Contents

Young stellar object

Star in its early stage of evolution

YSOs in the Rho Ophiuchi cloud complex

V1331 Cyg, a YSO surrounded by a reflection nebula

Star formation
Object classes
Interstellar medium Molecular cloud Bok globule Dark nebula Young stellar object Protostar Pre-main-sequence star T Tauri star Herbig Ae/Be star Herbig–Haro object
Theoretical concepts
Accretion Initial mass function Jeans instability Kelvin–Helmholtz mechanism Nebular hypothesis Planetary migration
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Young stellar object (YSO) denotes a star in its early stage of evolution. This class consists of two groups of objects: protostars and pre-main-sequence stars.

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 ${\displaystyle \alpha \,}$:^[1]

${\displaystyle \alpha ={\frac {d\log(\lambda F_{\lambda })}{d\log(\lambda )}}}$.

Here ${\displaystyle \lambda \,}$ is wavelength, and ${\displaystyle F_{\lambda }}$isflux density.

The ${\displaystyle \alpha \,}$ is calculated in the wavelength interval of 2.2–20 ${\displaystyle {\mu }m}$ (near- and mid-infrared region). Andre et al. (1993) discovered a class 0: objects with strong submillimeter emission, but very faint at ${\displaystyle {\lambda }<10{\mu }m}$.^[2] Greene et al. (1994) added a fifth class of "flat spectrum" sources.^[3]

• Class 0 sources – undetectable at ${\displaystyle {\lambda }<20{\mu }m}$
• Class I sources have ${\displaystyle {\alpha }>0.3}$
• Flat spectrum sources have ${\displaystyle 0.3>{\alpha }>-0.3}$
• Class II sources have ${\displaystyle -0.3>{\alpha }>-1.6}$
• Class III sources have ${\displaystyle {\alpha }<-1.6}$

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 ${\displaystyle 24{\mu }m}$) are known as transition-disk objects.

YSOs are also associated with early star evolution phenomena: jets and bipolar outflows, masers, Herbig–Haro objects, and protoplanetary disks (circumstellar disks or proplyds).

These stars may be differentiated by mass: Massive YSOs, intermediate-mass YSOs, and brown dwarfs.

• 
  Young stellar jet MHO 2147 ^[4]

• Bok globule

1. ^ 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 ${\displaystyle {\rho }}$ 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 ${\displaystyle {\rho }}$ 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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