eso0310 — Science Release
Really Hot Stars
Spectacular VLT Photos Unveil Mysterious Nebulae
9 April 2003
Quite a few of the most beautiful objects in the Universe are still shrouded in mystery. Even though most of the nebulae of gas and dust in our vicinity are now rather well understood, there are some which continue to puzzle astronomers. This is the case of a small number of unusual nebulae that appear to be the subject of strong heating - in astronomical terminology, they present an amazingly "high degree of excitation." This is because they contain significant amounts of ions, i.e., atoms that have lost one or more of their electrons. Depending on the atoms involved and the number of electrons lost, this process bears witness to the strength of the radiation or to the impact of energetic particles. But what are the sources of that excitation? Could it be energetic stars or perhaps some kind of exotic objects inside these nebulae? How do these peculiar objects fit into the current picture of universal evolution? New observations of a number of such unusual nebulae have recently been obtained with the Very Large Telescope (VLT) at the ESO Paranal Observatory (Chile). In a dedicated search for the origin of their individual characteristics, a team of astronomers - mostly from the Institute of Astrophysics & Geophysics in Liège (Belgium)  - have secured the first detailed, highly revealing images of four highly ionized nebulae in the Magellanic Clouds, two small satellite galaxies of our home galaxy, the Milky Way, only a few hundred thousand light-years away. In three nebulae, they succeeded in identifying the sources of energetic radiation and to eludicate their exceptional properties: some of the hottest, most massive stars ever seen, some of which are double. With masses of more than 20 times that of the Sun and surface temperatures above 90 000 degrees, these stars are truly extreme.
Four unique images of highly excited nebulae in the Magellanic Clouds
This collection of impressive VLT colour photos is unique. They show some of the highest excitation nebulae in the Magellanic Clouds (MCs), two satellite galaxies of our own Milky Way.
They may be enjoyed for their beauty alone. However, each of them also carries a message about the depicted objects, their properties and evolutionary state. In fact, they represent the spectacular and visible result of a dedicated research programme begun by an international team of astronomers from Belgium and the United States of America , and aimed at unravelling the secrets of unsually hot nebulae.
What makes them shine? From where come the enormous energies needed to make these nebulae glow in the light of ionized helium atoms?
Nebulae are huge clouds of gas and dust, the cosmic material from which stars and planets form, cf. the Appendix. Many of them emit their own light, and are then called emission nebulae. Astronomers distinguish between Planetary Nebulae (PNe), Supernova Remnants (SNRs) and "normal" emission nebulae or "HII regions" (pronounced "Eitch-two").
PNe result from the death of comparatively light stars, similar to our Sun, while SNRs originate from the explosive death of heavier stars. The collision between the surrounding interstellar matter and that ejected by the dying star, accompanied by the intense radiation from the hot stellar remnant (white dwarf, neutron star) excites the gas and makes it shine brightly.
But the radiation of young hot stars embedded in an interstellar cloud is also able to heat the surrounding gas, resulting in the apparition of another type of emission nebula, that shines mostly in the light of ionized hydrogen (H) atoms. Such nebulae are therefore often referred to as "HII regions". The well-known Orion Nebula is an outstanding example of that type of nebula.
Highly excited nebulae
The hotter the central object of an emission nebula, whether a white dwarf, a neutron star or just a young star, the hotter and more excited will be the surrounding nebula. The word "excitation" refers to the degree of ionization of the nebular gas. The more energetic the impinging particles and radiation, the more electrons will be lost and higher is the degree of excitation.
Only in the most excited nebulae is there enough ultraviolet energy to completely ionize the helium atoms. When these ions subsequently capture an electron, this process gives rise to the characteristic radiation of single ionized helium (HeII). A particularly useful way to trace the very highest excitation areas is thus to map the distribution of HeII by means of imaging or spectroscopic observations that are sensitive to the radiation from these helium ions, for example at a particular wavelength in blue light (468.6 nm).
It is common to detect the presence of HeII in Planetary Nebulae around extremely hot white dwarf stars, but not in "normal" HII regions. However, a few otherwise seemingly normal HII regions reveal the characteristics of high excitation. One of them is located in our own Milky Way galaxy, another has been found in the nearby galaxy IC 1613, and five others are situated in the Magellanic Clouds.
Astronomers have also detected the presence of HeII ions in a number of remote galaxies undergoing a phase of intense star formation ("starburst galaxies") and in the vicinity of ultraluminous X-ray sources in very distant galaxies. What is going on in those remote objects in the early Universe? Do we see the action of young and very hot stars or is something unknown going on? What can the existence of those hot nebulae in young galaxies tell about the evolution of our own Milky Way?
Searching for the energy source
We would like to know, but those distant nebulae are unfortunately too faint to be studied in any reasonable detail, even by means of the largest available telescopes. The only way forward is therefore to look closer at the nearest ones in the hope that they will provide clues about the processes leading to the observed high excitation and thus help to better understand their cousins in those distant galaxies.
There appears to be three possible answers to the basic question about the nature of the energetic sources that heat these strange emission nebulae:
- very fast particles: if there is in the area a fast-moving gas (more than 100 km/s), the shock created by the impact of this material is able to heat the ambient interstellar medium sufficiently to produce a HeII nebula;
- ultraviolet emission from massive stars: according to the most recent model calculations, even the most massive O-type stars do not emit enough ultraviolet light to ionize a sufficient number of helium atoms in the surrounding nebula to produce a detectable HeII nebula. However, some of the hottest stars of the so-called Wolf-Rayet (W-R) type (that are the evolved descendants of O-stars) may produce enough high energy emission to completely ionize the helium atoms in their surroundings;
- intense X-ray emission: close binary stars in which one component is a "compact" object (a white dwarf, a neutron star, or a black hole) and the other an "ordinary" star can produce an intense X-ray emission. This happens because the compact object is so dense and massive that it siphons off matter from its companion star - astronomers refer to this as an accretion process, sometimes also called "stellar cannibalism". When the "stolen" matter approaches the compact object, it gradually heats up and may reach temperatures of millions of degrees. It then emits X-rays. At the same time, ultraviolet radiation is also emitted, which may produce high excitation regions in the surrounding nebula. This scenario can also explain the association of HeII nebulae with ultraluminous X-ray sources in other galaxies.
VLT observations of highly excited nebulae in the MCs
Observations of a number of highly excited nebulae in the Magellanic Clouds were carried out by a team composed of Belgian and American astronomers  in January 2002, by means of the FORS1 multi-mode instrument at the 8.2-m VLT MELIPAL telescope. Detailed images were obtained through various special optical filters - they bring into light the complex structure of these nebulae and reveal for the first time the exact morphology of the high excitation zones.
Some of exposures have been combined to produce the colour photos shown in ESO Press Photos eso0310-d. Here, the blue colour traces the exceptional HeII emission, whilst the red and green correspond to the more common nebular emissions from atomic hydrogen and doubly-ionized oxygen, respectively.
All four nebulae shown were found to be associated with very hot stars. They carry rather prosaic names: BAT99-2 and BAT99-49, AB7 and N44C Star #2 .
The first three of these objects contain some of the highly evolved massive stars, of the so-called Wolf-Rayet (WR) type, while the fourth is an mid-age massive star, of type O. Massive stars, with masses more than 20 times that of the Sun, are very bright (100,000 to 10 million times brighter than the Sun), very blue and very hot, with surface temperatures of a few tens of thousands of degrees.
Another property of these exceptional stars is their very strong stellar winds: they continuously eject energetic particles - like the "solar wind" from the Sun - but some 10 to 1000 million times more intensely than our star! These powerful winds exert an enormous pressure on the surrounding interstellar material and forcefully shape those clouds into "bubbles".
These photos have now provided the astronomers with sufficient information to understand exactly what is going on in three of those unusual nebulae - while one case still remains ambiguous.
The nebulae around BAT99-2, BAT99-49 and AB7
BAT99-2 (cf. ESO Press Photo eso0310) is one of the hottest WR-stars known in the Large Magellanic Cloud (LMC). Before this star reached this phase of its short life, the strong stellar wind from its progenitor O-type star swept the interstellar medium and created a "bubble", much like a snowplough pushes aside the snow on a road.
Part of this "bubble" can still be seen as a large half-ring to the south of the star. When the star did become a WR, the increasingly intense stellar wind impacted on the material previously ejected from the star. This created a new bubble, now visible as a small arc-like structure to the north-west of the star. We are appparently witnessing an ongoing merger of these two bubbles.
With its strong ultraviolet (UV) radiation, BAT99-2 is strongly heating its immediate surroundings, in particular the above mentioned arc-like feature that, due to the resulting high excitation, is seen as a violet-pink region in the colour image. The entire field is very complex - the presence of a supernova remnant (SNR) is revealed by a few faint red filaments rather close to the high excitation nebula, to the north-west of the arc-like structure.
AB7 (ESO Press Photo eso0310) and BAT99-49 (ESO Press Photo eso0310) are both binary stars, consisting of one WR-star and a companion O-type star. Like in the case of BAT99-2, the strong UV-radiation from their WR-star has created HeII nebulae around them, well visible in the photos by their blue colour.
AB7 is particularly remarkable: the associated huge nebula and HeII region indicate that this star is one of the, if not THE, hottest WR-star known so far, with a surface temperature in excess of 120,000 degrees! Just outside this nebula, a small network of green filaments is visible - they are the remains of another supernova explosion.
The new VLT images, complemented with VLT spectra, demonstrate that these stars are indeed the source of the observed ionization. These very first maps of the HeII emission unveil the as yet undiscovered complex structure of those highly excited nebulae.
Moreover, the new observations provide the first accurate determination of the true ionizing power of these exceptional stars. They allow a direct measurement of the otherwise unobservable intensity of the far-UV emission of WR stars. The new observations have clearly identified the ultraviolet emission of very massive stars as the energy source in these three nebulae. Using the latest theoretical models to interpret these unique data, the Belgian astronomers and their American collaborator were also able to show that all of these stars are hotter than 90,000 degrees!
The N44C nebula
The fourth photo, ESO Press Photo eso0310, shows the very peculiar nebula N44C in the LMC. There is a beautiful (blue) HeII nebula near the two central stars. It is very different from the larger, "normal" HII region that is delimited by the light from atomic hydrogen (red) and doubly-ionized oxygen (green): this hot central region of N44C rather appears to "enshroud" the stars like a veil.
There is a mystery, though. With a temperature of "only" a few tens of thousand degrees, even the hottest of the two stars, an O-type star (the upper one), cannot possibly be responsible for this inner high excitation nebula . Moreover, no fast motions have so far been detected in the vicinity. Some astronomers have suggested that N44C is a "fossil X-ray nebula".
What does that mean ? It may well be that this O-type star is not alone, but actually possesses a compact companion. The X-ray emission from such a binary may not be constant. During their orbital motion, the two stars can move away from each other, and the larger separation may cause the X-ray emission to stop (because of the cessation of accretion of matter onto the compact object). In this case, the observed high excitation nebula could still persist for a short period of time as a "fossil" of the previous X-ray ionized nebula. Later, that part of the nebula would then gradually disappear.
However, to the astonishment of the astronomers, the present VLT observations show little or no variation in the HeII emission. Thus the above described "fossil X-ray nebula" explanation does not appear to be completely adequate and the cause of the high excitation in N44C remains a challenge to astronomers.
"You can't win them all", says Yaël Nazé. "We were able to fully understand three nebulae, but we must now look more closely at N44C. I would not be surprised, if we will be able to solve this riddle by means of additional VLT observations."
 The names of these stars refer to the research papers in which they were first decribed. BAT99-2 and BAT99-49 are nos. 2 and 49 in the list published by Breysacher, Azzopardi and Testor (A&AS, 137, 117, 1999), AB7 is star no. 7 in the list by Azzopardi and Breysacher (A&A, 75, 120, 1979) and N44C Star #2 is included in a paper by Stasinska, Testor and Heydari-Malayeri (A&A, 170, L4, 1986).
The information contained in this press release is based on two research articles to be published in the European research journal "Astronomy & Astrophysics", one of which is available at the preprint website at the Institut d'Astrophysique et de Géophysique de Liège (Belgium).
Appendix: Different types of nebulae
Nebulae are huge clouds of gas and dust, the cosmic material from which stars and planets form. Most of them belong to five main categories, each representing a different physical state. Two of these do not shine by their own light, but three others do.
Dark nebulae and reflection nebulae
If the gas does not emit visible light by itself, astronomers talk about dark nebulae or reflection nebulae.
The former block the light from objects behind them, and they are therefore seen as dark regions in the sky - famous examples are the Barnard 68 "globule" and the "Horsehead Nebula".
Contrarily, reflection nebulae appear as bright areas in the sky because their dust particles reflect the light emitted by nearby stars. A good example is the nebulae surrounding some of the brightest stars in the "Pleiades" stellar cluster or in the southern Chamaeleon I area.
Other nebulae emit visible light of their own. Astronomers distinguish between Planetary Nebulae (PNs), Supernova Remnants (SNRs) and "normal" emission nebulae or "HII regions" (pronounced "Eitch-two").
When stars die, they eject copious amounts of matter into neighbouring space. These ejecta collide with and heat the surrounding interstellar matter. This is sometimes accompanied by intense radiation from the hot stellar remnant at the centre. These processes excite the interstellar gas (and the ejecta) so that they shine brightly.
In the case of lighter stars like the Sun, the remnant object is a hot "white dwarf", a star barely larger than the Earth and the surrounding nebula is called a "Planetary Nebula (PN)". This historical term refers to the planet-like appearance of such a nebula in a small telescope. A fine example is the "Dumbbell Nebula", photographed by the VLT in 1998.
On the other hand, heavier stars explode violently - such dramatic events are seen as supernovae - and leave behind a exceedingly hot and dense, rotating "neutron star" of diameter 10-20 km (or, in the case of the heaviest stars, presumably a "black hole") as well as a surrounding nebula, the supernova remnant (SNR). A famous example is the"Crab Nebula" from the supernova that exploded in the year 1054.
Finally, the radiation of young hot stars embedded in an interstellar cloud is also able to heat the surrounding gas, resulting in the apparition of an emission nebula, that shines mostly in the light of ionized hydrogen (H) atoms. Such nebulae are therefore often referred to as "HII regions". The well-known Orion Nebula is an outstanding example of that type of nebula.
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