Class 0 proto-stars

In the classical view of star formation [21,26], young proto-stars are surrounded by an accretion disk, also called a circumstellar disk, which contributes to the feeding of the proto-star before it reaches its final mass. The so called class 0 proto-stars are deeply embedded and radiate mostly in the far infrared and sub-millimetre spectral range. At this stage, proto-stars are actively accreting, as testified by the presence of jets and molecular outflows. These outflows contribute to the release of energy and... 

Fig. 2.6 The molecular outflow created by the class 0 proto-star LI 157 viewed in the infrared by the Spitzer (left) and in the far infrared with Herschel (right) satellites. The Spitzer image taken with the IRAC camera is dominated by emission from the rotational lines of H2. The Herschel image taken with the PACS instrument at 179 pm, the wavelength of one of the strongest lines of water vapour (2j 2 — lo.i) shows the excellent agreement between the H2 and water vapour morphology [35]...

Class I obj ects also have bipolar outflows, but they are less powerful and less well collimated than those of Class 0 objects. This stage lasts 100 000 to 200 000 years. Class //objects, also known as classical T Tauri stars, are pre-main-sequence stars with optically thick proto-planetary disks. They are no longer embedded in their parent cloud, and they are observed in optical and infrared wavelengths. They still exhibit bipolar outflows and strong stellar winds. This stage lasts from 1-10 million years. Class ///objects are the so-called weak line or naked T-Tauri stars. They have optically thin disks, perhaps debris disks in some cases, and there are no outflows or other evidence of accretion. They are observed in the visible and near infrared and have strong X-ray emission. These stars may have planets around them, although they cannot be observed. 

---