Mathematics – Probability
Scientific paper
Jun 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eso..pres...15.&link_type=abstract
ESO Press Release, 06/2003
Mathematics
Probability
Scientific paper
Formation of Exceedingly Luminous and Hot Stars in Young Stellar Cluster Observed Directly
Summary
Based on a vast observational effort with different telescopes and instruments, ESO-astronomer Dieter Nürnberger 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°C), 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.
PR Photo 16a/03: Stellar cluster and star-forming region NGC 3603. PR Photo 16b/03: Region near very young, massive stars IRS 9A-C in NGC 3603 (8 bands from J to Q). 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 Nürnberger 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
ESO PR Photo 16a/03
ESO PR Photo 16a/03
[Preview - JPEG: 400 x 469 pix - 70k [Normal - JPEG: 800 x 938 pix - 544k]
Caption: PR Photo 16a/03 shows the sky region around NGC 3603, a cluster of young, hot stars (of the "OB-type") still partly embedded in its natal cloud of gas and dust. A small group of nascent, very massive stars has been found to the south of the cluster centre - here indicated as "IRS9". The area within the indicated square is shown in more detail in PR Photo 16b/03 (in different wavebands). The present photo was first published as PR Photo 38a/99; it was obtained with the ISAAC multi-mode instrument at the 8.2-m VLT ANTU telescope at Paranal. The orientation and the scale at the distance of NGC 3603 (22,000 light-years) are indicated.
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 unr
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