Herbig–Haro object

Herbig–Haro (HH) objects are bright patches of nebulosity associated with newborn stars. They are formed when narrow jets of partially ionized gas ejected by said stars collide with nearby clouds of gas and dust at speeds of several hundred kilometers 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. Most of them lie within about one parsec (3.26 light-years) of the source, although some have been observed several parsecs away. HH objects are transient phenomena that last around a few tens of thousands of years. They can change visibly over quite short timescales of a few years 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 the clumpy material of the interstellar medium.

First observed in the late 19th century by Sherburne Wesley Burnham, Herbig–Haro objects were not recognized 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 recognized that they were a by-product of the star formation process. Although HH objects are a visible wavelength phenomena, many remain undetectable at these wavelengths due to dust and gas envelope and can only be seen at infrared wavelengths. Such objects, when observed in near infrared, are called Molecular Hydrogen emission-line Objects (MHOs).

HH34
Herbig–Haro object HH 34 lies to the upper left of this image by the Hubble Space Telescope. It lies about 1,400 light-years away from the Sun, near the Orion Nebula.[1]

Discovery and history of observations

HH1 and HH2 imaged by WFPC2
HH objects HH 1 and HH 2 lie about a light year apart, symmetrically flanking a young star which is ejecting material along its polar axis

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.[2] 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.[3] 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 centers.[4]

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.[3]

Protostar HH-34
This three-color composite of the YSO HH 34 reveals the jet and the nebular emission.

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.[3]

Studies of the HH objects showed they were highly ionized, 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.[3]

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.[3][5]

Formation

HH object diagram
Schematic diagram of how HH objects arise

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.[6] Some of the accreting material is ejected out along the star's axis of rotation in two jets of partially ionized gas (plasma).[7]

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.[8][9] 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.[9] When these jets collide with the interstellar medium, they give rise to the small patches of bright emission which comprise HH objects.[10]

Properties

Electromagnetic emission from HH objects is caused when shock waves collide with the interstellar medium, creating what is called the "terminal working surfaces".[11] The spectrum is continuous, but also has intense emission lines of neutral and ionized species.[7] 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.[12][13] Spectroscopic observations of HH objects show they are moving away from the source stars at speeds of several hundred kilometers per second.[3][14] 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.[15][16] 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.[9][11] This is most likely because of the precession of the jets[17][18] and their pulsating, rather than steady, eruption from the parent stars.[10] 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.[19]

HH30
Hubble Space Telescope images of HH 30 over a period of five years. Discontinuities in the jet are caused by the pulsating nature of the eruptions. Variations in brightness can also be noted.
Hubble Space Telescope video shows evolution of HH 34 over 13 years period from 1994 to 2007.

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,[17] a very small amount of material compared to the mass of the stars themselves[20] but amounting to about 1–10% of the total mass accreted by the source stars in a year.[21] Mass loss tends to decrease with increasing age of the source.[22] The temperatures observed in HH objects are typically about 9,000–12,000 K,[23] similar to those found in other ionized nebulae such as H II regions and planetary nebulae.[24] 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,[23] compared to a few thousand particles per cm3 in most H II regions and planetary nebulae.[24]

Densities also decrease as the source evolves over time.[22] 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.[20] 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.[8]

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.[23] Shocking at the end of the jet can re-ionize some material, giving rise to bright "caps".[7]

Numbers and distribution

HH 47 HH 34 and HH 2
Episodically ejected by young stars like cannon salvos, the brightly glowing lobes travel through space at more than 700,000 kilometers per hour[25]
Ssc2003-06g
Infrared spectrum of the gaseous envelope of HH 46/47, obtained by NASA Spitzer Space Telescope. The medium in immediate vicinity of the star is silicate-rich.

HH objects are named approximately in order of their identification; HH 1 and HH 2 being the earliest such objects to be identified.[26] More than a thousand individual objects are now known.[8] They are ubiquitous in star-forming H II regions, and are often found in large groups.[10] 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.[8]

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,[27] 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.[22][23]

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.[8] 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.[8][28]

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.[8][29]

Source stars

Herbig-Haro object HH32
Herbig–Haro object HH 32 is one of the brightest HH objects

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.[30] 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.[31][30]

Class 0 objects are only a few thousand years old; so young that they are not yet undergoing nuclear fusion reactions at their centers. Instead, they are powered only by the gravitational potential energy released as material falls onto them.[32] They mostly contain molecular outflows with low velocities (less than a hundred kilometers per second) and weak emissions in the outflows.[18] 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.[33] Outflows from this class are dominated by ionized species and velocities can range up to 400 kilometers per second.[18] 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.[18] Class III objects (Weak-line T Tauri stars) have only trace remnants of their original accretion disk.[30]

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.[34] 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.[34][35]

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,[36] usually in the frequencies of hot molecular hydrogen or warm carbon monoxide emission.[37]

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.[38] 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.[39]

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.[38] The MHO catalog contains over 2000 objects.

See also

References

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External links

Bipolar outflow

A bipolar outflow comprises two continuous flows of gas from the poles of a star. Bipolar outflows may be associated with protostars (young, forming stars), or with evolved post-AGB stars (often in the form of bipolar nebulae).

Bright giant

The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between ordinary giants and supergiants, based on the appearance of their spectra.

HH1

HH1 or HH-1 may refer to:

HH-1 Huey, a military helicopter

PRR HH1, a steam locomotive

Sindlinger HH-1 Hawker Hurricane, a homebuilt aircraft

Tetulomab (HH1), an experimental cancer drug

HH 1, the first Herbig-Haro object identified

HHO

HHO may refer to:

Croatian Helsinki Committee (Croatian: Hrvatski helsinški odbor)

HHO gas, a fringe science term for oxyhydrogen with a 2:1 ratio of hydrogen and oxygen

HHO Multimedia, a British audio and video licensor

Home heating oil

HH 34

HH 34 is a Herbig–Haro object located in Orion Nebula at a distance of about 460 parsecs (1500 light-years). It is notable for its highly collimated jet and very symmetric bow shocks. Bipolar jet from young star is ramming into surrounding medium at supersonic speeds, heating the material to the point of ionization and emission at visual wavelengths. Source star is a class I protostar with a total luminosity of 45 L☉. Two bow shocks separated by 0.44 parsecs make the primary HH 34 system. Several larger and fainter bow shocks were later discovered on either side, making the extent of the system around 3 parsecs. The jet blows up the dusty envelope of the star, giving rise to 0.3 parsec long molecular outflow.

HL Tauri

HL Tauri (abbreviated HL Tau) is a very young T Tauri star in the constellation Taurus, approximately 450 light-years (140 pc) from Earth in the Taurus Molecular Cloud. The luminosity and effective temperature of HL Tauri imply that its age is less than 100,000 years. At apparent magnitude 15.1, it is too faint to be seen with the unaided eye. It is surrounded by a protoplanetary disk marked by dark bands visible in submillimeter radiation that may indicate a number of planets in the process of formation. It is accompanied by the Herbig–Haro object HH 150, a jet of gas emitted along the rotational axis of the disk that is colliding with nearby interstellar dust and gas.

IRAS 18162−2048

IRAS 18162-2048 is a far-infrared source discovered by IRAS spacecraft in 1983. It is associated with a massive (~10 solar masses) protostar, which accretes gas from a disk that surrounds it. IRAS 18162-2048 emits two collimated radio jets along its axis of rotation. The jets are made of chains of radio sources aligned in a southwest-northeast direction. The northern jet terminates in Herbig–Haro object HH 81N, while the southern one terminates in Herbig–Haro objects HH 80 and HH 81. The total luminosity of IRAS 18162-2048 is about 17,000 solar luminosities. The total extent of this system of jets and radio sources is about 5 pc.In 2010 HH 80–81 jet of IRAS 18162-2048 were found to emit polarized radio waves, which indicated that they were produced by relativistic electrons moving along the magnetic field estimated at 20 nT. This observation was the first of kind demonstrating that a protostar can have a magnetized jet.

Kelvin–Helmholtz mechanism

The Kelvin–Helmholtz mechanism is an astronomical process that occurs when the surface of a star or a planet cools. The cooling causes the pressure to drop, and the star or planet shrinks as a result. This compression, in turn, heats the core of the star/planet. This mechanism is evident on Jupiter and Saturn and on brown dwarfs whose central temperatures are not high enough to undergo nuclear fusion. It is estimated that Jupiter radiates more energy through this mechanism than it receives from the Sun, but Saturn might not. The latter process causes Jupiter to shrink at a rate of two centimetres each year.The mechanism was originally proposed by Kelvin and Helmholtz in the late nineteenth century to explain the source of energy of the Sun. By the mid-nineteenth century, conservation of energy had been accepted, and one consequence of this law of physics is that the Sun must have some energy source to continue to shine. Because nuclear reactions were unknown, the main candidate for the source of solar energy was gravitational contraction.

However, it soon was recognized by Sir Arthur Eddington and others that the total amount of energy available through this mechanism only allowed the Sun to shine for millions of years rather than the billions of years that the geological and biological evidence suggested for the age of the Earth. (Kelvin himself had argued that the Earth was millions, not billions, of years old.) The true source of the Sun's energy remained uncertain until the 1930s, when it was shown by Hans Bethe to be nuclear fusion.

Lead star

A lead star is a low-metallicity star with an overabundance of lead and bismuth as compared to other products of the S-process.

MAGPIE

MAGPIE (Mega Ampere Generator for Plasma Implosion Experiments) is a pulsed power generator based at Imperial College London, United Kingdom. The generator was originally designed to produce a current pulse with a maximum of 1.8 million amperes in 240 nanoseconds (150 nanoseconds rise time). At present the machine is operated with a maximum current of approximately 1.4 million amperes and operates as a z-pinch facility.

The generator consists of four voltage multipliers (Marx generators), each one containing 24 capacitors. At the maximum charging voltage of 100 kilo-volts, an output voltage of 2.4 million volts is produced and delivered into the load section.

Research at the MAGPIE generator has focused in the past on the field of inertial confinement fusion, but has recently seen significant adaptations for studies of Laboratory Astrophysics. In particular, the study of astrophysical jets in young stellar objects (see Herbig–Haro object) has been motivated by improved observational capabilities in the recent years. The simulation of such large-scale events has been undertaken at MAGPIE both from a computational point of view, through the GORGON code, and from an experimental one by means of the generator.MAGPIE is one of several similar pulsed power machines worldwide, of which the largest and most powerful is the Z-machine at Sandia National Laboratories, Albuquerque, New Mexico.

NGC 1555

NGC 1555, sometimes known as Hind's Variable Nebula, is a variable nebula, illuminated by the star T Tauri, located in the constellation Taurus. It is also in the second Sharpless catalog as 238. It is a Herbig–Haro object. The nebula was discovered on October 11, 1852 by John Russell Hind.

Pelican Nebula

The Pelican Nebula (also known as IC 5070 and IC 5067) is an H II region associated with the North America Nebula in the constellation Cygnus. The gaseous contortions of this emission nebula bear a resemblance to a pelican, giving rise to its name. The Pelican Nebula is located nearby first magnitude star Deneb, and is divided from its more prominent neighbour, the North America Nebula, by a molecular cloud filled with dark dust.

The Pelican is much studied because it has a particularly active mix of star formation and evolving gas clouds. The light from young energetic stars is slowly transforming cold gas to hot and causing an ionization front gradually to advance outward. Particularly dense filaments of cold gas are seen to still remain, and among these are found two jets emitted from the Herbig–Haro object 555. Millions of years from now this nebula might no longer be known as the Pelican, as the balance and placement of stars and gas will leave something that appears completely different.

Photometric-standard star

Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system measured very carefully. Other objects can be observed using CCD cameras or photoelectric photometers connected to a telescope, and the flux, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.A current set of photometric-standard stars for UBVRI photometry was published by Arlo U. Landolt in 1992 in the Astronomical Journal.

Protostar

A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low mass star (i.e. that of the Sun or lower), it lasts about 500,000 years The phase begins when a molecular cloud fragment first collapses under the force of self-gravity and an opaque, pressure supported core forms inside the collapsing fragment. It ends when the infalling gas is depleted, leaving a pre-main-sequence star, which contracts to later become a main-sequence star at the onset of Hydrogen fusion.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.

Starfield (astronomy)

A starfield refers to a set of stars visible in an arbitrarily-sized field of view, usually in the context of some region of interest within the celestial sphere. For example: the starfield surrounding the stars Betelgeuse and Rigel could be defined as encompassing some or all of the Orion constellation.

T 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. T Tauri appears from Earth amongst the Hyades cluster, not far from ε Tauri; but it is actually 420 light years behind it and was not formed with the rest of them. Faint nebulosity around T Tauri is a Herbig–Haro object called Burnham's Nebula or HH 255.

Like all T Tauri stars, it is very young, being only a million years old. Its distance from Earth is about 460 light years, and its apparent magnitude varies unpredictably from about 9.3 to 14.The T Tauri system consists of at least three stars, only one of which is visible at optical wavelengths; the other two shine in the infrared and one of them also emits radio waves. Through VLA radio observations, it was found that the young star (the "T Tauri star" itself) dramatically changed its orbit after a close encounter with one of its companions and may have been ejected from the system.

Physically nearby is NGC 1555, a reflection nebula known as Hind's Nebula or Hind's Variable Nebula. It is illuminated by T Tauri, and thus also varies in brightness. The nebula NGC 1554 was likewise associated with T Tauri and was observed in 1868 by Otto Wilhelm von Struve, but soon disappeared or perhaps never existed, and is known as "Struve's Lost Nebula".

The T Tauri wind, so named because this young star is currently in this stage, is a phase of stellar development between the accretion of material from the slowing rotating material of a solar nebula and the ignition of the hydrogen that has agglomerated into the protostar. A protostar is the denser parts of a cloud core, typically with a mass around 104 solar masses in the form of gas and dust, that collapses under its own weight/gravity, and continues to attract matter.

The protostar, at first, only has about 1% of its final mass. But the envelope of the star continues to grow as infalling material is accreted. After a few million years, thermonuclear fusion begins in its core, then a strong stellar wind is produced which stops the infall of new mass. The protostar is now considered a young star since its mass is fixed, and its future evolution is now set.

Yellow giant

A yellow giant is a luminous giant star of low or intermediate mass (roughly 0.5–11 solar masses (M)) in a late phase of its stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,200-7500 K. The appearance of the yellow giant is from white to yellow, including the spectral types F and G. About 10.6 percent of all giant stars are yellow giants.

Young stellar object

Young stellar object (YSO) denotes a star in its early stage of evolution. This class consists of two groups of objects: protostars and pre-main-sequence stars.

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