Transition disks or transitional disks are defined as objects whose inner disk regions have undergone substantial clearing.1 This inference is from the observations of their SEDs, which do not exhibit the so-called infrared excess. The excess is defined with respect to or is in with comparison with the infrared flux from the stellar photosphere. Because some dust that would have otherwise thermally emitted in the infrared or mm-wavelengths2 has now been cleared (in other words, missing), the SEDs exhibit a deficit in the infrared excess. The name comes from the fact that these disks are transitioning from optically thick objects to optically thin.
The first works which identified these disks like Strom et al. (1989)3 and Skrutskie et al. (1990)4 observed small NIR and/or MIR excesses but significant MIR and FIR excesses. The fact that the emission was less towards shorter wavelengths but larger wavelengths implies dust is missing from warmer regions. This is in accordance with Wein’s displacement law, where the peak shifts towards lower wavelengths as the temperature is increased.
See also
References
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An Observational Perspective of Transitional Disks: https://arxiv.org/pdf/1402.7103 ↩
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Transition Disks: https://pweb.cfa.harvard.edu/news/transition-disks ↩
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Strom K. M. et al. (1989) Astron. J., 97, 1451. ↩
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Skrutskie M. F. et al. (1990) Astron. J., 99, 1187. ↩