IR Compendium: Background: Galactic Science

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Overview

Spitzer will be able to study star and planet formation.

Star formation is ongoing in dark interstellar clouds. Most if not all stars are believed to have a circumstellar disk at some point in the star formation process. Planet formation, if it occurs for any given star, is thought to occur in these circumstellar disks, which are best seen in infrared light. Spitzer can study the evolution of disks in the key phase of planet formation.

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.

• The structure of our Galaxy as mapped in gas and dust clouds.
• Supernova remnants interacting with the interstellar medium.
• Planetary nebulae.

Images and Plots

Note: Note: This Compendium is a work in progess. We have used the best information available, including data from other missions, and will update these pages as soon as possible with the new information.

Representations of star formation process:

• Cartoon of star formation process from Greene, American Scientist, Jul-Aug 2001.
• Definitions of different classifications from Andre, with spectral energy distributions indicated.

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Representation of spectral energy distribution of protostar+disk. From Mike Werner's speaker's bureau slides, August 2001.

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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.)

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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.)

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Approximate size of a Spitzer pixel at the distance of Vega. (Image from JPL image library.)

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Infrared can allow us to see through dark clouds. Image courtesy ESO.

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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.

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.

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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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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For more information

Starting at the beginning:

• Basic IR Astro Links
• More information on star formation in the infrared
• More information on circumstellar disks in the infrared
• More information on interstellar dust and gas in the infrared
• More information on interstellar molecular clouds in the infrared
• More information on star clusters in the infrared
• More information on our Galaxy in the infrared
• More information on brown dwarfs in the infrared

More advanced:

• Mannings, Boss, and Russell (eds), Protostars and Planets IV, University of Arizona Press, 2000.
• Davy Kirkpatrick's archive of M, L, and T dwarfs
• Legacy team web pages - especially the three galactic teams:
  • GLIMPSE team - Galactic Legacy Infrared Mid-Plane Survey Extraordinare (Churchwell et al.)
  • c2d team - From Molecular Cores to Planet-Forming Disks (Evans et al.)
  • FEPS team - The Formation and Evolution of Planetary Systems: Placing our Solar System in Context (Meyer et al.)