An Etymological Dictionary of Astronomy and Astrophysics
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A stage in the evolution of low-mass → protostars resulting from a → Class I object about 10⁶ years after the initial → gravitational collapse. Most of the envelope has been removed and the embedded object becomes visible at infrared and optical wavelengths. At this stage, the bulk of the material has → accreted onto the central object. A flattened → circumstellar disk or → protoplanetary disk is present in which material moves inward at a decreasing rate. The disk contributes only about 1% of the total mass of the system. Material from a remaining envelope may still accrete onto the outer parts of the disk. The → spectral energy distribution (SED) at → near infrared wavelengths is dominated by the emission of the central protostar and typically peaks around 2 μm, corresponding to temperatures around 1000 to 2000 K. At longer wavelengths an → infrared excess is observed, originating from the disk. The SED has a negative → spectral index (-1.5 < α_IR < 0). Estimated disk masses and → accretion rates are 10^-3 to 10^-1 → solar masses and 10^-8 solar masses per year, respectively. This stage initiates the → pre-main sequence stage of a star. The object is referred to as a → classical T Tauri star. The stellar → photosphere is revealed at optical wavelengths accompanied by strong → emission lines and photometric variability, but the infrared luminosity is far larger than can be explained by the photometric temperature and radius.

→ class; → two.

An evolutionary stage in the formation of low-mass → protostars resulting from a → Class II object between 1 to 10 million years after the initial → gravitational collapse. At this stage → accretion has ceased completely and what remains from the → circumstellar disk is a → debris disk. The temperature and density of the → pre-main sequence star keep increasing as the object slowly contracts to its final size. Most of the → luminosity derives from protostellar contraction. The → spectral energy distribution (SED) resembles a stellar → blackbody, peaking at optical and infrared wavelengths. Minor → infrared excess is still observed. The SED has a negative → spectral index (α_IR < -1.5). Class III objects are sometimes called → weak-line T Tauri stars.

→ class; → three.

In the → Fanaroff-Riley classification, → radio sources with hotspots in their lobes at distances from the center which are such that R_FR > 0.5. The → radio jets in FR-II sources are expected to be highly → supersonic, allowing them to travel large distances.

→ Fanaroff-Riley classification; → class.