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A bipolar outflow comprises two continuous flows of gas from the poles of a star. Bipolar outflows may be associated with
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 5 ...
s (young, forming stars), or with evolved
post-AGB The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) l ...
stars (often in the form of
bipolar nebula A bipolar nebula is a type of nebula characterized by two lobes either side of a central star. About 10-20% of planetary nebulae are bipolar. Formation Though the exact causes of this nebular structure are not known, it is often thought to impl ...
e).


Protostars

In the case of a young star, the bipolar outflow is driven by a dense, collimated jet. These
astrophysical jet An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets bec ...
s are narrower than the outflow and very difficult to observe directly. However,
supersonic Supersonic speed is the speed of an object that exceeds the speed of sound ( Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...
shock fronts along the jet heat the gas in and around the jet to thousands of degrees. These pockets of hot gas radiate at
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
wavelengths and thus can be detected with telescopes like the
United Kingdom Infrared Telescope The United Kingdom Infra-Red Telescope (UKIRT) is a 3.8 metre (150 inch) infrared reflecting telescope, the second largest dedicated infrared (1 to 30 micrometres) telescope in the world. It is located on Mauna Kea, Hawai'i as part of Mauna ...
(UKIRT). They often appear as discrete knots or arcs along the beam of the jet. They are usually called molecular bow shocks, since the knots are usually curved like the bow wave at the front of a ship.


Occurrence

Typically, molecular bow shocks are observed in ro-vibrational emission from hot molecular hydrogen. These objects are known as molecular hydrogen emission-line objects, or MHOs. Bipolar outflows are usually observed in emission from warm carbon monoxide molecules with millimeter-wave telescopes like the
James Clerk Maxwell Telescope The James Clerk Maxwell Telescope (JCMT) is a submillimetre-wavelength radio telescope at Mauna Kea Observatory in Hawaii, US. The telescope is near the summit of Mauna Kea at . Its primary mirror is 15 metres (16.4 yards) across: it is the larg ...
, though other trace molecules can be used. Bipolar outflows are often found in dense, dark clouds. They tend to be associated with the very youngest stars (ages less than 10,000 years) and are closely related to the molecular bow shocks. Indeed, the bow shocks are thought to sweep up or "entrain" dense gas from the surrounding cloud to form the bipolar outflow. Jets from more evolved young stars -
T Tauri stars T Tauri stars (TTS) are a class of variable stars that are less than about ten million years old. This class is named after the prototype, T Tauri, a young star in the Taurus star-forming region. They are found near molecular clouds and iden ...
- produce similar bow shocks, though these are visible at
optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
wavelengths and are called
Herbig–Haro object Herbig–Haro (HH) objects are bright patches of nebula, nebulosity associated with newborn stars. They are formed when narrow jets of partially plasma (physics), ionised gas ejected by stars collide with nearby clouds of gas and dust at several ...
s (HH objects). T Tauri stars are usually found in less dense environments. The absence of surrounding gas and dust means that HH objects are less effective at entraining molecular gas. Consequently, they are less likely to be associated with visible bipolar outflows. The presence of a bipolar outflow shows that the central star is still accumulating material from the surrounding cloud via an
accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other fo ...
. The outflow relieves the build-up of angular momentum as material spirals down onto the central star through the accretion disk. The magnetised material in these protoplanetary jets is rotating and comes from a wide area in the protostellar disk. Bipolar outflows are also ejected from evolved stars, such as proto-planetary nebulae,
planetary nebulae A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated ...
, and
post-AGB The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) l ...
stars. Direct imaging of proto-planetary nebulae and planetary nebulae has shown the presence of outflows ejected by these systems. Large spectroscopic radial velocity monitoring campaigns have revealed the presence of high-velocity outflows or jets from post-AGB stars. The origin of these jets is the presence of a binary companion, where mass-transfer and accretion onto one of the stars lead to the creation of an accretion disk, from which matter is ejected. The presence of a magnetic field causes the eventual ejection and collimation of the matter, forming a bipolar outflow or jet. In both cases, bipolar outflows consist largely of molecular gas. They can travel at tens or possibly even hundreds of kilometers per second, and in the case of young stars extend over a
parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...
in length.


Galactic outflow

Massive galactic molecular outflows may have the physical conditions such as high gas densities to form stars. This star-formation mode could contribute to the morphological evolution of galaxies.


See also

*
Accretion disc An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other f ...
*
Astrophysical jet An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets bec ...
*
Bipolar nebula A bipolar nebula is a type of nebula characterized by two lobes either side of a central star. About 10-20% of planetary nebulae are bipolar. Formation Though the exact causes of this nebular structure are not known, it is often thought to impl ...
*
Herbig–Haro object Herbig–Haro (HH) objects are bright patches of nebula, nebulosity associated with newborn stars. They are formed when narrow jets of partially plasma (physics), ionised gas ejected by stars collide with nearby clouds of gas and dust at several ...
*
Planetary nebula A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated to ...


References

# Reipurth B., Bally J. (2001), "Herbig–Haro flows: probes of early stellar evolution", ''Annual Review of Astronomy and Astrophysics'', vol. 39, p. 403-455 # Davis C. J., Eisloeffel J. (1995), "Near-infrared imaging in H2 of molecular (CO) outflows from young stars", ''Astronomy and Astrophysics'', vol. 300, p. 851-869. # Kwok S. (2000), ''The origin and evolution of Planetary Nebulae'', Cambridge Astrophysics Series, Cambridge University Press. # Chen Z., Frank A., Blackman E. G., Nordhaus J. and Carroll-Nellenback J., (2017), "''Mass Transfer and Disc Formation in AGB Binary systems''", Monthly Notices of the Royal Astronomical Society, https://doi.org/10.1093/mnras/stx680


External links


''A General Catalogue of Herbig–Haro Objects''

''A Catalogue of Molecular Hydrogen Emission-Line Objects in Outflows from Young Stars: MHO Catalogue''
{{Star Stellar astronomy