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Herbig-Haro object

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   Herbig-Haro object HH47, imaged by the Hubble Space Telescope. The
   scale bar represents 1000 Astronomical Units, equivalent to about 20
   times the size of our solar system, or 1000 times the distance from the
   Earth to the Sun
   Enlarge
   Herbig-Haro object HH47, imaged by the Hubble Space Telescope. The
   scale bar represents 1000 Astronomical Units, equivalent to about 20
   times the size of our solar system, or 1000 times the distance from the
   Earth to the Sun

   Herbig-Haro objects are small patches of nebulosity associated with
   newly-born stars, 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 only a few thousand years
   at most. 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
   that 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 Tauri with the 36-inch 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 stars which have yet to reach a state of equilibrium
   between gravitational collapse and energy generation through nuclear
   fusion at their centres.
   Schematic diagram of how HH objects arise
   Enlarge
   Schematic diagram of how HH objects arise

   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 that it displayed an unusual electromagnetic spectrum, with
   prominent emission lines of hydrogen, sulphur 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
   Ambartsumian gave the objects their name, and based on their occurrence
   near young stars (a few hundred thousand years old), suggested that
   they might represent an early stage in the formation of T Tauri stars.

   Studies showed that HH objects were highly ionised, and early theorists
   speculated that 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 that the nebulae might contain
   protostars, but eventually HH objects came to be understood as material
   ejected by nearby young stars, and colliding at supersonic speeds with
   the interstellar medium (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
   ionized plasma perpendicular to the disk, known as polar jets. When
   these jets collide with the interstellar medium, they give rise to the
   small patches of bright emission which comprise HH objects ^.

Physical characteristics

   HH objects HH1 and HH2 lie about a light year apart, symmetrically
   opposite a young star which is ejecting material along its polar axis
   Enlarge
   HH objects HH1 and HH2 lie about a light year apart, symmetrically
   opposite a young star which is ejecting material along its polar axis

   Emission from HH objects is caused by shock waves 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 that 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 regions and planetary
   nebulae. They tend to be quite dense, with densities 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 that 1% of the mass
   of HH objects is made up of heavier chemical elements, 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 that 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 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 is still thought to be a very small proportion of the total
   number existing in our galaxy. Estimates suggest that 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
   parsecs of their parent star, with very few found more than 1 pc away.
   However, some are seen several parsecs away, perhaps implying that the
   interstellar medium is not very dense in their vicinity, allowing them
   to travel further from their source before dispersing.

Proper motions and variability

   Images taken over five years reveal the motion of material in HH object
   HH47. view detail.
   Images taken over five years reveal the motion of material in HH object
   HH47. view detail.

   Spectroscopic observations of HH objects show that 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.

Infrared counterparts

   Infrared image of molecular bow shocks associated with bipolar outflows
   in Orion. Credit: UKIRT/Joint Astronomy Centre
   Enlarge
   Infrared image of molecular bow shocks associated with bipolar outflows
   in Orion. Credit: UKIRT/Joint Astronomy Centre

   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.
   Infrared bow shocks travel at hundreds of kilometers per second,
   heating gas to hundreds or even thousands of degrees. 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 ^.
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