- Herbig Ae/Be stars
Herbig Ae/Be stars are
pre-main sequence star s - young (<10Myr) stars ofspectral type s A and B. They are still embedded in gas-dust envelopes and may be surrounded by circumstellar disks.Hydrogen andCalcium emission lines are observed in the spectra of these stars. They are 2-8 Solar mass objects, still existing in thestar formation (gravitational contraction) stage and approaching themain sequence (i.e. they are not burning hydrogen in their center). In theHR diagram these stars are located to the right of themain sequence . They are named after the American astronomerGeorge Herbig , who first distinguished them from otherstar s in 1960. The original Herbig criteria were:
*Spectral type earlier than F0 (in order to excludeT Tauri star s),
* Balmer emission lines in the stellar spectrum (in order to be similar toT Tauri star s),
* Projected location within the boundaries of a darkinterstellar cloud (in order to select really young stars near their birthplaces),
* Illumination of a nearby brightreflection nebula (in order to guarantee physical link withstar formation region).Nowadays there are known several isolated Herbig Ae/Be stars (i.e. not connected with dark clouds or nebulae). Thus the most reliable criteria now can be:
*Spectral type earlier than F0,
* Balmer emission lines in the stellar spectrum,
*Infrared radiation excess (in comparison with normal stars) due tocircumstellar dust (in order to distinguish from classicalBe star s, which have infrared excess due to free-free emission).Sometimes Herbig Ae/Be stars show significant brightness variability. They are believed to be due to clumps (
protoplanet s andplanetesimal s) 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 star s, are calledT Tauri star s. More massive (>8 Solar mass)star s 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 aremain sequence objects.References
* Pérez M.R., Grady C.A. (1997), "Observational Overview of Young Intermediate-Mass Objects: Herbig Ae/Be Stars", Space Science Reviews, Vol 82, p. 407-450
* Waters L. B. F. M., Waelkens, C. (1998), "HERBIG Ae/Be STARS", Annual Review of Astronomy and Astrophysics, Vol. 36, p. 233-266
* [http://astro.berkeley.edu/ay216/NOTES/Lecture27.pdf Herbig Ae/Be stars] (no longer available)
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