A Herbig-Haro object is a small patch of nebulosity associated with newly-born stars. Young stars are often surrounded by accretion disks, and emit jets of material from their poles known as polar jets. When these jets collide with the interstellar medium, they give rise to small patches of bright emission. Several Herbig-Haro objects are often seen around a single star, aligned along its rotational axis.

The objects are named after their independent discovers, George Herbig and Guillermo Haro .

Discovery and history of observations

The first Herbig-Haro object was observed in the late 19th century by Robert Burnham . However, it was noted merely as an emission nebula, later becoming known as Burnham's Nebula , and was not recognised as a distinct class of object.

50 years later, both Guillermo Haro and George Herbig made independent observations of several semi-stellar objects. Herbig also looked at Burnham's Nebula and found that it displayed an unusual spectrum, with prominent emission lines of hydrogen, sulphur and oxygen. Haro noted that all the objects of this type were invisible in infrared light. The Soviet astronomer Viktor Ambartsumian gave the objects their name, and based on their occurrence near young stars, suggested that they might represent an early stage in the formation of T Tauri stars.

Studies showed that HH objects were highly ionised, and early theorists speculated that they might contain low-luminosity hot stars. However, the absence of infra-red radiation from the nebulae argued against an ionising source within them. Later studies suggested that the nebulae might contain protostars, but eventually they came to be understood as material ejected by nearby young stars, and ionised by colliding supersonically with the interstellar medium.

In the early 1980s, observations revealed for the first time the jet-like nature of most HH objects. This led to the understanding that the material ejected to form HH objects is highly collimated , most likely arising from polar jets which suggest the existence of an accretion disk around the young star.

Numbers and distribution

Over 300 individual HH objects or groups are now known. They are ubiquitous in star-forming H II regions, and are often found in large groups. They are typically observed near to Bok globules and often emanate from them.

Proper motions and variability

Spectroscopic observations of HH objects show that they are moving away from the source stars at speeds of 100 to 1000 km/s. In recent years, the high optical resolution of Hubble Space Telescope observations has revealed the proper motion of many HH objects in observations spaced several years apart.

As they move away from the parent star, HH objects evolve significantly, varying in brightness on timescales of a few years. Individual knots within an object may brighten and fade or disappear entirely, while new knots have been seen to appear.

Physical conditions

Although emission from HH objects is caused by shockwaves when they collide with the interstellar medium, their spectra indicate rather lower impact speeds than are observed spectroscopically. This may mean that some of the material they are colliding with is also moving outwards, although at a slower speed.

The temperatures observed in HH objects are typically about 8000-12,000 K, similar to those found in other ionised nebulae such as H II regions and planetary nebulae. They tend to be quite dense, with densities ranging from a few thousand to a few tens of thousands of particles per cm³, compared to generally less than 1000/cm³ in H II regions and planetary nebulae. The total mass being ejected is estimated to be of the order of 1-20 Earth-masses.

Near to the source star, HH objects are typically about 20-30% ionised, with the proportion of ionised material decreasing at increasing distances. This implies that the material is ionised in the polar jet, and recombines as it moves away from the star. Shocking at the end of the jet can re-ionise the material, giving rise to bright 'caps' at the ends of the jets.

Abundances of elements in HH objects are generally consistent with average abundances in young Population I stars.

Multiplicity

Studies have shown that a large proportion of the stars giving rise to HH objects are binary or multiple systems, with an occurrence of multiple systems higher than that found for low mass stars on the main sequence. It does not seem that multiple systems preferentially form HH objects; instead it is likely that a sizable fraction of multiple systems are disrupted before they reach the main sequence.

References

1. Bacciotti F., Eislöffel J., (1999), Ionization and density along the beams of Herbig-Haro jets, Astronomy and Astrophysics, v.342, p.717-735
2. Bally J., Morse J., Reipurth B. (1995), The Birth of Stars: Herbig-Haro Jets, Accretion and Proto-Planetary Disks, Science with the Hubble Space Telescope -- II, Eds: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
3. Brugel E.W., Boehm K.H., Mannery E. (1981), Emission line spectra of Herbig-Haro objects, Astrophysical Journal Supplement Series, vol. 47, p. 117-138
4. Reipurth B., Heathcote S. (1997), 50 Years of Herbig-Haro Research. From discovery to HST, Herbig-Haro Flows and the Birth of Stars; IAU Symposium No. 182, Edited by Bo Reipurth and Claude Bertout. Kluwer Academic Publishers, 1997, p. 3-18

External links

• A catalogue of HH objects
• Entry in the Encyclopedia of Astrobiology, Astronomy, and Spaceflight
• Animations of HH object jets from HST observations