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 < αIR < 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.
See also: → class; → two.
