young stellar object

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 $\backslash alpha\; \backslash ,$:{{cite book |chapter=Star Formation: From OB Associations to Protostars |title=Star Forming Regions: Proceedings of the 115th Symposium of the International Astronomical Union Held in Tokyo, Japan, November 11–15, 1985 |publisher=D. Reidel |location=Dordrecht |first=Charles J. |last=Lada |editor1-first=Manuel |editor1-last=Peimbert |editor2-first=Jun |editor2-last=Jugaku |pages=1–17 |date=1987 |bibcode=1987IAUS..115....1L |isbn=978-90-277-2388-8}}

$\backslash alpha=\backslash frac\{d\backslash log(\backslash lambda\; F\_\backslash lambda)\}\{d\backslash log(\backslash lambda)\}$.

Here $\backslash lambda\; \backslash ,$ is wavelength, and $F\_\backslash lambda$ is flux density.

The $\backslash alpha\; \backslash ,$ is calculated in the wavelength interval of 2.2–20 $\{\backslash mu\}m$ (near- and mid-infrared region). Andre et al. (1993) discovered a class 0: objects with strong submillimeter emission, but very faint at .{{cite journal |title=Submillimeter Continuum Observations of $\{\backslash rho\}$ Ophiuchi A: The Candidate Protostar VLA 1623 and Prestellar Clumps |journal=The Astrophysical Journal, Part 1 |first1=Philippe |last1=Andre |first2=Derek |last2=Ward-Thompson |first3=Mary |last3=Barsony |volume=406 |issue=1 |pages=122–141 |date=March 1993 |doi=10.1086/172425 |bibcode=1993ApJ...406..122A|doi-access=free }} Greene et al. (1994) added a fifth class of "flat spectrum" sources.{{cite journal |title=Further Mid-infrared Study of the $\{\backslash rho\}$ Ophiuchi Cloud Young Stellar Population: Luminosities and Masses of Pre-main-sequence Stars |journal=The Astrophysical Journal, Part 1 |first1=Thomas P. |last1=Greene |first2=Bruce A. |last2=Wilking |first3=Philippe |last3=Andre |first4=Erick T. |last4=Young |first5=Charles J. |last5=Lada |volume=434 |issue=2 |pages=614–626 |date=October 1994 |doi=10.1086/174763 |bibcode=1994ApJ...434..614G}}

• Class 0 sources – undetectable at
• Class I sources have $\{\backslash alpha\}>0.3$
• Flat spectrum sources have $0.3>\{\backslash alpha\}>-0.3$
• Class II sources have $-0.3>\{\backslash alpha\}>-1.6$
• 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 $24\{\backslash mu\}m$) are known as transition-disk objects.

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

File:Opo0113i.jpg

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

File:Young stellar jet MHO 2147.jpg|Young stellar jet MHO 2147 {{cite web|title=Sidewinding Young Stellar Jets Spied by Gemini South|url=https://noirlab.edu/public/news/noirlab2204/|access-date=January 27, 2023}}

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• Bok globule

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