Herbig Haro object
Herbig Haro objects (after George Herbig and Guillermo Haro ) are small, foggy structures around young stars . They are created when gas ejected from the star hits clouds of dust . Herbig Haro objects are ubiquitous in regions where stars are formed . Often times they are seen around a single star where they are aligned along its axis of rotation .
Herbig-Haro objects are very short-lived, with a lifespan of a few thousand years at best. They can become visible in a very short time if they move quickly away from their original star into the gas cloud in interstellar space (also called interstellar matter ). The Hubble space telescope demonstrated a complex formation of the Herbig-Haro objects in just a few years. In that short time, some lit up as they collided with material in the interstellar medium, while others darkened.
Herbig Haro objects were first observed by Sherburne Wesley Burnham in the late 19th century , but were interpreted as emission nebulae until the 1940s . The first astronomers to study it in more detail were Herbig and Haro. Independently of each other, they found that these objects are part of the process of star formation .
Discovery and observation history
The first Herbig-Haro object was observed by Burnham in the 19th century. When he observed the star T Tauri with a telescope from the Lick Observatory , he noticed a small, misty shape around the star. It was cataloged as an emission nebula and was later named Burnhams Nebula . It has been found that T Tauri is a very young and changeable star that is currently in a balance between collapsing from its own weight and producing energy from nuclear fusion at the center. Such stars are counted to the group of T-Tauri stars .
Fifty years after Burnham's discovery, many similar nebulae were discovered, all so small that they could be appearances within a star system. In the 1940s, Herbig and Haro independently observed such objects. Herbig watched Burnham mist and found an unusual electromagnetic spectrum with hydrogen -, sulfur - and oxygen - emission lines . Haro found that all objects of this type were invisible in infrared light.
Following their independent discoveries Herbig and Haro met at an astronomy conference in Tucson ( Arizona ). Herbig had paid little attention to these objects, but after hearing about Haro's discoveries, that changed. The Soviet astronomer Viktor Ambartsumian gave the objects their names and added that because of their abundance in young stars (a few hundred thousand years old), they mark an early stage in the formation of T-Tauri stars.
Studies showed that Herbig-Haro objects are highly ionized , and early theorists speculated that they contained faintly glowing hot stars. However, this was refuted by the lack of infrared radiation. After that it was suggested that they contained protostars . According to today's opinion, they are material ejected by young stars that collides with interstellar matter at supersonic speed .
In the early 1980s, observations showed the jet- like shape of most Herbig-Haro objects. This made it clear that the material from them was concentrated in narrow jets, that is, highly collimated . Young stars are often surrounded by an accretion disk in their first hundred thousand years . The rapid rotation of the inner parts of this disk leads to the emission of narrow polar jets of partially ionized plasma , moving perpendicularly away from the disk . When these jets collide with the interstellar matter, this leads to formations of brightly radiating matter that contain the Herbig-Haro objects.
Physical Properties
The emissions from Herbig-Haro objects are caused by shock waves when they collide with interstellar matter. However, their movements are complicated. With the help of the Doppler effect, spectroscopic observations made it possible to determine their speed of a few hundred kilometers per second. However, the emission lines in the spectrum are too weak to have been caused by collisions at such high speeds. This possibly means that some material will also move outwards at a slower speed and then collide with the interstellar matter.
The measured temperature in Herbig-Haro objects is mostly 8,000–12,000 K and is thus about as great as in other ionized nebulae, H-II areas or planetary nebulae . The total mass ejected from a star to form a Herbig-Haro object is very small, at 1 to 20 Earth masses, compared to the total mass of the star. Herbig-Haro objects have a density of a few thousand to a few tens of thousands of particles per cubic centimeter, much higher than H-II areas or planetary nebulae with mostly less than 1,000 particles / cm³. They mainly consist of hydrogen (75% by mass) and helium (25% by mass). Heavier chemical elements occupy less than 1% of their mass, which roughly corresponds to the proportion found in young stars.
In the vicinity of its original star, 20–30% of a Herbig-Haro object is ionized, the proportion decreases with increasing distance. This assumes that the material that was ionized in the polar jet is then recombined and is hardly re-ionized afterwards through subsequent collisions. As a result of the collision at the end of the jet, some material can re-ionize, creating small, bright “caps” here.
Number and distribution
Today over 400 individual Herbig Haro objects or groups of them are known. This number has grown rapidly over the past few years, but it is still much less than the 150,000 estimated for our galaxy . It is believed that most of them are too far away to be seen with today's technology.
Herbig Haro objects are ubiquitous in star birthplaces such as H-II areas and are often found there in large groups. They are mostly observed in the vicinity of globules ( dark nebulae that contain very young stars) and often emerge from them. Often several Herbig-Haro objects are observed around a single energy source, along whose polar axes they form a chain.
Most Herbig-Haro objects are within half a parsec of their original star, only a few more than 1 pc away and an even smaller proportion at a distance of several parsecs. In these cases, it is believed that the interstellar medium has a very low density so that the Herbig-Haro objects can move on before they perish.
Proper movement
Spectroscopic observations of Herbig-Haro objects show that they are moving away at a speed of 100–1000 km / s. In recent years, high-resolution images from the Hubble space telescope have been used to investigate the proper motion of the Herbig-Haro objects over several years. These observations made it possible to determine the distance of some of these objects with the help of parallax .
When they move away from their original star, they develop decisively. They vary in their brightness within a few years. Individual nodes in the object can lighten, fade, or disappear entirely while new ones appear. Furthermore, interactions with the intergalactic medium and between jets of different speeds are also a reason for changes.
The jets created by the original star are not steady currents, but rather single eruptions . This creates jets that move in the same direction, but at different speeds, which leads to collisions. This creates shock waves.
Origin stars
The stars responsible for the creation of Herbig-Haro objects are always very young. The youngest of them are still protostars , formed from the surrounding gas. Astronomers divide these stars into classes 0, I, II and III, depending on the amount of infrared radiation emitted. A larger amount of infrared radiation suggests a larger amount of cold material around the star because its matter is still contracting. The numbering came about because class 0 objects (the youngest) had not yet been discovered when classes I, II and III had already been defined.
- Class 0 objects are only a few thousand years old. They are so young that nuclear fusion has not yet started in their centers . Instead, they only get their energy from the potential energy caused by gravity when matter falls inside.
- Nuclear fusion starts with Class I objects, but gas and dust still fall on the surface of them, too. They are still covered by a thick layer of dust that does not let any visible light through, so you can only observe them with wavelengths in the radio or infrared range.
- In the case of class II objects, the incidence of gas and dust is largely closed, but they are still enclosed by a disk of gas and dust.
- Only remnants of this disk can be found in class III stars.
Research has shown that about 80% of the stars in which Herbig-Haro objects have been found are double or multiple star systems. Because of this very large proportion, it is assumed that with multiple star systems, jets form much more frequently, from which the Herbig-Haro objects are then formed. An indication of this is that the largest objects can arise when multiple systems disintegrate. It is assumed that most stars originated from multiple systems, but that the smaller pieces are torn apart by gravitational influences before nuclear fusion occurs.
literature
- B. Reipurth, S. Heathcote: 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, pp. 3-18.
- J. Bally, J. Morse, B. Reipurth: 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, EJ Schreier. 1995.
- M. Dopita: The Herbig-Haro objects in the GUM Nebula. Astronomy and Astrophysics, B. 63, No. 1-2, Feb. 1978, pp. 237-241.
- EW Brugel, KH Boehm, E. Mannery: Emission line spectra of Herbig-Haro objects. Astrophysical Journal Supplement Series , Vol. 47, 1981, pp. 117-138.
- F. Bacciotti, J. Eislöffel: Ionization and density along the beams of Herbig-Haro jets. Astronomy and Astrophysics, Vol. 342, 1999, pp. 717-735.
- AL Giulbudagian: On a connection between Herbig-Haro objects and flare stars in the neighborhood of the sun. Astrofizika, Vol. 20, March / April 1984, pp. 277-281.
- CJ Lada: Star formation - From OB associations to protostars. In: Star forming regions; Proceedings of the Symposium, Tokyo, Japan, Nov. 11-15, 1985 (A87-45601 20-90). D. Reidel Publishing Co., Dordrecht 1987, pp. 1-17.
- P. Andre, D. Ward-Thompson, M. Barsony: Submillimeter continuum observations of Rho Ophiuchi A - The candidate protostar VLA 1623 and prestellar clumps. Astrophysical Journal, Vol. 406, 1993, pp. 122-141.
- B. Reipurth, LF Rodríguez, G. Anglada, J. Bally: Radio Continuum Jets from Protostellar Objects. Astronomical Journal, Vol. 127, 2004, pp. 1736-1746.
