Figure 4.3 Spectral energy distribution of three stars in the p Ophiuchi dark cloud complex. The dashed curve corresponds to a blackbody at 2300 K. From bottom to top, these broadband spectra exemplify Class I, II, and III sources, respectively.
The copious infrared emission from the first two sources stems from heated dust grains. Note from equation (2.32) that thermal radiation which peaks at 10 pm has an associated temperature near 300 K. The typical infrared excess does not display a pure blackbody spectrum, indicating a significant range of temperatures. Moreover, these temperatures are high enough that the dust in question must be relatively close to the star. It is natural to suppose that these grains are part of cloud material that is either participating in protostellar collapse or else was left behind after collapse ended. Such remnant matter gradually disappears with time.
Pursuing this line of reasoning, we may use the infrared excess as an empirical measure of stellar youth. We quantify matters by considering the infrared spectral index aIR:
It is conventional to evaluate the derivative by numerically differencing the flux between 2.2 and 10 pm. Infrared sources such as WL 12, with aIR > 0, are said to be in Class I. Such objects are generally associated with dense cores, as seen by NH3 emission. The less embedded star SR 24 is an example of a Class II source, for which -1.5 < aIR < 0. Class III stars like SR 20 have aIR < -1.5. Finally, a "Class 0" has been added to incorporate sources so deeply buried that they can only be detected at far-infrared and millimeter wavelengths. One example, shown in Figure 4.4, is the object known as L1448/mm. This star of 10 Lq lies inside a Bok globule in the Perseus region, some 300 pc distant. Notice how its spectral energy distribution is shifted to much longer wavelengths than those in Figure 4.3. Like all Class 0 objects, L1448/mm is driving a powerful molecular outflow.
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