Voici un premier essai sur la nébuleuse de la méduse en utilisant une combinaison de deux filtres :
– un filtre double bande permettant d’isoler les signaux H-alpha (Hydrogène) et O-III (Oxygène)
– un filtre dédié à la bande S-II (souffre)
En combinant les images couleur de ces deux filtres, on obtient des détails supplémentaires dans certaines zones (voir ci-dessous).
Le processus de combinaison consiste à extraire les 3 canaux rouge, vert et bleu de chacun des deux images puis de recomposer une image à partir de certains de ces canaux. Ainsi, dans cette image finale, les canaux sont composés ainsi :
– Rouge : canal rouge du filtre double bande (signal H-alpha maximum)
– Vert : addition des canaux vert et bleu du filtre double bande (approximation du signal O-III)
– Bleu : canal Rouge du filtre S-III (signal S-III maximum).
(Source: Wikipedia)
IC 443, ou SNR G189.0+03.0, est un rémanent de supernova situé dans la constellation des Gémeaux. De taille angulaire importante (45 minutes d’arc, soit une fois et demi le diamètre apparent de la Lune), et assez lumineux tant en X qu’en radio, il fait partie des rémanents de supernova les plus étudiés, une des raisons étant qu’il représente un prototype de rémanent interagissant avec le milieu interstellaire environnant. Pour autant tant la distance que l’âge exact du rémanent ne sont pas connus. L’étude de l’expansion du rémanent en optique plaide pour une distance modérée (entre 0,7 et 1,5 kpc), mais son association probable avec la structure appelée S249 suggère une distance plus grande (1,5 à 2,0 kpc).
Dans un instrument d’astronomie amateur, IC 443 apparaît comme une grande et faible nébuleuse en émission, accompagnée par quelques étoiles jeunes. Il faut l’observer au 200 mm de préférence avec un filtre Type OIII et des GR faibles à moyen selon la transparence atmosphérique. Sa structure laisse entrevoir deux lobes asymétriques faiblement reliés l’un a l’autre, le plus lumineux étant légèrement plus compact. Cette apparence évoque la forme d’une méduse, d’où le nom traditionnel (peu usité) de Nébuleuse de la Méduse (à ne pas confondre avec la Nébuleuse de la Méduse (Abell 21), une nébuleuse planétaire du même nom dans la même constellation).
Elle se trouve sur le bord de la constellation, près de la limite avec Orion/Taureau, et dans la voie lactée près de l’étoile η Geminorum (Tejat Prior), de magnitude apparente 3,3.
Des lycéens, en exploitant des données de Chandra, ont découvert une étoile à neutrons dans les restes de la supernova IC 4431.
(Source : Wikipedia)
IC 443 is an extended source, having an angular diameter of 50 arcmin (by comparison, the full moon is 30 arcmin across). At the estimated distance of 5,000 ly (1,500 parsec) from Earth, it corresponds to a physical size of roughly 70 light years (20 parsec).
The SNR optical and radio morphology is shell-like (e.g. a prototypical shell-like SNR is SN 1006), consisting of two connected sub-shells with different centers and radii. A third, larger sub-shell—initially attributed to IC 443—is now recognized as a different and older (100,000 years) SNR, called G189.6+3.3.[1]
Notably, IC 443 X-ray morphology is centrally peaked and a very soft X-ray shell is barely visible.[2] Unlike plerion remnants, e.g. the Crab Nebula, the inner X-ray emission is not dominated by the central pulsar wind nebula. It has indeed a thermal origin.[3] IC 443 shows very similar features to the class of mixed morphology[4] SNRs. Both optical and X-ray emission are heavily absorbed by a giant molecular cloud in the foreground, crossing the whole remnant body from northwest to southeast.
The remnant’s age is still uncertain. There is some agreement that the progenitor supernova happened between 3,000[3] and 30,000[5] years ago. Recent Chandra[6] and XMM-Newton[7] observations identified a plerion nebula, close to the remnant southern rim. The point source near the apex of the nebula is a neutron star, relic of a SN explosion. The location in a star forming region and the presence of a neutron star favor a Type II supernova, the ultimate fate of a massive star, as the progenitor explosion.
The SNR environment
IC 443 wide field image. The stars η (right) and μ (left) Geminorum, the diffuse emission from S249 (north), and the G189.6+3.3 partial shell (center) are visible.
The SNR IC 443 is located in the galactic anticenter direction (l=189.1°), close to the galactic plane (b=+3.0°). Many objects lie in the same region of sky: the HII region S249, several young stars (members of the GEM OB1 association), and an older SNR (G189.6+3.3).
The remnant is evolving in a rich and complex environment, which strongly affects its morphology. Multi-wavelength observations show the presence of sharp density gradients and different cloud geometries in the surroundings of IC 443. Massive stars are known to be short lived (roughly 30 million years), ending their life when they are still embedded within the progenitor cloud. The more massive stars (O-type) probably clear the circum-stellar environment by powerful stellar winds or photoionizing radiation. Early B-type stars, with a typical mass between 8 and 12 solar masses, are not capable of this, and they likely interact with the primordial molecular cloud when they explode. Thus, it is not surprising that the SNR IC 443, which is thought to be the aftermath of a stellar explosion, evolved in such a complex environment. For instance, an appreciable fraction of supernova remnants lies close to dense molecular clouds (~50 out of 265 in the Green catalogue[8]), and most of them (~60%) show clear signs of interaction with the adjacent cloud.
X-ray and the optical images are characterized by a dark lane, crossing IC 443 from northwest to southeast. Emission from quiescent molecular gas has been observed toward the same direction,[9] and it is likely due to a giant molecular cloud, located between the remnant and the observer. This is the main source of extinction of the low energy SNR emission.
In the southeast the blast wave is interacting with a very dense (~10,000 cm−3) and clumpy molecular cloud, such that the emitting shocked gas has a ring-like shape. The blast wave has been strongly decelerated by the cloud and is moving with an estimated velocity of roughly 30–40 km s−1.[10] OH (1720 MHz) maser emission, which is a robust tracer of interaction between SNRs and dense molecular clouds, has been detected in this region.[11] A source of gamma-ray radiation[12] is spatially coincident with IC 443 and the maser emission region, though is not well understood whether it is physically associated with the remnant or not.
In the northeast, where the brightest optical filaments are located, the SNR is interacting with a very different environment. The forward shock has encountered a wall of neutral hydrogen (HI), and is propagating into a less dense medium (~10-1,000 cm−3) with a much higher velocity (80–100 km s−1)[10] than in the southern ridge.
In the western region, the shock wave breaks out into a more homogeneous and rarefied medium.[2]