Herbig-Haro object

In Astronomy, objects Herbig-Haro (or objects HH ) are small nebulosities associated with some very young people star S, and which is formed when matter ejected by these incipient stars enters in collision with the clouds of surrounding gas and dust, at speeds of several hundred kilometers a second. The Herbig-Haro objects are omnipresent in the areas of stellar formation, and very often it is possible to observe several around the same star of them, aligned along its Axis of rotation.

These objects are transitory phenomena, having one lifespan of a few thousands of years at most. It is possible to see them evolving/moving over one relatively short period of time, whereas they move away from the star from which they result through the interstellar gas clouds. The observations of the Space telescope Hubble only show complex evolutions in a few years, certain parts weakening while others are illuminated, according to the Densité medium met.

The Herbig-Haro objects were observed for the first time by Sherburne Wesley Burnham at the end of the 19th century, but it is only in the Années 1950 that one understood that it was about a new type of Nébuleuse in emission. The first astronomers studied them in detail were George Herbig and Guillermo Haro, from where their name. Herbig and Haro studied independently the formation of stars when they analyzed these objects, and understood for the first time that those were the consequence of the formation process of stars.

Discovered and observations

The first Herbig-Haro object was discovered by Sherburne Wesley Burnham at the end of the 19th century: whereas it observed the star T Tauri with the glasses of 900 mm of the Observatoire Lick, Burnham seen just at side a very small nebulosity. This one was catalogued like a Nébuleuse in emission “standard”, named nebulous of Burnham thereafter (from now on HH 255), but on this date was not recognized like representing of a new class of objects. On the other hand one knew already at that time that T Tauri was a variable star very young, prototype of variable a star class called variable of type T Tauri. These stars are extremely young and did not reach the stage of balance yet where gravitational collapse is compensated by the reactions of nuclear Fusion in their center.

At the end of the Years 1940, is fifty years after the discovery of Burnham, a certain number of similar nebulas had been observed, the majority being so small which one could almost confuse them with stars. Guillermo Haro and George Herbig then carried out way independent of the observations on several of these objects. Herbig leant again on the nebula of Burnham and found that it had a electromagnetic Specter unusual, with emission lines very marked for the Hydrogène, the Soufre (II) and the Oxygène (II), and that other objects, such as HH 1, HH2 and 3, showed similar characteristics. On its Haro side in 1952-53 the discovery of many other objects of the same type announced, and showed that all were invisible in Infrarouge.

Following their observations, Herbig and Haro met with a conference of astronomy to Tucson in Arizona in 1949. Herbig had not paid much attention to the objects which it had observed, being especially pursued the study of young close stars, but by taking note of discovered of Haro, it decided to undertake a more thorough study. The Soviet astronomer Viktor Ambartsumian then gave their current name to these objects, and while basing itself on their proximity with the youngest stars (old of a few hundreds of thousands of years at most), suggested that the Herbig-Haro objects can be related to the first stages of the formation of the stars T Tauri.

The studies which followed showed that objects HH were strongly ionized, and the first theories advanced that they can contain hot stars of weak luminosity. However, the infra-red absence of radiation coming from nebula contradicted this assumption. One imagined later that nebula could contain Protostar S, the energy released during the process of Accrétion becoming the source of photoionization.

Progressively of the theoretical and observational projections, it became clear that objects HH was generated by the matter ejected by young close stars, this matter entering in collision at very high speeds with gas of the interstellar Milieu.

The beginning of the Years 1980, technological advances made it possible for the first time the observations to reveal the shape out of jets of objects HH. This naturally resulted in understanding that the ejected matter and who give rise to objects HH is concentrated out of very fine bipolar jets (collimated). Indeed, the incipient stars are surrounded, during the first thousands of years of their existence, by a Disque of accretion formed by remainders of the initial gas cloud. The fast rotation of the internal parts of this disc generates the emission of powerful partially ionized matter jets perpendicular to the disc plane. When these jets enters in collision with the interstellar Milieu, they give rise to small nebulas in emission, of which the Herbig-Haro objects.

Physical characteristics

The radiation emitted by the Herbig-Haro objects is due to the shock waves caused by the collision with the interstellar Milieu, but their movements are complicated. The spectroscopic observations of the shift Doppler indicate that the matter of the jets moves at speeds of several hundred kilometers a second, but the emission lines of the spectrum of these objects are too weak to be formed at such speeds of collision. This probably means that the matter with which the jets enters in collision is also moving, moving away it also from central star, but at a speed lower than that of the jets.

The estimate of the Mass total ejected necessary to form an object HH is about one to twenty terrestrial masses, that is to say a relatively small quantity compared to the total mass of star itself. The temperatures observed in objects HH are of approximately 8 000 with 12 000 Kelvin S, similar to those observed in other types of ionized nebulas, such as the areas HII or the planetary nebulas. They are relatively dense, their density varying of a few thousands to a few tens of thousands of particles per cm ³, areas HII having by comparison a density lower than 1000 particles per cm ³ in general. Objects HH are made up mainly of Hydrogène and of Hélium, respectively 75% and 25% mass some approximately. Less one pourcent total mass of those consists of heavier elements, and their abundance is in general similar to that measured in young close stars.

Near the star source, between 20 and 30% of gas of an object HH is ionized, but this proportion tends to decrease with the distance. This implies that the gas is ionized in the polar jet, and recombines then whereas it moves away from star, rather than to be ionized at the time of the collision in itself. The shock occurring at the end of the jet can however D-ionize part of the matter, giving rise to more brilliant “hats” at the end of the jets.

Number and distribution

More than 450 objects HH or groups of objects are currently indexed (2006). They are omnipresent in the areas of formation of stars, and very often present in groups. One very often observes them near the globules of Bok (of the dark nebulas containing very young stars), and they emanate besides often from these globules. It is frequent to observe several near one of them only stars, thus forming a kind of chain along a line representing the Axis of rotation of this one.

The number of known objects HH increased these last years quickly, but it is generally thought that this number actually represents only one very small proportion of the quantity really existing in the Galaxie. The estimates suggest that there are some until 150 000, great majority of between-them being too distant and too not very luminous to be able to be solved with the current instruments. The majority of objects HH are at a distance not exceeding 0,5 Parsec of the star source, some having been observed up to 1 parsec. There is however a small number being located several away parsecs, probably because the interstellar environment is far from dense in their vicinity, thus allowing the matter ejected to travel much further before being dispersed.

Own movement and variability

The spectroscopic observations of objects HH show that those move away from the star source at speeds varying from 100 to 1000 km/s. These last years, the Space telescope Hubble made it possible to measure the own Mouvement several objects HH, thanks to spaced observations several years.

When they move away from their star, objects HH only evolve/move to a significant degree, variable in luminosity over periods of a few years. Certain “nodes” inside object HH can light, weaken or disappear completely, while others appear at a different place.

The matter of the jets emitted by star is not ejected uninterrupted but rather by impulses. These pulsations can produce gas jets moving in the same direction but at different speeds, and the interactions between these various jets produce in their turn of the shock waves.

The stars sources

The stars which hide behind the creation of the Herbig-Haro objects are very young people, youngest of enters being still Protostar S in the course of formation starting from surrounding gas. The astronomers classify these stars in four classes, 0, I, I and III, according to the intensity of the radiation Infrarouge emitted by star. The more the infra-red radiation is raised, the more the star is surrounded by cold matter, which indicates that it is still at the stage of gravitational collapse.

The stars of class 0 have only a few thousands of years, and are so young that they did not begin yet the process of fusion nuclear. The reactions of fusion started in the heart of the objects of class I, but the gas and dust continue to be accrétés by the star in formation. At this stage these stars generally are still wrapped in a dense gas cloud and dust, which darkens the visible light and makes that they can be observed only in the ranges infra-red and radio. The accretion of gas and the particles of dust is mainly finished for stars of class II, but they are always surrounded by a disc of gas and dust, while the stars of class III have nothing any more but some traces of their original accretion disc.

Studies showed that approximately 80% of stars giving rise to Herbig-Haro objects are in fact of the double stars or multiples, this proportion being much larger than that found for stars of low mass on the principal sequence. This seems to indicate that the binary systems are more favourable with the formation of the jets which will give rise to then objects HH.

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