Universe Today

Image credit: ESO

On March 29, 2003 NASA’s High Energy Transient Explorer detected a bright burst of gamma rays, and shortly after telescopes from around the world focused in on the object; now called GRB 030329 and measured to be 2.6 billion light-years away. By measuring the afterglow of the explosion, astronomers realized that it matches the spectrum of a hypernova – explosions of extremely large stars, at least 25 times larger than our own Sun. By matching the spectra, astronomers have compelling evidence that there is some connection between gamma ray bursts and the explosions of very large stars.

A very bright burst of gamma-rays was observed on March 29, 2003 by NASA’s High Energy Transient Explorer (HETE-II), in a sky region within the constellation Leo.

Within 90 min, a new, very bright light source (the “optical afterglow”) was detected in the same direction by means of a 40-inch telescope at the Siding Spring Observatory (Australia) and also in Japan. The gamma-ray burst was designated GRB 030329, according to the date.

And within 24 hours, a first, very detailed spectrum of this new object was obtained by the UVES high-dispersion spectrograph on the 8.2-m VLT KUEYEN telescope at the ESO Paranal Observatory (Chile). It allowed to determine the distance as about 2,650 million light-years (redshift 0.1685).

Continued observations with the FORS1 and FORS2 multi-mode instruments on the VLT during the following month allowed an international team of astronomers [1] to document in unprecedented detail the changes in the spectrum of the optical afterglow of this gamma-ray burst. Their detailed report appears in the June 19 issue of the research journal “Nature”.

The spectra show the gradual and clear emergence of a supernova spectrum of the most energetic class known, a “hypernova”. This is caused by the explosion of a very heavy star – presumably over 25 times heavier than the Sun. The measured expansion velocity (in excess of 30,000 km/sec) and the total energy released were exceptionally high, even within the elect hypernova class.

From a comparison with more nearby hypernovae, the astronomers are able to fix with good accuracy the moment of the stellar explosion. It turns out to be within an interval of plus/minus two days of the gamma-ray burst. This unique conclusion provides compelling evidence that the two events are directly connected.

These observations therefore indicate a common physical process behind the hypernova explosion and the associated emission of strong gamma-ray radiation. The team concludes that it is likely to be due to the nearly instantaneous, non-symmetrical collapse of the inner region of a highly developed star (known as the “collapsar” model).

The March 29 gamma-ray burst will pass into the annals of astrophysics as a rare “type-defining event”, providing conclusive evidence of a direct link between cosmological gamma-ray bursts and explosions of very massive stars.

What are Gamma-Ray Bursts?
One of the currently most active fields of astrophysics is the study of the dramatic events known as “gamma-ray bursts (GRBs)”. They were first detected in the late 1960’s by sensitive instruments on-board orbiting military satellites, launched for the surveillance and detection of nuclear tests. Originating, not on the Earth, but far out in space, these short flashes of energetic gamma-rays last from less than a second to several minutes.

Despite major observational efforts, it is only within the last six years that it has become possible to pinpoint with some accuracy the sites of some of these events. With the invaluable help of comparatively accurate positional observations of the associated X-ray emission by various X-ray satellite observatories since early 1997, astronomers have until now identified about fifty short-lived sources of optical light associated with GRBs (the “optical afterglows”).

Most GRBs have been found to be situated at extremely large (“cosmological”) distances. This implies that the energy released in a few seconds during such an event is larger than that of the Sun during its entire lifetime of more than 10,000 million years. The GRBs are indeed the most powerful events since the Big Bang known in the Universe, cf. ESO PR 08/99 and ESO PR 20/00.

During the past years circumstantial evidence has mounted that GRBs signal the collapse of massive stars. This was originally based on the probable association of one unusual gamma-ray burst with a supernova (“SN 1998bw”, also discovered with ESO telescopes, cf. ESO PR 15/98). More clues have surfaced since, including the association of GRBs with regions of massive star-formation in distant galaxies, tantalizing evidence of supernova-like light-curve “bumps” in the optical afterglows of some earlier bursts, and spectral signatures from freshly synthesized elements, observed by X-ray observatories.

VLT observations of GRB 030329
On March 29, 2003 (at exactly 11:37:14.67 hrs UT) NASA’s High Energy Transient Explorer (HETE-II) detected a very bright gamma-ray burst. Following identification of the “optical afterglow” by a 40-inch telescope at the Siding Spring Observatory (Australia), the redshift of the burst [3] was determined as 0.1685 by means of a high-dispersion spectrum obtained with the UVES spectrograph at the 8.2-m VLT KUEYEN telescope at the ESO Paranal Observatory (Chile).

The corresponding distance is about 2,650 million light-years. This is the nearest normal GRB ever detected, therefore providing the long-awaited opportunity to test the many hypotheses and models which have been proposed since the discovery of the first GRBs in the late 1960’s.

With this specific aim, the ESO-lead team of astronomers [1] now turned to two other powerful instruments at the ESO Very Large Telescope (VLT), the multi-mode FORS1 and FORS2 camera/spectrographs. Over a period of one month, until May 1, 2003, spectra of the fading object were obtained at regular rate, securing a unique set of observational data that documents the physical changes in the remote object in unsurpassed detail.

The hypernova connection
Based on a careful study of these spectra, the astronomers are now presenting their interpretation of the GRB 030329 event in a research paper appearing in the international journal “Nature” on Thursday, June 19. Under the prosaic title “A very energetic supernova associated with the gamma-ray burst of 29 March 2003”, no less than 27 authors from 17 research institutes, headed by Danish astronomer Jens Hjorth conclude that there is now irrefutable evidence of a direct connection between the GRB and the “hypernova” explosion of a very massive, highly evolved star.

This is based on the gradual “emergence” with time of a supernova-type spectrum, revealing the extremely violent explosion of a star. With velocities well in excess of 30,000 km/sec (i.e., over 10% of the velocity of light), the ejected material is moving at record speed, testifying to the enormous power of the explosion.

Hypernovae are rare events and they are probably caused by explosion of stars of the so-called “Wolf-Rayet” type [4]. These WR-stars were originally formed with a mass above 25 solar masses and consisted mostly of hydrogen. Now in their WR-phase, having stripped themselves of their outer layers, they consist almost purely of helium, oxygen and heavier elements produced by intense nuclear burning during the preceding phase of their short life.
“We have been waiting for this one for a long, long time”, says Jens Hjorth, “this GRB really gave us the missing information. From these very detailed spectra, we can now confirm that this burst and probably other long gamma-ray bursts are created through the core collapse of massive stars. Most of the other leading theories are now unlikely.”
A “type-defining event”

His colleague, ESO-astronomer Palle M?ller, is equally content: “What really got us at first was the fact that we clearly detected the supernova signatures already in the first FORS-spectrum taken only four days after the GRB was first observed – we did not expect that at all. As we were getting more and more data, we realised that the spectral evolution was almost completely identical to that of the hypernova seen in 1998. The similarity of the two then allowed us to establish a very precise timing of the present supernova event”.

The astronomers determined that the hypernova explosion (designated SN 2003dh [2]) documented in the VLT spectra and the GRB-event observed by HETE-II must have occurred at very nearly the same time. Subject to further refinement, there is at most a difference of 2 days, and there is therefore no doubt whatsoever, that the two are causally connected.

“Supernova 1998bw whetted our appetite, but it took 5 more years before we could confidently say, we found the smoking gun that nailed the association between GRBs and SNe” adds Chryssa Kouveliotou of NASA. “GRB 030329 may well turn out to be some kind of ‘missing link’ for GRBs.”

In conclusion, GRB 030329 was a rare “type-defining” event that will be recorded as a watershed in high-energy astrophysics.

What really happened on March 29 (or 2,650 million years ago)?
Here is the complete story about GRB 030329, as the astronomers now read it.

Thousands of years prior to this explosion, a very massive star, running out of hydrogen fuel, let loose much of its outer envelope, transforming itself into a bluish Wolf-Rayet star [3]. The remains of the star contained about 10 solar masses worth of helium, oxygen and heavier elements.

In the years before the explosion, the Wolf-Rayet star rapidly depleted its remaining fuel. At some moment, this suddenly triggered the hypernova/gamma-ray burst event. The core collapsed, without the outer part of the star knowing. A black hole formed inside, surrounded by a disk of accreting matter. Within a few seconds, a jet of matter was launched away from that black hole.

The jet passed through the outer shell of the star and, in conjunction with vigorous winds of newly formed radioactive nickel-56 blowing off the disk inside, shattered the star. This shattering, the hypernova, shines brightly because of the presence of nickel. Meanwhile, the jet plowed into material in the vicinity of the star, and created the gamma-ray burst which was recorded some 2,650 million years later by the astronomers on Earth. The detailed mechanism for the production of gamma rays is still a matter of debate but it is either linked to interactions between the jet and matter previously ejected from the star, or to internal collisions inside the jet itself.

This scenario represents the “collapsar” model, introduced by American astronomer Stan Woosley (University of California, Santa Cruz) in 1993 and a member of the current team, and best explains the observations of GRB 030329.

“This does not mean that the gamma-ray burst mystery is now solved”, says Woosley. “We are confident now that long bursts involve a core collapse and a hypernova, likely creating a black hole. We have convinced most skeptics. We cannot reach any conclusion yet, however, on what causes the short gamma-ray bursts, those under two seconds long.”

Original Source: ESO News Release

Interested in space and astronomy but you’ve never actually looked through a telescope? Until you’ve actually gone out and done some actual observing with your own two eyes, you won’t know what you’re missing. In this article, Fraser gives you a kick in the pants to get out there under the skies and start enjoying the heavens above. You don’t need any special equipment or advanced university degrees, just some enthusiasm, a little time and the ability to look up.

I know there are a lot of subscribers interested in space and astronomy, but I’m wondering how many of you have actually taken a look through a telescope and seen some of the objects I talk about with your own eyes.

One of my fondest memories was when I was 13 years old, and set up my 4″ telescope at my Dad’s birthday party. I was in a darkish corner of our property and would sneak away a few partygoers to show them Saturn. Fortunately the rings were at their greatest angle, and people looking through the eyepiece couldn’t believe their eyes. Looking at pictures taken by Hubble is one thing, but when you’re actually looking through the eyepiece at Saturn, it’s an incredible experience.

Stargazing has since played a big part of my life: I organized a star party, hit on my future wife by pointing out constellations, and started a space-related website, but I’m still amazed at the number of people who’ve never actually gone out there and gotten to know their sky.

With all the new observatories and space news, I think that people are starting to think that astronomy is one of those sciences reserved for people with the expensive instruments, but that couldn’t be further from the truth. It’s one of the few sciences that amateurs still make valuable contributions, and it costs absolutely nothing to get started – you just need your eyes, and a little knowledge.

Find your community
The first thing you need to do is make a commitment to get involved in astronomy. It’s not as easy as just turning on your television; you’ve got to get organized; make some phone calls; set aside some time to explore.

I’ll bet you didn’t know, but there’s an astronomical society lurking in almost every population centre on the planet. We’ve got dozens just here in Canada, and there are literally thousands in the US. The members of the society will usually meet on a regular basis and will have observing nights where they all get together and point their telescopes at different objects. This is a great way to quickly see what the night sky has to offer.

Do a search on Google with the search terms: yourtown astronomical society. For example, I would do a search for: Vancouver astronomical society. If nothing turns up for your specific location, broaden the search a bit. Eventually you should come up with something. Find the contact information for the society and drop them an email or give them a phone call. Trust me, they’ll be happy to give you more information and have you join them for an evening.

Next, see if there’s an observatory in your region. Although most of the largest telescopes are fully booked up for years in advance, some of the smaller ones have open nights where people can come down, ask to see stuff and they’ll move the scope around. Often these open nights are run by the local astronomical society. Once again, contact the society and find out if they can recommend an observatory to check out. Or, you can do a search on Google (search for: yourtown observatory) and contact them directly.

Learn your constellations
Whether you actually contact a society or just decide to go solo is up to you, but your first step is to learn some of your constellations. Maybe you already know the Big Dipper or Orion’s belt, but there are 88 constellations in Northern and Southern hemispheres. It’s pretty cool to be able to ask a person what their sign is, and then point it out in sky.

Learning your constellations is also the first step to finding some of the more interesting stuff to look at in the night sky. They’re like your guides. For example, our nearest galaxy, Andromeda (aka M31) is easily visible in binoculars or a telescope. It’s just a little up from the middle of the constellation Andromeda, which is just above Aries. I can spot M31 in a second whenever a look up in the sky (at the right time of year). Once you start to learn your constellations, they all start to fit together like a puzzle. And the great thing is the knowledge never goes away, even if it’s been a few years since you’ve done any observing.

There are many great resources for learning your constellations. One option is to do a search, once again on Google, for the term: astronomy sky charts. Some of these are fairly detailed, however, and make it hard to just learn the basic constellations.

The book that taught me, and I can’t recommend it highly enough is Nightwatch, by Terrence Dickinson. The book breaks the night sky into seasons and then has single pages for each chunk of sky with clearly defined stars and constellations – similar to one of those road maps that sit open on the car seat next to you. The book also has fabulous information on starting equipment, etc. (Order Nightwatch from Amazon.com – $20.97)

Another handy tool is Astronomy magazine. The middle of each issue is a star chart for the current month. The advantage of using a magazine like Astronomy is that it also has the current positions of the planets. (Click here to get a subscription to Astronomy for 32% off the newsstand price)

Finally, you can use a software product like Starry Nights, which lets you define your location and time to produce a custom star chart that includes the locations of the planets. (Click here for more information on Starry Nights)

Once you’ve got your sky chart together, I suggest you also get a flashlight with a red-light filter. You can usually pick them up at camping stores or army surplus. This way you can look at your charts without ruining your night vision.

Now, hit the road! If you live in an area with reasonably dark skies, you can just turn out the lights in your house and head into your back yard. If you live in a city, you’ll have to get a little ways out. Even a dark park or dimly lit suburb will be a vast improvement over the downtown core. City lights cause two problems: the streetlights will send a glare up into your night sky, dimming your visibility; and the lights will ruin your night vision directly.

Give yourself a couple of hours, and by the end of it you’ll be familiar with most of the constellations in the sky. You’ll probably also see a few meteors and even some satellites. Quality family entertainment if you ask me.

Improving your stargazing experience
Astronomy is one of those hobbies that you can enjoy for free, but you can really improve your experience with some basic equipment.

Binoculars
Chances are you’ve already got one of the most useful pieces of stargazing equipment already in your home: binoculars. Anywhere you look in the night sky is significantly improved by a simple pair of astronomical binoculars, from the Moon to star clusters. In fact, some stuff looks better in binoculars than a more powerful telescope.

Binoculars generally have two measurements: magnification and field of view. For example, a common kind is 7×35. This means it has a 7x magnification and 35mm field of view. For astronomy, power isn’t necessarily a good thing. Some go as high as 20x or even 30x, but this usually creates a very small field of view. And since you’re holding the binoculars with your hands, it can get very shaky.

It’s much better to go with a lower power set of binoculars with a large field of view: 8×50 is a perfect combination of power and field of view.

Obviously it’s important to have good quality optics, but that’s one of those things that you should experience with first to get a sense of the equipment you already have. If it’s too high-power, or you can’t focus the image to get really crisp stars, you might want to consider upgrading your gear.

It’s also really useful to have a tripod adapter hole on the bottom of your binoculars. This will let you screw them onto the top of a tripod and then let other people come and take a look through the eyepieces to share your view.

Here are some links to Binoculars.com for some good astronomical binoculars:

Celestron 7×50 Enduro. Straightforward pair of binoculars with good magnification and field of view. $57.40 USD

Bausch & Lomb 10×50 Legacy. Higher magnification with 50mm field of view. $111.00 USD.

Canon 15×50 IS. Pretty much the best binoculars you can get. Higher resolution but image stabilization keeps the image from shaking. $899.00 USD

Telescope
If you’re thinking of buying a telescope, then you’ve really got the bug. However, don’t just run down and purchase a telescope from a department or toy store. These usually have low quality optics, a jiggly mount and generally stink for astronomy – those “in the know” call them “Christmas trash scopes”.

For the same price or a little more you can purchase a real telescope with quality optics and mount and have a much better experience with the night sky.

There are many different kinds of telescopes, and explaining the differences of how to select a good telescope can fill a book so I won’t go into the details here. Remember your contacts at your local astronomical society? Let them know your budget and objectives and they can probably recommend a good telescope. They might even know someone who’s selling one used. Of course, these folks are going to be astronomy fans, so they might have bigger ideals than what you’re looking for.

There are two main kinds of telescopes: refractors and reflectors.

Refractors work through a series of lenses which focus light into the telescope’s eyepiece (think of your traditional ship captain’s spyglass) and typically have a main lens between 70mm and 100mm. These can be solid telescopes, but the optics can make them more expensive than reflectors.

One example refractors would be the Meade EXT-70AT ($298.00 USD). A small portable refractor with with a computer-controlled mount. Put the telescope on a flat surface, align it with the sky and then it can automatically pick out targets in the sky. These automated telescopes can take some of the fun out of stargazing, but it definitely speeds things up.

Reflectors use a big mirror to reflect and focus incoming light to the telescope’s eyepiece. They’re usually shorter and fatter than a refractor, starting at 4 inches and going up from there. I started, and still use a 4″ telescope, which is perfectly fine to see the major planets and all kinds of astronomical objects.

An example reflector is a Celestron 4.5″ Firstscope ($149.00). No computer on this telescope, so you’ll get a chance to learn the location of sky objects on your own.

Bigger telescopes gather more light, so they can display fainter objects, but they come with a higher price. My recommendation is to start small, get some experience before considering a higher-end telescope.

Probably the best starting telescope is something like a 6″ Dobsonian reflector. Unlike most telescopes you’ve seen, the Dobsonians have their mount down at the base and then point up. They’re solid, inexpensive, and easy to use. Some of the largest, most powerful amateur-built telescopes are Dobsonians.

Here’s a link to a Swift Instruments 6″ Dobsonian telescope ($382.95 USD).

An a link to a much larger Meade Starfinder 16″ Dobsonian ($1,386.00 USD).

Now Get Out There!
Enough reading, start sky watching. Early Summer is a great time to get involved in astronomy (and a terrible time to watch TV) – warm summer nights and stargazing go hand in hand. Do a little research, grab some supplies, gather the friends and family, and get out under the stars. And please, email me your summer experiences. Trust me, you’ll get some memories you’ll never forget.

Image credit: ESO

A new series of photographs taken by the European Southern Observatory show a rare look into the very early stages of heavy star formation. This time in a star’s life is usually obscured from sight because of thick clouds of gas and dust, but in star cluster NGC 3603, the stellar wind from hot stars are blasting away the obscuring material. Inside this cluster, astronomers are finding massive protostars which are only 100,000 years old. This is a valuable discovery because it helps astronomers understand how the early stages of heavy star formation begins – is it through gravity pulling together gas and dust, or something more violent, like smaller stars colliding together.

Based on a vast observational effort with different telescopes and instruments, ESO-astronomer Dieter N?rnberger 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.

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 N?rnberger 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 ?m) 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 ?C to 0?C). Their own temperatures are quite high, of the order of 20,000 – 22,000 degrees.

What do the massive protostars tell us?
Dieter N?rnberger 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.

Original Source: ESO News Release