Interferometric observations of pre-main sequence disks - Caltech ...

1.1 Star formation 7
the mid-infrared. Class II stars are characterized by a SED composed by the protostellar
emission and the flux arising from the circumstellar disk composed by gas and dust;
in this phase most of the circumstellar material is in the disk. The SED peaks in the
near infrared and decreases roughly as a power law from the mid infrared to millimeter
wavelengths. Finally, for the Class III objects the emission of the circumstellar material
becomes negligible and the SED has a pure photospheric shape.
The protostellar phase ends when the core has either accreted onto the star+disk or
dissipated by winds and jets. At this point the mass of the central star is practically
fixed and the mass accretion rate ˙M decreases to very low values (typically ˙M< ∼ 10 −7 M⊙
yr −1 ). The star evolves by slow contraction on the Kelvin-Helmholtz timescale (tKH∼
GM 2 ⋆ /R⋆L⋆) toward the ZAMS, where the stellar luminosity is due to the hydrogen
burning. During this pre-main sequence phase, the stellar luminosity is dominated by
the gravitational energy released in the contraction while the accretion luminosity is
negligible.
Pre-main sequence stars are located on the Herzsprung-Russell diagram in a region
to the right of the ZAMS and evolutionary tracks have been computed by several au-
thors (Stahler and Palla, 2005). By comparing the position of any individual star to the
evolutionary tracks, it is possible to determine its age and mass (a more direct way to
determine the mass of the central star will be discussed in Sec. 1.2). In this way it has
been found that the age of pre-main sequence stars range from 10 5 to 10 7 yr. It is also
generally accepted to call T Tauri stars (TTS) the low-mass (M⋆< 1.5M⊙) pre-main se-
quence stars, while Herbig Ae/Be stars (HAe/Be, since the work of George H. Herbig in
1960) are the intermediate mass pre-main sequence stars (1.5M⊙< M⋆< 8M⊙). Higher-
mass stars reach the ZAMS while still accreting and do not have a pre-main-sequence
phase (Beuther et al., 2006).
Both TTS and HAe/Be stars show a strong stellar activity in the form of continuum
excess emission in the UV and IR, emission lines, strong photometric and spectroscopic
variability, ejection of matter as wind and jets (see the review of Bally et al., 2006). In
many cases the activity is strictly related to the presence of circumstellar disks, which