Young stellar object explained

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.

Classification by spectral energy distribution

\alpha :^[1] \alpha = (d log ( λ F _{λ)})/(d log ( λ )) .

Here

λ is wavelength, and F _{λ} is flux density.

The

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

• Class 0 sources – undetectable at

{ λ }< 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.

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

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

Characteristics

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

Classification of YSOs by mass

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

See also

• Bok globule

Notes and References

1. Book: Lada, Charles J. . Star Formation: From OB Associations to Protostars . Star Forming Regions: Proceedings of the 115th Symposium of the International Astronomical Union Held in Tokyo, Japan, November 11–15, 1985 . D. Reidel . Dordrecht . Manuel . Peimbert . Jun . Jugaku . 1–17 . 1987 . 1987IAUS..115....1L . 978-90-277-2388-8.
2. Submillimeter Continuum Observations of {\rho} Ophiuchi A: The Candidate Protostar VLA 1623 and Prestellar Clumps . The Astrophysical Journal, Part 1 . Philippe . Andre . Derek . Ward-Thompson . Mary . Barsony . 406 . 1 . 122–141 . March 1993 . 10.1086/172425 . 1993ApJ...406..122A. free .
3. Further Mid-infrared Study of the {\rho} Ophiuchi Cloud Young Stellar Population: Luminosities and Masses of Pre-main-sequence Stars . The Astrophysical Journal, Part 1 . Thomas P. . Greene . Bruce A. . Wilking . Philippe . Andre . Erick T. . Young . Charles J. . Lada . 434 . 2 . 614–626 . October 1994 . 10.1086/174763 . 1994ApJ...434..614G.