Herbig–Haro (HH) objects are small patches of nebulosity associated with newly born stars, and are formed when narrow jets of partially ionized gas ejected by those stars collide with nearby clouds of gas and dust at speeds of several hundred kilometres per second. Herbig–Haro objects are ubiquitous in star-forming regions, and several are often seen around a single star, aligned with its rotational axis.
HH objects are transient phenomena that last less than a few thousand years. They can evolve visibly over quite short astronomical timescales as they move rapidly away from their parent star into the gas clouds of interstellar space (the interstellar medium or ISM). Hubble Space Telescope observations have revealed the complex evolution of HH objects over the period of a few years, as parts of the nebula fade while others brighten as they collide with clumpy material of the interstellar medium.
The objects were first observed in the late 19th century by Sherburne Wesley Burnham, but were not recognised as being a distinct type of emission nebula until the 1940s. The first astronomers to study them in detail were George Herbig and Guillermo Haro, after whom they have been named. Herbig and Haro were working independently on studies of star formation when they first analysed the objects, and recognised that they were a by-product of the star formation process.
Discovery and history of observations
The first HH object was observed in the late 19th century by Sherburne Wesley Burnham, when he observed the star T Tauri with the 36-inch (910 mm) refracting telescope at Lick Observatory and noted a small patch of nebulosity nearby. However, it was catalogued merely as an emission nebula, later becoming known as Burnham's Nebula, and was not recognised as a distinct class of object. T Tauri was found to be a very young and variable star, and is the prototype of the class of similar objects known as T Tauri stars which have yet to reach a state of hydrostatic equilibrium between gravitational collapse and energy generation through nuclear fusion at their centres.
Fifty years after Burnham's discovery, several similar nebulae were discovered which were so small as to be almost star-like in appearance. Both Haro and Herbig made independent observations of several of these objects in Orion Nebula during the 1940s. Herbig also looked at Burnham's Nebula and found it displayed an unusual electromagnetic spectrum, with prominent emission lines of hydrogen, sulfur and oxygen. Haro found that all the 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 initially paid little attention to the objects he had discovered, being primarily concerned with the nearby stars, but on hearing Haro's findings he carried out more detailed studies of them. The Soviet astronomer Viktor Hambardzumyan gave the objects their name, and based on their occurrence near young stars (a few hundred thousand years old), suggested they might represent an early stage in the formation of T Tauri stars.
Studies of the HH objects showed they were highly ionized, and early theorists speculated they might contain low-luminosity hot stars. However, the absence of infrared radiation from the nebulae meant there could not be stars within them, as these would have emitted abundant infrared light. Later studies suggested the nebulae might contain protostars, but eventually HH objects were understood to be material ejected from nearby young T Tauri stars that was colliding at supersonic speeds with the interstellar medium, with the resulting shock waves generating visible light.
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 that formed HH objects is highly collimated (concentrated into narrow jets).
Stars form by gravitational collapse of interstellar gas clouds. As the collapse increases the density, radiative energy loss decreases due to increased opacity. This raises the temperature of the cloud which prevents further collapse, and a hydrostatic equilibrium is established. Gas continues to fall towards the core in a rotating disk. It is called a protostar. Some of the accreting material is focused into collimated bipolar jets of partially ionized gas (plasma) parallel to the axis of rotation. The mechanism is not entirely understood, but it is believed that interaction between accretion disk and stellar magnetic field accelerates some of accreting material away from the disk plane from within a few astronomical units of the star. At these distances the outflow is divergent, but it becomes increasingly collimated at distances of tens to hundreds of astronomical units from the source as its expansion is constrained. When these jets collide with the interstellar medium, they give rise to the small patches of bright emission which comprise HH objects.
Electromagnetic emission from HH objects is caused when shock waves collide with the interstellar medium, creating what is called the "terminal working surfaces". Spectroscopic observations of their doppler shifts indicate velocities of several hundred kilometres per second, but the emission lines in those spectra are weaker than what would be expected from such high speed collisions. This suggests that some of the material they are colliding with is also moving along the beam, although at a lower speed. Spectroscopic observations of HH objects show they are moving away from the source stars at speeds of several hundred km/s. In recent years, the high optical resolution of Hubble Space Telescope 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. This is because of precession of jets and their pulsating, rather than steady, eruption from the parent stars. Faster jets catch up with earlier slower jets creating the so-called "internal working surfaces", where streams of gas collide and generate shock waves and consequently emissions.
The total mass being ejected to form typical HH objects is estimated to be of the order of 10−8–10−6 M☉ per year, a very small amount of material compared to the mass of the stars themselves but amounts to about 1–10% of the total mass accreted in a year. Mass loss tends to decrease with increasing age of the source. The temperatures observed in HH objects are typically about 8000–12,000 K, similar to those found in other ionized nebulae such as H II regions and planetary nebulae. Densities, on the other hand, are relatively higher; 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.
Densities also decrease as the source evolves over time. HH objects consist mostly of hydrogen and helium, which account for about 75% and 24% respectively of their mass. Around 1% of the mass of HH objects is made up of heavier chemical elements, including oxygen, sulfur, nitrogen, iron, calcium and magnesium. Abundances of these elements, determined from emission lines of respective ions, are generally similar to their cosmic abundances. Several chemical compounds have been identified in the outflows, including water (ice), methanol, methane, carbon monoxide, carbon dioxide (dry ice) and various silicates. Many chemical compounds found in surrounding interstellar medium, which are not present in the source material, such as metal hydrides, are believed to have been produced by shock induced chemical reactions.
Near to the source star, about 20–30% of the gas in HH objects is ionized, but this proportion decreases at increasing distances. This implies the material is ionized in the polar jet, and recombines as it moves away from the star, rather than being ionized by later collisions. Shocking at the end of the jet can re-ionize some material, however, giving rise to bright "caps" at the ends of the jets.
Numbers and distribution
About 500 individual HH objects are now known. They are ubiquitous in star-forming H II regions, and are often found in large groups. They are typically observed near Bok globules (dark nebulae which contain very young stars) and often emanate from them. Frequently, several HH objects are seen near a single energy source, forming a string of objects along the line of the polar axis of the parent star.
The number of known HH objects has increased rapidly over the last few years, but that is a very small proportion of the estimated up to 150,000 in the Milky Way, the vast majority of which are too far away to be resolved. Most HH objects lie within 0.5 parsecs of their parent star, with very few found more than 1 parsec away. Some, however, are seen several parsecs away.
The stars from which HH objects are emitted are all very young stars, the youngest of which are still protostars in the process of collecting from their surrounding gases. Astronomers divide these stars into classes 0, I, II and III, according to how much infrared radiation the stars emit. A greater amount of infrared radiation implies a larger amount of cooler material surrounding the star, which indicates it is still coalescing. The numbering of the classes arises because class 0 objects (the youngest) were not discovered until classes I, II and III had already been defined.
Class 0 objects are only a few thousand years old, so young that they are not yet undergoing nuclear fusion reactions at their centres. Instead, they are powered only by the gravitational potential energy released as material falls onto them. Nuclear fusion has begun in the cores of Class I objects, but gas and dust are still falling onto their surfaces from the surrounding nebula, and most of luminosity is accounted for by gravitational energy. They are generally still shrouded in dense clouds of dust and gas, which obscure all their visible light and as a result can only be observed at infrared and radio wavelengths. The in-fall of gas and dust has largely finished in Class II objects (Classical T Tauri stars), but they are still surrounded by disks of dust and gas, while class III objects (Weak-line T Tauri stars) have only trace remnants of their original accretion disk.
About 80% of the stars giving rise to HH objects are in fact binary or multiple systems (two or more stars orbiting each other), which is a much higher proportion than that found for low mass stars on the main sequence. This may indicate that binary systems are more likely to generate the jets which give rise to HH objects, and evidence suggests the largest HH outflows might be formed when multiple star systems disintegrate. It is thought that most stars originate from multiple star systems, but that a sizable fraction are disrupted before they reach the main sequence by gravitational interactions with nearby stars and dense clouds of gas.
HH objects associated with very young stars or very massive protostars are often hidden from view at optical wavelengths by the cloud of gas and dust from which they form. The intervening material can diminish the visual magnitude by factors of tens or even hundreds at optical wavelengths. Such deeply embedded objects can only be observed at infrared or radio wavelengths, usually in the frequencies of hot molecular hydrogen or warm carbon monoxide emission.
In recent years, infrared images have revealed dozens of examples of "infrared HH objects". Most look like bow waves (similar to the waves at the head of a ship), and so are usually referred to as molecular "bow shocks". The physics of infrared bow shocks can be understood in much the same way as that of HH objects, since these objects are essentially the same – supersonic shocks driven by collimated jets from the opposite poles of a protostar. It is only the conditions in the jet and surrounding cloud that are different, causing infrared emission from molecules rather than optical emission from atoms and ions.
In 2009 the acronym "MHO", for Molecular Hydrogen emission-line Object, was approved for such objects, detected in near infrared, by the International Astronomical Union Working Group on Designations, and has been entered into their on-line Reference Dictionary of Nomenclature of Celestial Objects. The MHO catalogue (see external links below) contains over 2000 objects.
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