A star forms by accumulation of material that falls in to a protostar from a circumstellar disk or envelope. Material in the disk is cooler than the surface of the protostar, so it radiates at longer wavelengths of light producing excess infrared emission. As material in the disk is depleted, the infrared excess decreases. Thus, YSOs are usually classified into evolutionary stages based on the slope of their spectral energy distribution in the mid-infrared, using a scheme introduced by Lada (1987). He proposed three classes (I, II and III), based on the values of intervals of spectral index:
The is calculated in the wavelength interval of 2.2–20 (near- and mid-infrared region). Andre et al. (1993) discovered a class 0: objects with strong submillimeter emission, but very faint at . Greene et al. (1994) added a fifth class of "flat spectrum" sources.
Class 0 sources – undetectable at
Class I sources have
Flat spectrum sources have
Class II sources have
Class III sources have
This classification schema roughly reflects evolutionary sequence. It is believed that most deeply embedded Class 0 sources evolve towards Class I stage, dissipating their circumstellar envelopes. Eventually they become optically visible on the stellar birthline as pre-main-sequence stars.
Class II objects have circumstellar disks and correspond roughly to classical T Tauri stars, while Class III stars have lost their disks and correspond approximately to weak-line T Tauri stars. An intermediate stage where disks can only be detected at longer wavelengths (e.g., at ) are known as transition-disk objects.
^Lada, Charles J. (1987). "Star Formation: From OB Associations to Protostars". In Peimbert, Manuel; Jugaku, Jun (eds.). Star Forming Regions: Proceedings of the 115th Symposium of the International Astronomical Union Held in Tokyo, Japan, November 11–15, 1985. Dordrecht: D. Reidel. pp. 1–17. Bibcode:1987IAUS..115....1L. ISBN 978-90-277-2388-8.
^Andre, Philippe; Ward-Thompson, Derek; Barsony, Mary (March 1993). "Submillimeter Continuum Observations of Ophiuchi A: The Candidate Protostar VLA 1623 and Prestellar Clumps". The Astrophysical Journal, Part 1. 406 (1): 122–141. Bibcode:1993ApJ...406..122A. doi:10.1086/172425.
^Greene, Thomas P.; Wilking, Bruce A.; Andre, Philippe; Young, Erick T.; Lada, Charles J. (October 1994). "Further Mid-infrared Study of the Ophiuchi Cloud Young Stellar Population: Luminosities and Masses of Pre-main-sequence Stars". The Astrophysical Journal, Part 1. 434 (2): 614–626. Bibcode:1994ApJ...434..614G. doi:10.1086/174763.
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