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IR Compendium: Background: Galactic Science |
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Even when a planet itself is too faint to see directly, its gravitational influence on its star's dust disk can still be visible, either directly, just as small moons sculpt Saturn's rings, or indirectly, via structure in the observed spectral energy distribution. Spitzer will provide the first images of many nearby circumstellar disks, and spectral energy distributions for many more. Holes, clumps, or sharp edges in these disks may betray the presence of planets.
Representations of star formation process:
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Representation of spectral energy distribution of protostar+disk. From Mike Werner's speaker's bureau slides, August 2001. |
 Click for larger image | Mid-IR spectra of several stages of star formation, with major molecular species marked; PAH=Polycyclic Aromatic Hydrocarbons. Plot courtesy the c2d Legacy team. |
| Orginal plots of IRAS data indicating standard stars (left column) and stars with (then) newly-discovered far-IR excesses (right column). (This copy of these plots from Backman and Paresce, 1993, "Protostars and Planets III"; originally from a paper by Fred Gillett, 1986, in "Light on Dark Matter" volume.) |
 Click for full-size complete image
| Representation of model spectral energy distributions of stars with debris disks as a function of spectral type. The model has 0.1 Mmoon of 30-micron size dust grains in a disk from 30-60 AU. The bars are 3 sigma limits. (The model is based on disks around A stars.) Plot courtesy D. Koerner, NAU. |
| Sensitivity to Evolution of Circumstellar Dust at 30 pc for IRAS, ISO, and Spitzer (approximate levels). Plot courtesy D. Backman, M. Meyer, and the FEPS Legacy team. |
| Spectral Energy Distributions (SED) for star with disk as a function of time, with IRAS, ISO, and Spitzer sensitivity levels indicated. Plot courtesy D. Backman, M. Meyer, and the FEPS Legacy team. |
 | Simulated MIPS 70 micron observations of (left to right) Vega (which is known to have a remnant dust disk), 1/10th as much dust as Vega, and a point source. All are on a logarithmic brightness scale. Simulations courtesy N. Wright, UCLA. |
 | Simulated MIPS 24 micron observations of (left to right) Vega (which is known to have a remnant dust disk), and the difference of 1/100th as much dust as Vega and a point source. Both are on a logarithmic brightness scale. Simulations courtesy N. Wright, UCLA. |
| Approximate size of a Spitzer pixel at the distance of alpha Cen. (Image from JPL image library.) |
 Click for larger image | Approximate size of a Spitzer pixel at the distance of Vega. (Image from JPL image library.) |
 Click for larger image | Infrared can allow us to see through dark clouds. Image courtesy ESO. |
 Click for larger version of plot  Click for larger version of plot |
As is the case in the far-red and near-IR, the brightness temperature,
Tbr, of brown dwarfs varies strongly with wavelength due to the high
contrast between opacity windows and molecular absorption bands. This is
still true in the mid-IR up to about 11 microns. Thereafter, Tbr gradually
decreases to stabilize at ~75% of the effective temperature, Teff, beyond
18 microns. Only near 6 and near 10 microns does Tbr become as large as
the effective temperature. Exposure times should be calculated
appropriately. The attached figures present a model mid-infrared spectral sequence of brown dwarfs with Teff ranging from 600 to 2400 K (from bottom to top) in steps of 200 K. All models have solar metallicity, log g = 5, include the effect of iron and silicate clouds, and are shown at a spectral resolution of 200. The bandpasses of the four IRAC filters are shown by thick solid lines at the top of the first figure. Additional details can be found in Saumon, Marley, and Lodders (2003). Figures courtesy Didier Saumon and Mark Marley.
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| As brown dwarfs age and cool, their molecular bands become very prominent. The IRAC 3.6 and 4.5 micron filters are tuned to methane-dominated brown dwarf spectra. Here are predicted spectral energy distribution of two brown dwarfs and Jupiter at different distances; red dots are IRAC sensitivities (5 sigma in 12 sec exposure). Plot courtesy N. Wright, UCLA. |
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Left: Image of Gl229B from Hubble. Right: Image of Jupiter at 5 microns (from J. L. Ortiz, et al. 1998. Evolution and persistence of 5-um hot spots at the Galileo Probe entry latitude, J. Geophys. Res. 103, 23051 - 23069). |
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full-size image. | Comparison of the ISO spectrum of the young star HD 100546 and that of comet Hale-Bopp in the mid-IR. The similarity in features indicates a possible bombardment of comets in the early stages of the disk forming around HD 100546. From Malfait et al. 1998, AandA, 332, 25 |
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help@spitzer.caltech.edu http://ssc.spitzer.caltech.edu/documents/compendium/galsci/ This file was last modified on Tue Oct 3 15:55:56 2006.
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