Young stellar object

Star formation

Interstellar medium Molecular cloud Bok globule Dark nebula Young stellar object Protostar T Tauri star Herbig Ae/Be stars Nebular hypothesis
Object classes
Herbig–Haro object
Theoretical concepts
Initial mass function Jeans instability Kelvin–Helmholtz mechanism
Star portal

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. Sometimes they are divided by mass - massive YSO (MYSO), intermediate mass YSO and brown dwarfs.

YSO are usually classified using criteria based on the slope of their SED, introduced by Lada C.J. in 1987. He proposed three classes (I, II and III), based on the values of intervals of spectral index $\alpha \,$ :

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

Here $\lambda \,$ is wavelength, and $F_\lambda$ is flux density.

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

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

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.

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

References

Lada, C. J. (1987), IAU Symposium 115: Star Forming Regions, 115, 1
Greene, Thomas P.; Wilking, Bruce A.; Andre, Philippe; Young, Erick T.; Lada, Charles J. (1994), Further mid-infrared study of the rho Ophiuchi cloud young stellar population: Luminosities and masses of pre-main-sequence stars, The Astrophysical Journal, vol. 434, pp. 614–626
Andre, Philippe; Ward-Thompson, Derek; Barsony, Mary (1993), Submillimeter continuum observations of Rho Ophiuchi A - The candidate protostar VLA 1623 and prestellar clumps, The Astrophysical Journal, vol. 406, pp. 122–141

Star

Evolution	Formation Pre–main sequence Main sequence Horizontal branch Asymptotic giant branch Dredge-up Instability strip Red clump PG1159 star Mira variable Planetary nebula Protoplanetary nebula Luminous red nova Luminous blue variable Wolf–Rayet star Supernova impostor Supernova Hypernova Hertzsprung–Russell diagram Color–color diagram

Protostars	Molecular cloud H II region Bok globule Young stellar object Herbig–Haro object Hayashi track Hayashi limit Henyey track Orion T Tauri FU Orionis Herbig Ae/Be

Luminosity class	Subdwarf Dwarf Blue Red Subgiant Giant Blue Red Bright giant Supergiant Blue Red Yellow Hypergiant Yellow Blue straggler

Spectral classification	O B A F G K M Be OB Subdwarf B Late-type Peculiar Am Ap/Bp Oscillating Barium Carbon CH Extreme helium Lambda Boötis Lead Mercury-manganese S Shell Technetium

Remnants	White dwarf Black dwarf Helium planet Neutron star Pulsar Magnetar Stellar black hole Compact star Quark Exotic Stellar core: EF Eridani B

Failed and theoretical stars	Substellar object Brown dwarf Sub-brown dwarf Planetar Boson star Dark-matter star Quasistar Thorne–Żytkow object Iron star

Nucleosynthesis	Alpha process Triple-alpha process Proton–proton chain Helium flash CNO cycle Lithium burning Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova Remnants

Structure	Core Convection zone Microturbulence Oscillations Radiation zone Photosphere Starspot Chromosphere Corona Stellar wind Bubble Asteroseismology Eddington luminosity Kelvin–Helmholtz mechanism

Properties	Designation Dynamics Effective temperature Kinematics Magnetic field Magnitude Absolute Mass Metallicity Rotation UBV color Variability

Star systems	Binary Contact Common envelope Multiple Accretion disc Planetary system Earth's Solar System

Earth-centric observation of	Pole star Circumpolar star Magnitude Apparent Photographic Color Radial velocity Proper motion Parallax Photometric-standard star

Lists	Star names Arabic names Chinese names Most massive Least massive Largest Brightest Historical Most luminous Nearest Nearest bright Stars with exoplanets Brown dwarfs Planetary nebulae Novae Notable supernovae Supernova remnants Supernova candidates Timeline of stellar astronomy

Related articles	Planet Star cluster Association Open cluster Globular cluster Galaxy Supercluster Helioseismology Guest star Constellation Asterism Gravity Intergalactic star Infrared dark cloud

Star portal

Young stellar object

References

See also