An Etymological Dictionary of Astronomy and Astrophysics
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An imaginary continuous curve drawn in a fluid so that the tangent at every point of it at any instant of time coincides with the direction of the motion of the fluid at that point. The component of velocity at right angles to the streamline is always zero. If a number of streamlines is considered at a particular instant, the pattern they form gives a good indication of the flow then occurring. Same as → flow line. See also → path line, → stream tube.

→ stream; → line.

A thin → transition  → layer inside → Sun, between the → differentially rotating  → convection zone and the uniformly rotating → radiative zone. Discovered through → helioseismology, it has raised considerable interest, but its thinness remains still to be explained. Due to its strong → shear, it is believed to play a crucial role in the generation of the → solar magnetic field.

Tachocline, from tacho- a combining form meaning "speed," → tachyon + → -cline "slope." The term was first coined by Edward A. Spiegel and Jean-Paul Zahn (1992, A&A 265, 106), by analogy to the oceanic → thermocline.

Tondâšib, from tondâ, → velocity, + šib, → -cline.

A collisionally excited spectral line formed in high density condition well above the → critical density. At such densities the → excitation temperature is at (or very near) the → kinetic temperature of the gas. At low densities, below the critical density, the excitation temperature will be only slightly above the radiation temperature and the emission line will be practically invisible.

Thermalized, p.p. of → thermalize; → line.

A layer in a large body of water, such as a lake, in which temperature changes more rapidly with depth than it does in the layers above or below.

→ thermo- + → -cline.

An instability in the ocean water that occurs when a layer of warm salt water is above a layer of fresh cold water of slightly higher density. In this process the hot salt water cools off and then, after having reached a higher density than the fresh water, sinks down even in the presence of stabilizing temperature gradients. This phenomenon explains the large-scale water movements in the oceans called themohaline circulation. First discussed by Melvin E. Stern (1960, Tellus 12, 172). → thermohaline mixing.

Thermohaline, from → thermo- + haline, from Gk. hals (genitive halos) "salt, sea;" cf. L. sal; O.Ir. salann; Welsh halen; O.C.S. sali "salt;" O.E. sealt; cf. O.N., O.Fris., Goth. salt, Du. zout, Ger. Salz from PIE *sal- "salt."

Garmâšur, from garmâ-→ thermo- + šur "salty" (Mid.Pers. šôr "salty," šorag "salt land;" cf. Skt. ksurá- "razor, sharp knife;" Gk. ksuron "razor;" PIE base *kseu- "to rub, whet").

In stars, an instability phenomenon, reminiscent of the → thermohaline convection in the oceans, that takes place when layers of higher molecular weight occur above a region of lower molecular weight. A situation of heavier material being above lighter gas in a star can occur during the → helium flash when → helium burning does not start in the center but in the shell. Similarly, in → close binary systems it may happen that helium-rich material is transferred to a → main sequence star. Then a helium-rich outer layer is formed and the instability occurs at the interface between that layer and the original stellar material. This process can explain several surface abundance variations in stars. First discussed by S. Kato (1966, Publ. Astron. Soc. Japan 18, 374).

→ thermohaline; → mixing.

A forbidden line emitted by interstellar ionized gas by several atomic species (O, O⁺, O⁺⁺, N⁺, S⁺⁺, etc.) corresponding to the transition from the electronic state 1S to 3P. Examples are the ultraviolet line of the doubly ionized oxygen [O III] at 2321 A and [N II] 3063 A. → auroral line; → forbidden line; → nebular line.

→ trans-; → auroral line.

A spectral line whose origin is not clearly established. → line identification.

→ unidentified; → line.

A network of ten 25-m radio telescopes for → very-long-baseline interferometry (VLBI), operated by the U.S. National Radio Astronomy Observatory. Eight of the VLBA telescopes are distributed across the continental United States, while the other two are in Hawaii and the Virgin Islands, giving a maximum baseline of about 8,000 km and a resolution better than a milliarcsecond at its shortest wavelength.

→ very; → large; → baseline; → array.

A technique in radio interferometry in which the individual telescopes are not directly connected together, but instead make their observations separately with very accurate timings. The data are later sent to a central correlator to be combined. With this technique the individual telescopes can be arbitrarily far apart, and so the technique provides the highest resolution images in astronomy, typically down to a few milliarcseconds.

→ very; → large; → baseline; → interferometry.

A member of a class of cntral stars of planetary nebula, → CSPN, which have weaker and narrower emission lines than → Wolf-Rayet-like CSPNe (Tylenda et al. 1993, A&AS 102, 595).

→ weak; → emission; → line.

A T Tauri star that lacks strong emission lines in its optical spectrum, and lacks both strong → stellar wind and → infrared excess. These objects are believed to be → pre-main sequence stars without obvious signs for disk → accretion. Weak-line T Tauri stars result from the evolution of → classical T Tauri stars.

→ weak; → line; → T Tauri star.

In relativity, the path traced out in four-dimensional → space-time that represents a continuous sequence of events relating to a given particle. A point on a world line is called an → event. Any straight world line corresponds to an → inertial motion. Curved world lines represent → accelerated motion. A world line that curves corresponds to an accelerated observer. World lines are shown on space-time diagrams.

→ world; → line.

The → fine structure → emission line of → carbon at 157.74 μm, with transition ²P_3/2-²P_1/2 (rest frame frequency 1900.54 GHz). [C II] is a major → cooling line in → molecular clouds and is produced in low density → photodissociation regions (PDRs) (n_H≤ 10³ cm^-3). In higher density PDRs the [O I] 63 μm line becomes an important → coolant. The [C II] line traces → neutral gas exposed to → ultraviolet  → photons from → young stars. The [C II] line is used to probe the stellar → radiation field and find out how it affects the physical conditions of the gas. The bulk of the [C II] emission line is believed to originate from PDRs, and the remainder from → X-ray Dissociation regions (XDRs), and, in a lesser extent, by cosmic-ray-dominated regions (CRDRs), → H II regions, low-density warm gas and/or diffuse H I clouds.

→ carbon; → line.