Hubble Space Telescope images of HH 32 (left; top) and HH 34 (right; upper left) – colourful nebulae are typical of Herbig–Haro objects
First observed in the late 19th century by Sherburne Wesley Burnham, Herbig–Haro objects were recognised as a distinct type of emission nebula in 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. Although HH objects are a visible wavelength phenomena, many remain invisible at these wavelengths due to dust and gas, and can only be detected at infrared wavelengths. Such objects, when observed in near infrared, are called molecular hydrogen emission-line objects (MHOs).
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. It was thought to be an emission nebula, later becoming known as Burnham's Nebula, and was not recognized 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 with almost star-like appearance. Both Haro and Herbig made
independent observations of several of these objects in the 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
in December 1949. 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 Ambartsumian
gave the objects their name (Herbig–Haro objects, normally shortened to
HH objects), 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 ionised, and early theorists speculated that they were reflection nebulae
containing low-luminosity hot stars deep inside. But the absence of
infrared radiation from the nebulae meant there could not be stars
within them, as these would have emitted abundant infrared light. In
1975 American astronomer R. D. Schwartz theorized that winds from T Tauri stars produce shocks in the ambient medium on encounter, resulting in generation of visible light. With the discovery of the first proto-stellar jet in HH 46/47, it became clear that HH objects are indeed shock-induced phenomena with shocks being driven by a collimated jet from protostars.
Formation
HH
objects are formed when accreted material is ejected by a protostar as
ionized gas along the star's axis of rotation, as exemplified by HH 34
(right).
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. The core of this system is called a protostar. Some of the accreting material is ejected out along the star's axis of rotation in two jets of partially ionised gas (plasma).
The mechanism for producing these collimated bipolar jets is not
entirely understood, but it is believed that interaction between the
accretion disk and the stellar magnetic field accelerates some of the accreting material from within a few astronomical units
of the star away from the disk plane. At these distances the outflow is
divergent, fanning out at an angle in the range of 10−30°, but it
becomes increasingly collimated at distances of tens to hundreds of
astronomical units from the source, as its expansion is constrained. The jets also carry away the excess angular momentum
resulting from accretion of material onto the star, which would
otherwise cause the star to rotate too rapidly and disintegrate. When these jets collide with the interstellar medium, they give rise to the small patches of bright emission which comprise HH objects.
Properties
Infrared spectrum of HH 46/47 obtained by the Spitzer Space Telescope, showing the medium in immediate vicinity of the star being silicate-rich
Electromagnetic emission from HH objects is caused when shock waves collide with the interstellar medium, creating what is called the "terminal working surfaces". The spectrum is continuous, but also has intense emission lines of neutral and ionized species. Spectroscopic observations of HH objects' doppler shifts indicate velocities of several hundred kilometers 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 kilometres per second. In recent years, the high optical resolution of the Hubble Space Telescope has revealed the proper motion (movement along the sky plane) 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 compact knots or clumps within an object may brighten and
fade or disappear entirely, while new knots have been seen to appear. This is most likely because of the precession of the 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 by stars to form typical HH objects is estimated to be of the order of 10−8 to 10−6 M☉ per year, a very small amount of material compared to the mass of the stars themselves but amounting to about 1–10% of the total mass accreted by the source stars in a year. Mass loss tends to decrease with increasing age of the source. The temperatures observed in HH objects are typically about 9,000–12,000 K, similar to those found in other ionized nebulae such as H II regions and planetary nebulae.
Densities, on the other hand, are higher than in other nebulae, ranging
from a few thousand to a few tens of thousands of particles per cm3, compared to a few thousand particles per cm3 in most 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% of their mass respectively. 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. Many chemical compounds found in the surrounding interstellar medium, but not present in the source material, such as metal hydrides, are believed to have been produced by shock-induced chemical reactions.
Around 20–30% of the gas in HH objects is ionized near the source star,
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-ionise some material, giving rise to bright "caps".
Numbers and distribution
HH
1/2 (top), HH 34 (left), and HH 47 (right) were numbered in order of
their discovery; it is estimated that there are up to 150,000 such
objects in the Milky Way.
HH objects are named approximately in order of their identification;
HH 1 and HH 2 being the earliest such objects to be identified. More than a thousand individual objects are now known. They are always present 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. Several HH
objects have been 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 about one parsec of their parent star. Many, however, are seen several parsecs away.
HH 46/47 is located about 450 parsecs (1,500 light-years) away from the Sun and is powered by a class I protostar binary.
The bipolar jet is slamming into the surrounding medium at a velocity
of 300 kilometers per second, producing two emission caps about 2.6
parsecs (8.5 light-years) apart. Jet outflow is accompanied by a 0.3
parsecs (0.98 light-years) long molecular gas outflow which is swept up
by the jet itself. Infrared studies by Spitzer Space Telescope have revealed a variety of chemical compounds in the molecular outflow, including water (ice), methanol, methane, carbon dioxide (dry ice) and various silicates. Located around 460 parsecs (1,500 light-years) away in the Orion nebula, HH 34
is produced by a highly collimated bipolar jet powered by a class I
protostar. Matter in the jet is moving at about 220 kilometers per
second. Two bright bow shocks,
separated by about 0.44 parsecs (1.4 light-years), are present on the
opposite sides of the source, followed by series of fainter ones at
larger distances, making the whole complex about 3 parsecs (9.8
light-years) long. The jet is surrounded by a 0.3 parsecs (0.98
light-years) long weak molecular outflow near the source.
Source stars
The stars from which HH jets are emitted are all very young stars, a
few tens of thousands to about a million years old. The youngest of
these 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. They mostly contain molecular outflows with low velocities (less than a hundred kilometres per second) and weak emissions in the outflows.
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 their 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. Outflows from this class are dominated by ionized species and velocities can range up to 400 kilometres per second.
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, and produce weak outflows of low luminosity. 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 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 of these systems are disrupted before their
stars reach the main sequence due to gravitational interactions with nearby stars and dense clouds of gas.
Infrared counterparts
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 catalog contains over 2000 objects.
