The protoplanetary disk's matter accretion rate may be estimated from the excess UV or IR radiation from the protostar. The infrared excess comes partly from the dissipation of the potential energy released when the matter spirals towards the interior of the disk (Lynden-Bell and Pringle, 1974). However, other processes contribute to the infrared emission.
The UV radiation, by contrast, comes directly from the dissipation of the gravitational energy from the transfer of material from the inner edge of the disk onto the star. It is therefore a more reliable indicator of the accretion rate. According to the most recent models, the star accretes the material along the protostar's magnetic-field lines, and not through a continuous boundary region (Fig. 5.8). This theory is based on the presence of P-Cygni profiles (which are characterized by an asymmetrical line, revealing the presence of a stellar wind, like that found in the star P Cygni, where the phenomenon was discovered) that are observed in certain Balmer lines, and which may be interpreted as the signature of material falling onto the star. Transfer models that simulate magnetic accretion manage to account for all the observed spectral characteristics. However, these observations may only be carried out on stars that have become optically visible, i.e., T-Tauri type stars, which have freed themselves from the primordial cloud.
The stellar object HH 30 (Fig. 5.9), which has been observed several times by the HST, reveals a remarkable example of a protoplanetary disk following dispersal of the primordial cloud. The opaque disk, the dark outline of which is silhouetted against the light from the star, is seen edge-on. Its diameter is several hundred AU. One jet, perpendicular to the disk, is clearly visible. It should be noted, however, that it is far less intense than in the collapse phase, an example of which we have seen in the object HH 211 (Fig. 5.4). Observations of the 12CO(2-1) and 13CO(2-1) transitions have enabled the velocity of the gas in the jet to be determined.
i—r—j—i—i—i—i—|—7—i—i" i ' |—i i—i 1—j—i—i—r
Fig. 5.8 The UV and optical spectrum of the star BP Tau (solid line) compared with that of another T-Tauri star, LkCa7 (dashed line). The spectrum of BP Tau shows a UV excess, the sign of material falling onto the protostar. The other T-Tauri star, LkCa7, does not exhibit this sign of accretion (After Hartmann, 1998)
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