Fig. 5.13 Evolution of the spectrum of a protostar during its four evolutionary phases (Classes 0 to III) (After Andre and Montmerle, 1994)
• Objects of Class II (t = about 106 years) are characterized by a thick disk, which absorbs part of the light from the protostar and re-emits it in the infrared. The spectrum of the protostar is usually maximum in the near infrared, whereas the disk component, which is colder, provides a contribution at longer wavelengths. Some T-Tauri stars show a flat spectrum over the infrared range.
• Finally, in objects of Class III (t = about 107 years), the disk has lost most of its mass because of the violent stellar wind, which has swept the material out into space. The spectrum of the star predominates. It peaks in the near infrared (which corresponds to a temperature of 4000 K). The peak shifts towards visible wavelengths as the star's temperature increases.
Figure 5.14 similarly shows how the infrared excess of a young star decreases with age, as the quantity of dust contained within the disk decreases.
It should be noted that the spectra of young stars may exhibit more complex forms than the simple combination of black-body emissions described above. This is particularly the case for the T-Tauri star, GM Aurigae, which exhibits, in common with a number of T-Tauri stars, a spectrum that is relatively flat over a range of frequencies (Fig. 5.15). To explain this type of spectrum, a model with a double-layer disk has been devised by Chiang and Goldreich (1997). According to this model, at a certain distance from the star, the stellar radiation is absorbed by the dust in the disk's outer layer, converted into thermal energy, and re-emitted in the form of infrared radiation both to space and towards the equatorial plane of the disk (Fig. 5.16). The dust grains, which are too small to convert all the energy that they receive, heat up to a temperature that is greater than that of the gas, producing a spectral component that is shifted towards the blue, relative to the emission from the disk.
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