This Echo is READ ONLY !   NO Un-Authorized Messages Please!
 ~~~~~~~~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

             Information from the European Southern Observatory

ESO Press Release 15/03

16 June 2003                                               [ESO Logo]

For immediate release
---------------------------------------------------------------------

Curtain-Lifting Winds Allow Rare Glimpse into Massive Star Factory

Part 1 of 3

Formation of Exceedingly Luminous and Hot Stars in Young Stellar
Cluster Observed Directly

Based on a vast observational effort with different telescopes and
instruments, ESO-astronomer Dieter Nuernberger has obtained a first
glimpse of the very first stages in the formation of heavy stars.

These critical phases of stellar evolution are normally hidden from
the view, because massive protostars are deeply embedded in their
native clouds of dust and gas, impenetrable barriers to observations
at all but the longest wavelengths. In particular, no visual or
infrared observations have yet "caught" nascent heavy stars in the
act and little is therefore known so far about the related processes.

Profiting from the cloud-ripping effect of strong stellar winds from
adjacent, hot stars in a young stellar cluster at the center of the
NGC 3603 complex, several objects located near a giant molecular
cloud were found to be bona-fide massive protostars, only about
100,000 years old and still growing.

Three of these objects, designated IRS 9A-C, could be studied in more
detail. They are very luminous (IRS 9A is about 100,000 times
intrinsically brighter than the Sun), massive (more than 10 times the
mass of the Sun) and hot (about 20,000 degrees). They are surrounded
by relative cold dust (about 0 degC), probably partly arranged in
disks around these very young objects.

Two possible scenarios for the formation of massive stars are
currently proposed, by accretion of large amounts of circumstellar
material or by collision (coalescence) of protostars of intermediate
masses. The new observations favour accretion, i.e. the same process
that is active during the formation of stars of smaller masses.

The full text of this Press Release, with four photos (ESO PR Photos
16a-d/03) and all related links, is available at:

http://www.eso.org/outreach/press-rel/pr-2003/pr-15-03.html

How do massive stars form?

This question is easy to pose, but so far very difficult to answer.
In fact, the processes that lead to the formation of heavy stars [1]
is currently one the most contested areas in stellar astrophysics.

While many details related to the formation and early evolution of
low-mass stars like the Sun are now well understood, the basic
scenario that leads to the formation of high-mass stars still remains
a mystery. It is not even known whether the same characterizing
observational criteria used to identify and distinguish the
individual stages of young low-mass stars (mainly colours measured at
near- and mid-infrared wavelengths) can also be used in the case of
massive stars.

Two possible scenarios for the formation of massive stars are
currently being studied. In the first, such stars form by accretion
of large amounts of circumstellar material; the infall onto the
nascent star varies with time. Another possibility is formation by
collision (coalescence) of protostars of intermediate masses,
increasing the stellar mass in "jumps".

Both scenarios impose strong limitations on the final mass of the
young star. On one side, the accretion process must somehow overcome
the outward radiation pressure that builds up, following the ignition
of the first nuclear processes (e.g., deuterium/hydrogen burning) in
the star's interior, once the temperature has risen above the
critical value near 10 million degrees.

On the other hand, growth by collisions can only be effective in a
dense star cluster environment in which a reasonably high probability
for close encounters and collisions of stars is guaranteed.

Which of these two possibilties is then the more likely one?

Massive stars are born in seclusion

There are three good reasons that we know so little about the
earliest phases of high-mass stars:

First, the formation sites of such stars are in general much more
distant (many thousands of light-years) than the sites of low-mass
star formation.  This means that it is much more difficult to observe
details in those areas (lack of angular resolution).

Next, in all stages, also the earliest ones (astronomers here refer
to "protostars"), high-mass stars evolve much faster than low-mass
stars. It is therefore more difficult to "catch" massive stars in the
critical phases of early formation. 

And, what is even worse, due to this rapid development, young
high-mass protostars are usually very deeply embedded in their natal
clouds and therefore not detectable at optical wavelengths during the
(short) phase before nuclear reactions start in their interior. There
is simply not enough time for the cloud to disperse - when the
curtain finally lifts, allowing a view of the new star, it is already
past those earliest stages.

Is there a way around these problems? "Yes", says Dieter Nuernberger
of ESO-Santiago, "you just have to look in the right place and
remember Bob Dylan...!". This is what he did.

"The answer, my friend, is blowing by the wind..."

Imagine that it would be possible to blow away most of the obscuring
gas and dust around those high-mass protostars! Even the strongest
desire of the astronomers cannot do it, but there are fortunately
others who are better at it!

Some high-mass stars form in the neighbourhood of clusters of hot
stars, i.e., next to their elder brethren. Such already evolved hot
stars are a rich source of energetic photons and produce powerful
stellar winds of elementary particles (like the "solar wind" but many
times stronger) which impact on the surrounding interstellar gas and
dust clouds. This process may lead to partial evaporation and
dispersion of those clouds, thereby "lifting the curtain" and letting
us look directly at young stars in that region, also comparatively
massive ones at a relatively early evolutionary stage.

The NGC 3603 region

Such premises are available within the NGC 3603 stellar cluster and
star-forming region that is located at a distance of about 22,000
light-years in the Carina spiral arm of the Milky Way galaxy.

NGC 3603 is one of the most luminous, optically visible "HII-regions"
(i.e.  regions of ionized hydrogen - pronounced "eitch-two") in our
galaxy. At its centre is a massive cluster of young, hot and massive
stars (of the "OB-type") - this is the highest density of evolved
(but still relatively young) high-mass stars known in the Milky Way,
cf. ESO PR 16/99.

 - Continued -

@Message posted automagically by IMTHINGS POST 1.30
---