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A bipolar outflow represents 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).
In the case of a young star, the bipolar outflow is driven by a dense, collimated jet. These astrophysical jets are narrower than the outflow and very difficult to observe directly. However, supersonic 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 wavelengths and thus can be detected with telescopes like the United Kingdom Infrared Telescope (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.
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, 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 - produce similar bow shocks, though these are visible at optical wavelengths and are called Herbig–Haro objects (HH objects). T Tauri stars are usually found in less dense environments. The absence of surrounding gas and dust mean 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.
Bipolar outflows from evolved stars probably start out as spherically-symmetric winds (called post-AGB winds), ejected from the surface of a red giant star as it cools and fades. These are focused into cones of gas by magnetic fields or a binary companion in a process that is not yet well understood. The bipolar outflows from post-AGB stars eventually grow to form a planetary nebula.
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 in length.
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