Infrared excess

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 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... 

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 sensitivity of the Infrared Array Camera (IRAC) camera on board the Spitzer Space Telescope (Fazio et al. 2004) recently allowed to characterize in detail the decrease in disk frequency with stellar age and trace dust slightly cooler than that observed in the L-band, out to about 1AU from T Tauri stars. Figure 9.2 shows the fraction of T Tauri stars (mostly K and M stars) with infrared excess at IRAC wavelengths (3.6, 4.5, 5.8, and 8 pm, full circles). In addition to the data (and references) presented in Hernandez et al. (2008) we have included the disk statistics... 

Figure 9.5 Summary of the timescales for the formation of chondrules, asteroids, and planets in the Solar System compared to the lifetime of disks around young stars. The Solar System chronology is based on the dating of the CAIs, which, we assume, formed within the first Myr of disk evolution. The inner-disk frequency is from infrared excess measurements of stars in different stellar groups (see Section 9.1.1). The timescale for the outer-disk dispersal is discussed in Sections 9.1.1 and 9.1.2. The Solar System chronology is summarized in Section 9.3. For the formation timescales of giant planets, we used those in Desch (2007) with the assumption that outer-disk planetesimals formed 2 Myr after CAIs.

Figure 1 The ratio of infrared excess/stellar luminosity is a measure of the fraction of starlight absorbed by circumstellar dust and re-radiated in the infrared. The plot from Spangler et al. (2001) shows the temporal decline of dust around Vega-like stars (points) and stars in clusters with measured ages (circles). At least for the longer ages, the dust is most probably generated by comets.

In addition to their irregular fadings thought to be due to directed mass-ejections, R CrB stars show low-amplitude variability with periods of 30 - 60 d and associated with pulsations. All show a substantial infrared excess attributed to a warm dust shell, while a few show evidence of a more extended nebula. It will be seen that these stars are extraordinarily rich in exotic behaviour [132]. 

Another interesting correlation is that noticed by Churchwell et al. (1974) between the ratio of the abundances He /H (derived from radio recombination line observations) and the infrared excess. They find that the ratio He /H decreases as the IRE increases, and... 

AN OPTICAL-INFRARED COLOR-COLOR DIAGRAM FOR FINDING YOUNG STARS WITH INFRARED EXCESS... 

FINDING YOUNG STARS WITH INFRARED EXCESS... 

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