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 the results of a program to acquire photometry for 86 late M, L, and T dwarfs using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We examine the behavior of these cool dwarfs in various color-color and color-magnitude diagrams composed of near-IR and IRAC data. The T dwarfs exhibit the most distinctive positions in these diagrams. In M 5.8 versus [5.8]À[8.0], the IRAC data for T dwarfs are not monotonic in either magnitude or color, giving the clearest indication yet that the T dwarfs are not a one-parameter family in T eff. Because metallicity does not vary enough in the solar neighborhood to act as the second parameter, the most likely candidate then is gravity, which in turn translates to mass. Among objects with similar spectral type, the range of mass suggested by our sample is about a factor of 5 ($70M J to $15M J), with the less massive objects making up the younger members of the sample. We also find the IRAC 4.5 m fluxes to be lower than expected, from which we infer a stronger CO fundamental band at $4.67 m. This suggests that equilibrium CH 4 /CO chemistry underestimates the abundance of CO in T dwarf atmospheres, confirming earlier results based on M-band observations from the ground. In combining IRAC photometry with near-IR JHK photometry and parallax data, we find the combination of K s , IRAC 3.6 m, and 4.5 m bands to provide the best color-color discrimination for a wide range of M, L, and T dwarfs. Also noteworthy is the M K s versus K s À[4.5] relation, which shows a smooth progression over spectral type, and splits the M, L, and T types cleanly.

No abstract

We present an optical and near-infrared study of a 45 0 Â 45 0 field in NGC 2264, which includes both S Mon and the Cone Nebula. We report photometry at optical (UBVR C I C ) and near-infrared (JHK) wavelengths for $5600 stars and spectroscopic classifications for $400 of these stars. We identify circumstellar disk candidates using three techniques: excess ultraviolet (UÀV ) emission, excess near-IR (IÀK and HÀK) emission, and H emission-line equivalent widths for those stars with spectra. We find generally good correlation between disk indicators thought to originate from different physical processes. We find little if any evolution of disk fraction with stellar age or mass. However, when we derive mass accretion rates ( _ M M) from the excess emission at U, we find that _ M M decreases with age over the age range spanned by our data, $0.1-5 Myr, and increases with mass over the range $0.25-1 M . These findings are comparable to results found previously by us in the Orion Nebula cluster flanking fields.

We report the discovery of T dwarf companions to the nearby stars HN Peg (G0V, 18.4 pc, τ ∼ 0.3 Gyr) and HD 3651 (K0V, 11.1 pc, τ ∼ 7 Gyr). During an ongoing survey of 5 ′ ×5 ′ fields surrounding stars in the solar neighborhood with the Infrared Array Camera aboard the Spitzer Space Telescope, we identified these companions as candidate T dwarfs based on their mid-infrared colors. Using near-infrared spectra obtained with SpeX at the NASA Infrared Telescope Facility, we confirm the presence of methane absorption that characterizes T dwarfs and measure spectral types of T2.5±0.5 and T7.5±0.5 for HN Peg B and HD 3651 B, respectively. By comparing our Spitzer data to images from the Two-Micron All-Sky Survey obtained several years earlier, we find that the proper motions of HN Peg B and HD 3651 B are consistent with those of the primaries, confirming their companionship. HN Peg B and HD 3651 B have angular separations of 43. ′′ 2 and 42. ′′ 9 from their primaries, which correspond to projected physical separations of 795 and 476 AU, respectively. A comparison of their luminosities to the values predicted by theoretical evolutionary models implies masses of 0.021 ± 0.009 and 0.051 ± 0.014 M ⊙ for HN Peg B and HD 3651 B. In addition, the models imply an effective temperature for HN Peg B that is significantly lower than the values derived for other T dwarfs at similar spectral types, which is the same behavior reported by Metchev & Hillenbrand for the young late-L dwarf HD 203030 B. Thus, the temperature of the L/T transition appears to depend on surface gravity. Meanwhile, HD 3651 B is the first substellar companion directly imaged around a star that is known to harbor a close-in planet from radial velocity surveys. The discovery of this companion supports the notion that the high eccentricities of close-in planets like the one near HD 3651 may be the result of perturbations by low-mass companions at wide separations.