Herbig–Haro object

Source: Wikipedia, the free encyclopedia.

HH 24 is located in the Orion B molecular cloud
HH 32 looks like a star due to its intense brightness. The surrounding gas appears like clouds around a full moon.
Hubble Space Telescope images of HH 24 (left) and HH 32 (right; top) – colourful nebulae are typical of Herbig–Haro objects

Herbig–Haro (HH) objects are bright patches of

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 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

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

emission lines of hydrogen, sulfur and oxygen. Haro found that all the objects of this type were invisible in infrared light.[2]

Following their independent discoveries, Herbig and Haro met at an astronomy

collimated jet from protostars.[2][4]

An image of a question mark associated with the object was reported on 18 August 2023 in The New York Times.[5]

Formation

Main articles: Star formation and Astrophysical jet
Illustration depicting two arrows of matter moving outwards in opposite directions from a star-disk system, and creating bright emission caps at the ends, where they collide with the surrounding medium
Yellow-green emission cap produced by red jet from a star in a deep green nebula
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

axis of rotation in two jets of partially ionised 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

Plot of light intensity vs wavelength featuring several dips, caused by absorption of light emitted from the star by the molecules in surrounding medium
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 their associated

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 kilometres per second.[2][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] These arise likely because of the precession of their jets,[17][18] along with the pulsating and intermittent eruptions from their 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 consequent emissions.[19]

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.

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-ionise some material, giving rise to bright "caps".[7]

Numbers and distribution

HH 47
(top) 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 1/2 being the earliest such objects to be identified.[25] More than a thousand individual objects are now known.[8] They are always present 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,[26] 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]

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][28]

Source stars

Thirteen-year timelapse of material ejecting from a class I protostar, forming the Herbig–Haro object HH 34

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.[29] 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.[30][29]

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

visible light and as a result can only be observed at infrared and radio wavelengths.[32] Outflows from this class are dominated by ionized species and velocities can range up to 400 kilometres 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.[29]

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

gravitational interactions with nearby stars and dense clouds of gas.[33][34]

The first and currently only (as of May 2017) large-scale Herbig-Haro object around a proto-brown dwarf is HH 1165, which is connected to the proto-brown dwarf Mayrit 1701117. HH 1165 has a length of 0.8 light-years (0.26 parsec) and is located in the vicinity of the sigma Orionis cluster. Previously only small mini-jets (≤0.03 parsec) were found around proto-brown dwarfs.[35][36]

Infrared counterparts

HH 49/50 seen in infrared by the Spitzer Space Telescope

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

supersonic shocks driven by collimated jets from the opposite poles of a protostar.[39] 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.[40]

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. As of 2010, almost 1000 objects are contained in the MHO catalog.[39]

Ultraviolet Herbig-Haro objects

HH objects have been observed in the ultraviolet spectrum.[41]

See also

References

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