Herbig–Haro object

Herbig - Haro objects are small nebulous structures around young stars. They arise when the star ejected gas strikes dust clouds. In regions where stars are forming, Herbig - Haro objects are ubiquitous. Often they will be seen by a single star, where they are aligned along the axis of rotation. Herbig -Haro objects are very short-lived with a lifetime of at best a few thousand years. They can be visible in a very short time, when they rapidly from its parent star off (also called Interstellar matter) in the cloud of gas in interstellar space inside to move. The Hubble Space Telescope showed a complex formation of the Herbig - Haro objects in just a few years. In this short time, some could darken, while others have been brightened when she collided with the material in the interstellar medium.

These objects were first observed in the late 19th century by Sherburne Wesley Burnham, but were interpreted until the 1940s as an emission nebula. The first astronomers, who examined in detail, were George Herbig and Guillermo Haro, according to which the objects are named. Herbig Haro and worked independently in studies of the star formation when they first analyzed the Herbig -Haro objects, and then found that they are a part of the star formation process.

Discovery and observation history

The first Herbig -Haro object was observed in the 19th century by Burnham. When he observed the star T Tauri with a telescope of the Lick Observatory, a little foggy structure fell on him in the vicinity of the star. It was cataloged as an emission nebula and later received the name Burnhamsnebel. It was found that T Tauri is a very young and variable star, who is currently in an equilibrium between the collapse of its own weight and the energy through nuclear fusion in the center. Such stars are counted in the group of T Tauri stars.

50 years after Burnham's discovery of many similar fog were discovered, all of which are so small that they could be symptoms within a star system. In the 1940s, Herbig and Haro made ​​independent observations of these objects. Herbig watched the Burnham 's nebula and found an unusual electromagnetic spectrum with hydrogen, sulfur and oxygen emission lines. Haro found that all objects of this type were invisible infrared light.

After their independent discoveries to Herbig and Haro met at an astronomy conference in Tucson ( Arizona). Herbig had paid little attention to these objects, but after he had heard of Haros discoveries, this changed. The Soviet astronomer Viktor Ambartsumian gave the objects their names, and added, that they mark an early stage in the formation of T Tauri stars because of their frequency in young stars ( a few hundred thousand years old ).

Studies showed that Herbig - Haro objects are highly ionized, and early theorists speculated that they contain weak luminous hot stars. However, it has been shown by their non-existing infra-red radiation, that this is not the case. Then it was assumed that they contain protostars. According to current opinion, they are ejected from young stars material collides with supersonic with the interstellar matter.

In the early 1980s, observations showed the jet -like shape of most Herbig -Haro objects. This one had recognized that the material of them concentrated in narrow jets, so it is highly collimated. Young Stars in their first hundred thousand years, often surrounded by an accretion disk. The rapid rotation of the inner parts of the disc results in the emission of narrow, vertically moving away from the disk polar jets of partially ionized plasma. When these jets collide with the interstellar medium, this leads to structures of light radiant matter, which include the Herbig -Haro objects.

Physical Properties

Emissions of Herbig -Haro objects are caused by shock waves when they collide with the interstellar matter. However, their movements are complicated. Spectroscopic observations could be determined from a few hundred kilometers per second using the Doppler effect the speed. However, the emission lines are too weak that they could be the result of collisions at such high speeds in this range. This may mean that even some material at a lower rate moves outward and then collides with the interstellar matter.

The total mass that is ejected from the star to form a Herbig -Haro object is 1 to 20 Earth masses. This is compared to the total mass of the star very little. The temperature measured in the objects is usually 8000 12.000K and is therefore the same as in other ionized nebulae, in H II regions or planetary nebulae in about. However, you have a very large density of a few thousand to a few tens of thousands of particles per cubic centimeter, while H II regions or planetary nebulae usually contain less than 1000 particles / cm ³. Herbig - Haro objects are composed mainly of hydrogen and helium with a mass fraction of 75% and 25%. Less than 1 % of the mass assume the heavier chemical elements, which corresponds to approximately the same proportion of heavy elements in young stars.

Near the origin of the star Herbig - Haro 20-30% of the object are ionized. However, the proportion decreases with increasing distance. This assumes that the material was ionized in Polarjet, will then recombined again, and then ionized hardly again by subsequent collisions. The collision at the end of the Jets something material can ionize again, making small bright "caps" arise at the end of the jet.

Number and distribution

There are over 400 known individual Herbig - Haro objects or groups. They are ubiquitous in stellar nurseries like in H II regions, where they are often found in large groups. They are usually near globules ( dark nebulae, the very young stars include ) observed and often go out of them. Often several Herbig -Haro objects are observed by a single power source, where they form a chain of objects along the polar axis of the source star

The number of known Herbig - Haro objects increased rapidly over the last few years, but is still much lower than the number that is estimated for our galaxy. It is considered that exist in our Milky Way 150,000, with most of today are too far away, that one could observe them with today's technology. Most of Herbig - Haro objects are within a half parsecs of their parent star, and only a few have been found that are further away than 1 pc. An even smaller proportion was found in a distance of several parsecs to the parent star. It is believed that in these cases the interstellar medium has a very low density, so that the Herbig -Haro objects can move further before they pass away.

Proper motion

Spectroscopic observations of Herbig -Haro objects show that they are moving away at a speed of 100-1000 km / s. In recent years, the proper motion of the Herbig - Haro objects has been studied for several years by high-resolution images from the Hubble Space Telescope. By this observation, the removal of some of these objects could also be determined with the help of parallax.

If they move away from their parent star, they develop critical. They vary in brightness within a few years. Individual nodes in the object can brighten, fade or disappear, while new ones emerge. Further interactions with the intergalactic medium and between jets of different speeds are also a reason for change.

The jets that are generated by the parent star, are not uniform currents, but rather individual eruptions. This creates jets, although moving in the same direction but at different speeds, resulting in clashes. This creates shock waves.

Origin stars

The stars that are responsible for the formation of Herbig - Haro objects are always very young. The youngest are still protostars that form from the surrounding gas. Astronomers divide these stars to the classes 0, I, II and III, depending on how much emits infrared radiation of the star. From a larger amount of infrared radiation allows conclusions to a larger amount of cold material around the star, what does it matter that their matter is still contracting. The numbering arose because class 0 objects ( youngest) had not yet been discovered when they had the classes I, II and III already defined.

Class 0 objects have an age of only a few thousand years. That's so young that has not been used in their centers, nuclear fusion. Your energy they receive instead only from the conditional by the gravitational Potential energy when matter falls into the interior. Nuclear fusion begins with Class I objects, but with them gas and dust still falls on its surface. You are still covered by a thick layer of dust, which lets through visible light. They can therefore only observe with wavelengths in the radio or infrared. The incidence of gas and dust is then largely completed in class 2 objects. However, they are still surrounded by a disk of gas and dust. From this disc are in Class III stars only remains to find.

Through research it has been found that approximately 80 % of the stars, which you have found Herbig - Haro objects, double or multiple star systems. As this is a very large proportion, it is assumed that in case of multiple star systems much more frequently arise jets, from which then form the Herbig -Haro objects. One indication of this is that the largest objects can arise when multiple systems disintegrate, because it is believed that most of the stars formed from multiple systems, but that the smaller pieces, are torn apart by gravitational influences before it comes to nuclear fusion.