluster members and others which happen to be seen in this direction
("field stars"). It was possible to measure the extent of the NGC
3603 cluster which was found to be about about 18 light-years, or 2.5
times larger than assumed before. These observations also served to
show that the spatial distributions of low- and high-mass cluster
stars are different, the latter being more concentrated towards the
centre of the cluster core.

Millimeter observations were made by means of the Swedish-ESO
Submillimeter Telescpe (SEST) at the La Silla Observatory.
Large-scale mapping of the distribution of the CS-molecule showed the
structure and motions of the dense gas in the giant molecular cloud,
from which the young stars in NGC 3603 originate. A total of 13
molecular clumps were detected and their sizes, masses and densities
were determined. These observations also showed that the intense
radiation and strong stellar winds from the hot stars in the central
cluster have "carved a cavity" in the molecular cloud; this
comparatively empty and transparent region now measures about 8
light-years across.

Mid-infrared imaging (at wavelengths 11.9 and 18 microns) was made of
selected regions in NGC 3603 with the TIMMI 2 instrument mounted on
the ESO 3.6-m telescope. This constitutes the first sub-arcsec
resolution mid-IR survey of NGC 3603 and serves in particular to show
the warm dust distribution in the region. The survey gives a clear
indication of intense, on-going star formation processes. Many
different types of objects were detected, including extremely hot
Wolf-Rayet stars and protostars; altogether 36 mid-IR point sources
and 42 knots of diffuse emission were identified. In the area
surveyed, the protostar IRS 9A is found to be the most luminous point
source at both wavelengths; two other sources, designated IRS 9B and
IRS 9C in the immediate vicinity are also very bright on the TIMMI 2
images, providing further indication that this is the site of an
association of protostars in its own right.

The collection of high-quality images of the IRS 9 area shown in PR
Photo 16b/03 is well suited to investigate the nature and the
evolutionary status of the highly obscured objects located there, IRS
9A-C. They are situated on the side of the massive molecular cloud
core NGC 3603 MM 2 that faces the central cluster of young stars (PR
Photo 16a/03) and were apparently only recently "liberated" from most
of their natal gas and dust environment by strong stellar winds and
energetic radiation from the nearby high-mass cluster stars.

The combined data lead to a clear conclusion: IRS 9A-C represent the
brightest members of a sparse association of protostars, still
embedded in circumstellar envelopes, but in a region of the pristine
molecular cloud core, now largely "blown-free" from gas and dust. The
intrinsic brightness of these nascent stars is impressive: 100,000,
1000 and 1000 times that of the Sun for IRS 9A, IRS 9B and IRS 9C,
respectively.

Their brightness and infrared colours give information about the
physical properties of these protostars. They are very young in
astronomical terms, probably less than 100,000 years old. They are
already quite massive, though, more than 10 times heavier than the
Sun, and they are still growing - comparison to the currently most
reliable theoretical models suggests that they accrete material from
their envelopes at the relatively high rate of up to 1 Earth mass per
day, i.e., the mass of the Sun in 1000 years.

The observations indicate that all three protostars are surrounded by
comparatively cold dust (temperature around 250 - 270 K, or -20 degC
to 0 degC).  Their own temperatures are quite high, of the order of
20,000 - 22,000 degrees.

A high-resolution NACO image of the IRS 9 area

In March and April 2003, very sharp images were obtained of the IRS 9
association of nascent high-mass stars by means of the NACO adaptive
optics camera at the 8.2-m VLT YEPUN telescope at Paranal. They have
been combined into impressive colour composite photos in PR Photo
16c/03 and PR Photo 16d/03. These images reveal a wealth of objects
in this region, support the classification of IRS 9A-C as protostars
and, in addition, at the same time confirm unambiguously their
membership of the NGC 3603 complex.

What do the massive protostars tell us?

Dieter Nuernberger is pleased: "We now have convincing arguments to
consider IRS 9A-C as a kind of Rosetta Stones for our understanding
of the earliest phases of the formation of massive stars. I know of
no other high-mass protostellar candidates which have been revealed
at such an early evolutionary stage - we must be grateful for the
curtain-lifting stellar winds in that area! The new near- and
mid-infrared observations are giving us a first look into this
extremely interesting phase of stellar evolution."

The observations show that criteria (e.g., infrared colours) already
established for the identification of very young (or proto-) low-mass
stars apparently also hold for high-mass stars. Moreover, with
reliable values of their brightness (luminosity) and temperature, IRS
9A-C may serve as crucial and discerning test cases for the currently
discussed models of high-mass star formation, in particular of
accretion models versus coagulation models.

The present data are well consistent with the accretion models and no
objects of intermediate luminosity/mass were found in the immediate
neighbourhood of IRS 9A-C. Thus, for the IRS 9 association at least,
the accretion scenario is favoured against the collision scenario.

 - Continued -

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