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.
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.
More information
The research described in this press release is presented in a
research article, which recently appeared in the European research
journal Astronomy & Astrophysics (“Infrared observations of NGC 3603
III. The enigmatic, highly reddened sources of IRS 9” by Dieter
Nuernberger). It is available on the web as HTML-, PDF- and
PS-formatted files.
Notes
[1] According to a somewhat arbitrary definition, astronomers talk
about low-mass stars (less than 3 solar masses), intermediate-mass
stars (from 3 to 8 solar masses) and high-mass stars (more than 8
solar masses). 1 solar mass =3D 1.99 10^30 kg.
[2] Individual parts of this major research project were performed by
Dieter N=FCrnberger in collaboration with Leonardo Bronfman (Universidad
de Chile, Santiago, Chile), Monika Petr-Gotzens, Thomas Stanke (MPIfR,
Bonn, Germany), Harold Yorke (JPL, Pasadena, California, USA) and Hans
Zinnecker (AIP, Potsdam, Germany).
Contact
Dieter Nuernberger
ESO-Santiago
Chile
Phone: +56 2 463-3080
email: dnuernbe@eso.org
