Infrared excess emission

Observations of near-infrared excess emission from hundreds of disks with ages covering the first 10 Myr demonstrate fundamental structural evolution and the eventual loss of the fine dust from the inner disk (< 1AU). The declining fraction of stars with dust disks suggests a disk half-life of 3 to 5 Myr (see Chapter 9, e.g. Hernandez et al. 2007). Longer-wavelength infrared observations, primarily from the Spitzer Space Telescope, show a similar picture for the intermediate disk radii (1-5 AU). The combination of these lines of evidence is interpreted as a rapid (< 1-3 Myr) dispersal of the fine dust in most systems, probably progressing inside-out. 

Figure 9.1 Examples of spectral energy distributions from young Sun-like stars with circumstellar dust disks. Optically thick dust disks (solid line) have excess emission relative to the stellar photosphere over a broad wavelength range, from near-infrared to millimeter wavelengths. Transition disks (dashed line) lack near-infrared excess emission, but have large mid- and far-infrared emission. Debris disks (dotted line) have small excess emission starting at wavelengths typically longer than 10 pm. Primordial and transition disks often show a prominent 10 pm silicate emission feature from warm dust grains in the disk atmosphere.

The above discussion provides the basis for using the infrared excess relative to the photospheric flux as a tool to detect primordial dust disks and determine the timescale over which they disperse. We should note that emission at infrared... 

In all four clusters, the very young stars and protostars were identified by their excess emission in the mid-infrared and their distribution over multi-... 

Over the course of fluorescence, which accompanies energy relaxation, the molecule can keep part of the energy it received in the form of vibrational energy of the ground state. This excess vibrational energy is dissipated by collisions or other non-radiative processes called vibrational relaxation. The emission of lower energy photons is also possible and gives rise to fluorescence in the mid infrared. 

It is also possible that the molecule will dispose of excess vibrational energy radiatively, that is, by infrared emission, however this is not very likely in condensed phases because relaxation to solvent degrees of freedom is usually much faster. Even in low-pressure samples the relaxation due to collisions with the walls is usually more efficient than the infrared emission route. 

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