Classification of Protostellar Objects

In order to help categorise the protostellar objects that have been observed a classification system is commonly used which is based on the amount and type of radiation emitted (and is thus constant with changing theories about what that radiation represents in the clumps).

Class 0

^1.4 These are the earliest form of protostars and have the majority of their total luminosity emitted in the submillimetre region (defined as $\lambda>350\mu$m) - ie. $\frac{L_{\rm bol}}{L_{\rm submm}}<1$. This is due to them still being very cold and surrounded in large amounts of gas and dust which has yet to accrete onto the central protostar.

Class I

The remaining three class descriptions depend upon the shape of the infra-red emission spectrum which can be described in terms of $\lambda F_\lambda$ where $\lambda$ is the wavelength and $F_\lambda$ is the flux at that wavelength. For the wavelength range 2-100$\mu$m $\lambda F_\lambda \propto \lambda^x$. An object is a Class I source if $x>0$ which implies that there is more flux at the longer wavelengths. These are objects with a hot emitting protostar at the centre but still surrounded by a dusty infalling envelope that obscures the central source. The envelope is heated by the central protostar and therefore re-emits in the infra red.

Class II

These are defined as having $-\frac{4}{3}<x<0$ and are stars where the envelope has collapsed to form a rotating disk. The disk is heated by the central star and emits large amounts of infra-red radiation, however, as the surrounding envelope has gone the central star is now visible.

Class III

Once the value of $x$ drops below $-\frac{4}{3}$ an object becomes a Class III source. This signifies that there is no longer an infra-red excess of radiation and that the dusty disk has either been blown away or accreted onto the star. The star is now about to join the main-sequence where it will stay until it has used up all its available hydrogen. Stars in the Class II or III phase are also known as T-Tauri stars which are low mass pre-mainsequence stars. T-Tauri stars themselves are divided into two categories, Weakline T-Tauri Stars (WTTS) and Classical T-Tauri Stars (CTTS) which are loosely Class II and III sources respectively.

Examples of the classification system.  

Figure 1.3^1.5 shows examples of one object in each of the three classes I, II & III (respectively IRAS 04016+2610, AA Tau and LkCa 7). It can be clearly seen the way that the younger objects have considerably more infra-red emission - consistent with them being cloaked in dust which absorbs all the visible light and re-emits it in the infra red. The class III source on the other hand has blown away the dust and is emitting much more like a black body (ie. a Planck curve).