Infrared emissions from dust disks

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. 

The composition of dust constituents and ices in disks are studied by ground-based and space-borne infrared spectroscopy, and by observing various vibrational bands in absorption (when a disk is seen edge-on) or emission (when a disk is seen face-on). The results from the Infrared Space Observatory and Spitzer Space... 

The earliest detailed studies of silicate dust in protoplanetary disks targeted those brightest in the mid-infrared, where high quality spectra could be obtained even by severely flux-limited observations. Cohen Wittebom (1985) reported the earliest detection of crystalline silicate emission from the environment of young stars and interpreted it as evidence for dust having been transformed from its pristine state in the interstellar medium to the material known to be contained in the comets and perhaps primitive meteorites. Interestingly, this observation and explanation pre-dated the evidence that young stars are surrounded by disks and not by spherical envelopes. 

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 thermal emission from the protoplanetary disks can be observed, as well as the scattered light from the dust grains in the surfaces of these disks which is comparable to the light received from stars. This disk radiation appears in the spectral energy distribution of a young star as an excess of infrared radiation. 

Ground-based, airborne and space-borne observations have shown that the IR spectra of bright sources with associated dust and gas are dominated by relatively broad emission features at 3.3, 6.2, 7.7, 8.6 and 11.3 pm, which always appear together. Because the carriers of these bands remained unidentified for almost a decade, these bands have become collectively known as the unidentified infrared (UIR) bands. These bands are now unequivocally identified with an aromatic carrier and this name is somewhat of a misnomer. Nevertheless, the abbreviation has stuck and is still widely used. As an example. Figure 7 shows three mid-infrared spectra of carbon-rich outflows from stars in the latest stages of their evolution. It is now known that these emission features are not limited to stellar outflows but are also present in the spectra of disks of newly formed... 

---