| 词条 | Herbig Ae/Be star |
| 释义 |
A Herbig Ae/Be star (HAeBe) is a pre-main-sequence star – a young (<10Myr) star of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks.[1] Hydrogen and calcium emission lines are observed in their spectra. They are 2-8 Solar mass ({{Solar mass|link=y}}) objects, still existing in the star formation (gravitational contraction) stage and approaching the main sequence (i.e. they are not burning hydrogen in their center). In the Hertzsprung–Russell diagram these stars are located to the right of the main sequence. They are named after the American astronomer George Herbig, who first distinguished them from other stars in 1960. The original Herbig criteria were:
There are now several known isolated Herbig Ae/Be stars (i.e. not connected with dark clouds or nebulae). Thus the most reliable criteria now can be:
Sometimes Herbig Ae/Be stars show significant brightness variability. They are believed to be due to clumps (protoplanets and planetesimals) in the circumstellar disk. In the lowest brightness stage the radiation from the star becomes bluer and linearly polarized (when the clump obscures direct star light, scattered from disk light relatively increases – it is the same effect as the blue color of our sky). Analogs of Herbig Ae/Be stars in the smaller mass range (<2 {{Solar mass}}) – F, G, K, M spectral type pre-main-sequence stars – are called T Tauri stars. More massive (>8 {{Solar mass}}) stars in pre-main-sequence stage are not observed, because they evolve very quickly: when they become visible (i.e. disperses surrounding circumstellar gas and dust cloud), the hydrogen in the center is already burning and they are main-sequence objects. PlanetsPlanets around Herbig Ae/Be stars include:
GalleryReferences1. ^V. Mannings & A. Sargent (2000) High-resolution studies of gas and dust around young intermediate-mass stars: II. observations of an additional sample of Herbig Ae/Be systems. Astrophysical Journal, vol. 529, p. 391 2. ^{{cite web|title=A stellar fingerprint|url=http://www.spacetelescope.org/images/potw1609a/|accessdate=29 February 2016}} Sources
4 : Star types|Star formation|Herbig Ae/Be stars|1960 in science |
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