The Infrared Array Camera (IRAC) is one of three focal plane instruments in the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 µm. Two nearly adjacent 5.2×5.2 arcmin fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 µm; 4.5 and 8 µm). All four detector arrays in the camera are 256×256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.

We present mid-infrared spectra of T Tauri stars in the Taurus star-forming region obtained with the Spitzer Infrared Spectrograph ( IRS). For the first time, the 5-36 m spectra of a large sample of T Tauri stars belonging to the same star-forming region is studied, revealing details of the mid-infrared excess due to dust in circumstellar disks. We analyze common features and differences in the mid-IR spectra based on disk structure, dust grain properties, and the presence of companions. Our analysis encompasses spectral energy distributions from the optical to the far-infrared, a morphological sequence based on the IRS spectra, and spectral indices in IRS wave bands representative of continuum emission. By comparing the observed spectra to a grid of accretion disk models, we infer some basic disk properties for our sample of T Tauri stars and find additional evidence for dust settling.

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The Infrared Spectrograph (IRS) is one of three science instruments on the Spitzer Space Telescope. The IRS comprises four separate spectrograph modules covering the wavelength range from 5.3 to 38 m with spectral resolutions, R ¼ k=Ák % 90 and 600, and it was optimized to take full advantage of the very low background in the space environment. The IRS is performing at or better than the prelaunch predictions. An autonomous target acquisition capability enables the IRS to locate the mid-infrared centroid of a source, providing the information so that the spacecraft can accurately offset that centroid to a selected slit. This feature is particularly useful when taking spectra of sources with poorly known coordinates. An automated data-reduction pipeline has been developed at the Spitzer Science Center.

We also mis-reported the temperature of the silicate and carbon grains in our fit to the HR 7012 IRS spectrum; the grains have a temperature 550 K, not 520 K as reported previously. Lastly, the composition of the enstatite used to fit the HR 7012 spectrum is Mg 0.7 Fe 0.3 SiO 3 , not Mg 0.7 Fe 0.3 SiO 4 . In addition, we noticed an error in the minimum blow-out size for silicate, carbon, and silica grains around HD 113766 and HR 7012; the blow-out sizes are smaller than previously estimated. For HD 113766, we originally estimated minimum silicate and carbon sizes of 1.4 and 1.9 m, respectively; we now estimate 0.35 and 0.46 m, respectively. With the exception of forsterite, all of the grains used to model the HD 113766 spectrum are larger than the minimum grain sizes. The forsterite grains (submicron) possess radii that are similar to the minimum silicate grain size. For HR 7012, we originally estimated minimum silicate, carbon, and silica sizes of 1.1, 1.4, and 1.6 m, respectively; we now estimate 0.9, 1.2, and 1.3 m, respectively. The enstatite and cristobalite grains used to model the infrared HR 7012 spectrum are still smaller than the minimum grain size. We had previously concluded that the minimum grain sizes (>1 m) were inconsistent with presence of submicron-sized grains inferred from the structure of the silicate emission features, suggesting that a recent massive collision must have occurred around HD 113766 and HR 7012. Our new minimum grain size estimates are more consistent with our models for the infrared spectra and do not require a recent massive collision around HD 113766. However, our models do indicate the presence of submicron-sized particles significantly smaller than the blow-out size around HR 7012, suggesting that a recent massive collision may have occurred in this system.