The Formation of Binary Systems

The existence of a stellar magnetic field partially explains the transfer of angular momentum, but is not always sufficient to account for the observed slow rotation rate. The mechanism is, in fact, effective during the early phases of the evolution of the cloud, when the magnetic field is strongly coupled to the gas. Ambipolar diffusion should, however, cause the gas to be decoupled from the magnetic field, which should lead to the conservation of the angular momentum during the later stages of collapse (Larson, 2003). Other mechanisms are thus necessary to account for the transfer of the angular momentum.

A very efficient mechanism is the collapse of the cloud in a binary, or even multiple, system. Most of the angular momentum is then transformed into the orbital motions of the different stars. The angular momentum of a typical molecular core is comparable with that of a binary system. Numerous theoretical studies suggest that the collapse of a molecular cloud frequently leads to the formation of binary and multiple systems (see Matsumoto and Hanawa, 2003, in particular).

Observations show that about two-thirds of all Main-Sequence stars are binary or multiple systems. Distant binaries may form directly from the collapse of molecular core. Close binaries are probably formed by more complex mechanisms, including

Fig. 5.11 The binary system GG Tau, observed by millimetric interferometry by IRAM (Plateau de Bure). The two pre-Main-Sequence stars are separated by 38 AU (distance as projected on the sky). The observations were made using the 13CO(2-1) line at 220 GHz. The emission shown in yellow, observed in the continuum at 1.3 mm, corresponds to the thermal emission from dust and indicates a disk-like structure surrounding the binary system. Material has been lost from the central region of the disk through tidal effects, caused by the interaction of the two components of the binary (© IRAM)

Fig. 5.11 The binary system GG Tau, observed by millimetric interferometry by IRAM (Plateau de Bure). The two pre-Main-Sequence stars are separated by 38 AU (distance as projected on the sky). The observations were made using the 13CO(2-1) line at 220 GHz. The emission shown in yellow, observed in the continuum at 1.3 mm, corresponds to the thermal emission from dust and indicates a disk-like structure surrounding the binary system. Material has been lost from the central region of the disk through tidal effects, caused by the interaction of the two components of the binary (© IRAM)

dynamical interactions with the disk and through tidal effects (see Chap. 6). As for isolated stars, they may result from the evolution of unstable triple systems, that subsequently divide into an isolated star and a binary system (Larson, 2003). Figure 5.11 shows the young binary system GG Tau as observed by millimetric interferometry.

5.2.5 The Principal Stages of Star Formation

The major stages in the formation of stars may be summarized as follows (Larson,

• collapse of the cold cloud (known as a 'class 0' object)

• protostar, an object ('class I') that is still buried within the collapsing cloud

• T-Tauri phase, a pre-Main-Sequence object ('class II'), optically visible, surrounded by a thick protoplanetary disk and with a characteristic, intense stellar wind

• young Main-Sequence object ('class III'), surrounded by a thin disk (a debris disk).

Fig. 5.12 A time/radius diagram of the contraction phases of a molecular cloud The abscissa shows the density in g/cm3, and the ordinate log T (K) (After Acker, 2005)

The collapse of a molecular cloud may be described in terms of several phases

• Isothermal phase: This is the first step of gravitational contraction. The temperature is about a few tens of Kelvin. The gas is tenuous enough for the gravitational energy to dissipate through the radiation coming from the thermal excitation of the atoms. The temperature thus remains low and constant. The object, with a size of a few hundred AU, is observable through its infrared thermal emission.

• Adiabatic phase: as the density increases, the opacity of the clouds increases, and the energy released by the contraction of the cloud cannot escape by radiation, and the temperature rises. The contraction takes place adiabatically, without any exchange of heat with the exterior. The increase in temperature leads to the dissociation and ionization of the hydrogen molecules. The ionization phase is accompanied by a reduction in the rate of heating and an acceleration of the contraction phase.

• Appearance of the star: at the end of the hydrogen-ionization phase, the temperature again rises rapidly, until it reaches several million degrees. This temperature is sufficient to initiate the first thermonuclear reactions. Henceforward, the star shines in the visible region.

The evolution of the protoplanetary disk may be followed by the evolution of its electromagnetic spectrum, from the UV to the radio region (Fig. 5.13):

• In the collapse phase (less than 10000 years), only the cold cloud is detectable from radiation by the dust as a black body that peaks in the far infrared; the spectrum is said to be of 'Class 0'.

• In spectra of Class I (t = 10 000 years), the increase in the temperature at the centre of the cloud produces a shift in the black-body spectrum towards the near infrared. Cold material continues to fall onto the protostar, producing another component that peaks in the far infrared. This cold material accumulates in an equatorial disk. A UV component, very close to the protostar, may also be present, and is the signature of matter falling onto the central object along its lines of magnetic force (see Sect. 5.2.2).

log Tc

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