Herbig-Haro object
Encyclopedia
Herbig–Haro objects are small patches of nebulosity
associated with newly born star
s, and are formed when gas ejected by young stars collides with clouds of gas and dust nearby 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 along its rotational axis.
HH objects are transient phenomena, lasting not more than a few thousand years. They can evolve visibly over quite short timescales as they move rapidly away from their parent star into the gas clouds in interstellar space (the interstellar medium
or ISM). Hubble Space Telescope
observations reveal complex evolution of HH objects over a few years, as parts of them fade while others brighten as they collide with clumpy material in 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 Herbig–Haro objects, and recognised they were a by-product of the star formation process.
with the 36 inches (914.4 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. However, 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 star
s 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 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 showed HH objects were highly ionised
, 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 protostar
s, but eventually HH objects came to be understood as material ejected by nearby young stars, and colliding at supersonic
speeds with the ISM, 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 to form HH objects is highly collimated (concentrated into narrow jets). Stars are often surrounded by accretion disks
in their first few hundred thousand years of existence, which form as gas falls onto them, and the rapid rotation of the inner parts of these disks leads to the emission of narrow jets of partially ionised plasma
perpendicular to the disk, known as polar jet
s. When these jets collide with the interstellar medium, they give rise to the small patches of bright emission
which comprise HH objects.
s when they collide with the interstellar medium, but their motions are complicated. Spectroscopic observations of their doppler shifts indicate velocities of several hundred kilometres per second, but the emission lines in the spectra
of HH objects are too weak to have been formed in such high speed collisions. This probably means some of the material they are colliding with is also moving outwards, although at a slower speed.
The total mass being ejected to form typical HH objects is estimated to be of the order of 1–20 Earth
-masses, a very small amount of material compared to the mass of the stars themselves. 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 region
s and planetary nebula
e. They tend to be quite dense, 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. HH objects consist mostly of hydrogen and helium
, which account for about 75% and 25% respectively of their mass. Less than 1% of the mass of HH objects is made up of heavier chemical element
s, and the abundances of these are generally similar to those measured in nearby young stars
.
Near to the source star, about 20–30% of the gas in HH objects is ionised, but this proportion decreases at increasing distances. This implies the material is ionised in the polar jet, and recombines as it moves away from the star, rather than being ionised by later collisions. Shocking at the end of the jet can re-ionise some material, however, giving rise to bright "caps" at the ends of the jets.
s (dark nebula
e 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 is still thought to be a very small proportion of the total number existing in the Milky Way
. Estimates suggest up to 150,000 exist, the vast majority of which are too far away to be resolved with current technological capabilities. Most HH objects lie within 0.5 parsec
s of their parent star, with very few found more than 1 pc away. However, some are seen several parsecs away, perhaps implying the interstellar medium is not very dense in their vicinity, allowing them to travel further from their source before dispersing.
Spectroscopic
observations of HH objects show 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. These observations have also allowed estimates of the distances of some HH objects via the expansion parallax
method.
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. As well as changes caused by interactions with the ISM, interactions between jets moving at different speeds within HH objects also cause variations.
The eruption of jets from the parent stars occurs in pulses rather than as a steady stream. The pulses may produce jets of gas moving in the same direction but at different speeds, and interactions between different jets create so-called "working surfaces", where streams of gases collide and generate shock waves.
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. They are generally still shrouded in dense clouds of dust and gas, which obscure all their visible light and mean they can only be observed at infrared and radio
wavelengths. Infall of gas and dust has largely finished in Class II objects, but they are still surrounded by disks of dust and gas, while class III objects have only trace remnants of their original accretion disk.
Studies have shown that 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 systems disintegrate. It is thought that most stars form as multiple systems, but that a sizable fraction are disrupted before they reach the main sequence, by gravitation
al interactions with nearby stars and dense clouds of gas.
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 sailing ship), and so are usually referred to as molecular "bow shocks". Like HH objects, these supersonic shocks are driven by collimated jets from the two poles of a protostar. They sweep up or "entrain" the surrounding dense molecular gas to form a continuous flow of material, which is referred to as a bipolar outflow
. Infrared bow shocks travel at hundreds of kilometers per second, heating gas to hundreds or even thousands of kelvins. Because they are associated with the youngest stars, where accretion is particularly strong, infrared bow shocks are usually associated with more powerful jets than their optical HH cousins.
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 – 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 these objects 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 1000 objects.
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
associated with newly born star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s, and are formed when gas ejected by young stars collides with clouds of gas and dust nearby 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 along its rotational axis.
HH objects are transient phenomena, lasting not more than a few thousand years. They can evolve visibly over quite short timescales as they move rapidly away from their parent star into the gas clouds in interstellar space (the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
or ISM). Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
observations reveal complex evolution of HH objects over a few years, as parts of them fade while others brighten as they collide with clumpy material in the interstellar medium.
The objects were first observed in the late 19th century by Sherburne Wesley Burnham
Sherburne Wesley Burnham
Sherburne Wesley Burnham was an American astronomer.He worked at Yerkes Observatory. All his working life, he served during the day as a court reporter and was an amateur astronomer, except for four years as a full-time astronomer at Lick Observatory.He served as a military stenographer in the...
, but were not recognised as being a distinct type of emission nebula
Emission nebula
An emission nebula is a cloud of ionized gas emitting light of various colors. The most common source of ionization is high-energy photons emitted from a nearby hot star...
until the 1940s. The first astronomers to study them in detail were George Herbig
George Herbig
George Howard Herbig is an astronomer at the University of Hawaii Institute for Astronomy. He is perhaps best known for the discovery of Herbig-Haro objects....
and Guillermo Haro
Guillermo Haro
Professor Guillermo Haro was born in Mexico City where he grew during the time of the Mexican Revolution. He studied philosophy at the National Autonomous University of Mexico...
, after whom they have been named. Herbig and Haro were working independently on studies of star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
when they first analysed Herbig–Haro objects, and recognised they were a by-product of the star formation process.
Discovery and history of observations
The first Herbig–Haro object was observed in the late 19th century by Burnham, when he looked at the star T TauriT Tauri
T Tauri is a variable star in the constellation Taurus, the prototype of the T Tauri stars. It was discovered in October 1852 by John Russell Hind...
with the 36 inches (914.4 mm) refracting telescope
Refracting telescope
A refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image . The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses...
at Lick Observatory
Lick Observatory
The Lick Observatory is an astronomical observatory, owned and operated by the University of California. It is situated on the summit of Mount Hamilton, in the Diablo Range just east of San Jose, California, USA...
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. However, 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 star
T Tauri star
T Tauri stars are a class of variable stars named after their prototype – T Tauri. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines.-Characteristics:...
s which have yet to reach a state of hydrostatic equilibrium
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...
between gravitational collapse
Gravitational collapse
Gravitational collapse is the inward fall of a body due to the influence of its own gravity. In any stable body, this gravitational force is counterbalanced by the internal pressure of the body, in the opposite direction to the force of gravity...
and energy generation through nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
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 during the 1940s. Herbig also looked at Burnham's Nebula and found it displayed an unusual electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
, with prominent emission lines of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. Haro found that all the objects of this type were invisible in infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
light.
Following their independent discoveries, Herbig and Haro met at an astronomy conference
Academic conference
An academic conference or symposium is a conference for researchers to present and discuss their work. Together with academic or scientific journals, conferences provide an important channel for exchange of information between researchers.-Overview:Conferences are usually composed of various...
in Tucson
Tucson, Arizona
Tucson is a city in and the county seat of Pima County, Arizona, United States. The city is located 118 miles southeast of Phoenix and 60 miles north of the U.S.-Mexico border. The 2010 United States Census puts the city's population at 520,116 with a metropolitan area population at 1,020,200...
, Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
. 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
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
astronomer Viktor Hambardzumyan
Viktor Hambardzumyan
Viktor Hambardzumyan was a Soviet Armenian scientist, and one of the founders of theoretical astrophysics. He worked in the field of physics of stars and nebulae, stellar astronomy, dynamics of stellar systems and cosmogony of stars and galaxies, contributed to Mathematical physics...
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 showed HH objects were highly ionised
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
, 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 protostar
Protostar
A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...
s, but eventually HH objects came to be understood as material ejected by nearby young stars, and colliding at supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
speeds with the ISM, 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 to form HH objects is highly collimated (concentrated into narrow jets). Stars are often surrounded by accretion disks
Accretion disc
An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of...
in their first few hundred thousand years of existence, which form as gas falls onto them, and the rapid rotation of the inner parts of these disks leads to the emission of narrow jets of partially ionised plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
perpendicular to the disk, known as polar jet
Polar jet
A polar jet is a phenomenon often seen in astronomy, where streams of matter are emitted along the axis of rotation of a compact object. It is usually caused by the dynamic interactions within an accretion disc. When matter is emitted at speeds approaching the speed of light, these jets are called...
s. When these jets collide with the interstellar medium, they give rise to the small patches of bright emission
Emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....
which comprise HH objects.
Physical characteristics
Emission from HH objects is caused by shock waveShock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
s when they collide with the interstellar medium, but their motions are complicated. Spectroscopic observations of their doppler shifts indicate velocities of several hundred kilometres per second, but the emission lines in the spectra
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
of HH objects are too weak to have been formed in such high speed collisions. This probably means some of the material they are colliding with is also moving outwards, although at a slower speed.
The total mass being ejected to form typical HH objects is estimated to be of the order of 1–20 Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
-masses, a very small amount of material compared to the mass of the stars themselves. The temperatures observed in HH objects are typically about 8000–12,000 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
, similar to those found in other ionized nebulae such as H II region
H II region
An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...
s and planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
e. They tend to be quite dense, 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. HH objects consist mostly of hydrogen and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, which account for about 75% and 25% respectively of their mass. Less than 1% of the mass of HH objects is made up of heavier chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s, and the abundances of these are generally similar to those measured in nearby young stars
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
.
Near to the source star, about 20–30% of the gas in HH objects is ionised, but this proportion decreases at increasing distances. This implies the material is ionised in the polar jet, and recombines as it moves away from the star, rather than being ionised by later collisions. Shocking at the end of the jet can re-ionise some material, however, giving rise to bright "caps" at the ends of the jets.
Numbers and distribution
Over 400 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 Bok globuleBok globule
Bok globules are dark clouds of dense cosmic dust and gas in which star formation sometimes takes place. Bok globules are found within H II regions, and typically have a mass of about 2 to 50 solar masses contained within a region about a light year or so across...
s (dark nebula
Dark nebula
A dark nebula is a type of interstellar cloud that is so dense that it obscures the light from the background emission or reflection nebula or that it blocks out background stars . The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of larger...
e 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 is still thought to be a very small proportion of the total number existing in the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
. Estimates suggest up to 150,000 exist, the vast majority of which are too far away to be resolved with current technological capabilities. Most HH objects lie within 0.5 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s of their parent star, with very few found more than 1 pc away. However, some are seen several parsecs away, perhaps implying the interstellar medium is not very dense in their vicinity, allowing them to travel further from their source before dispersing.
Proper motions and variability
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
observations of HH objects show they are moving away from the source stars at speeds of 100 to 1000 km/s. In recent years, the high optical resolution
Optical resolution
Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...
of Hubble Space Telescope observations has revealed the proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
of many HH objects in observations spaced several years apart. These observations have also allowed estimates of the distances of some HH objects via the expansion parallax
Cosmic distance ladder
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth...
method.
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. As well as changes caused by interactions with the ISM, interactions between jets moving at different speeds within HH objects also cause variations.
The eruption of jets from the parent stars occurs in pulses rather than as a steady stream. The pulses may produce jets of gas moving in the same direction but at different speeds, and interactions between different jets create so-called "working surfaces", where streams of gases collide and generate shock waves.
Source stars
The stars which are behind the creation of Herbig–Haro objects are all very young stars, the youngest of which are still protostars in the process of forming from the surrounding gases. Astronomers divide these stars into classes 0, I, II and III, according to how much infrared radiation the stars give off. 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. They are generally still shrouded in dense clouds of dust and gas, which obscure all their visible light and mean they can only be observed at infrared and radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
wavelengths. Infall of gas and dust has largely finished in Class II objects, but they are still surrounded by disks of dust and gas, while class III objects have only trace remnants of their original accretion disk.
Studies have shown that about 80% of the stars giving rise to HH objects are in fact binary
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
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
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
. 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 systems disintegrate. It is thought that most stars form as multiple systems, but that a sizable fraction are disrupted before they reach the main sequence, by gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
al interactions with nearby stars and dense clouds of gas.
Infrared counterparts (MHOs)
Herbig–Haro (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. This surrounding natal material can produce tens or even hundreds of magnitudes of extinction at optical wavelengths. Such deeply embedded objects can only be observed at infrared or radio wavelengths, usually in the light 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 sailing ship), and so are usually referred to as molecular "bow shocks". Like HH objects, these supersonic shocks are driven by collimated jets from the two poles of a protostar. They sweep up or "entrain" the surrounding dense molecular gas to form a continuous flow of material, which is referred to as a bipolar outflow
Bipolar outflow
A bipolar outflow represents two continuous flows of gas from the poles of a star. Bipolar outflows may be associated with protostars , or with evolved post-AGB stars ....
. Infrared bow shocks travel at hundreds of kilometers per second, heating gas to hundreds or even thousands of kelvins. Because they are associated with the youngest stars, where accretion is particularly strong, infrared bow shocks are usually associated with more powerful jets than their optical HH cousins.
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 – 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 these objects by the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
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 1000 objects.