"Hot Jupiter" extrasolar planets are expected to be tidally locked because they are close (<0.05 astronomical units, where 1 AU is the average Sun-Earth distance) to their parent stars, resulting in permanent daysides and nightsides. By observing systems where the planet and star periodically eclipse each other, several groups have been able to estimate the temperatures of the daysides of these planets 1-3 . A key question is whether the atmosphere is able to transport the energy incident upon the dayside to the nightside, which will determine the temperature at different points on the planet's surface. Here we report observations of HD 189733, the closest of these eclipsing planetary systems 4-6 , over half an orbital period, from which we can construct a 'map' of the distribution of temperatures. We detected the increase in brightness as the dayside of the planet rotated into view. We estimate a minimum brightness temperature of 973±33 K and a maximum brightness temperature of 1212±11 K at a wavelength of 8 µm, indicating that energy from the irradiated dayside is efficiently redistributed throughout the atmosphere, in contrast to a recent claim for another hot Jupiter 7 . Our data indicate that the peak hemisphere-integrated brightness occurs 16±6 degrees before opposition, corresponding to a hot spot shifted east of the substellar point. The secondary eclipse (when the planet moves behind the star) occurs 120±24 s later than predicted, which may indicate a slightly eccentric orbit.We monitored HD 189733 continuously over a 33.1 hour period using the 8 µm channel of the InfraRed Array Camera (IRAC) 8 on the Spitzer Space Telescope 9 , observing in subarray mode 1 with a cadence of 0.4 s. Our observations spanned slightly more than half of the planet's orbit, beginning 2.6 hours before the start of the transit (when the planet moves in front of the star) and ending 1.9 hours after the end of the secondary eclipse. This gave us a total of 278,528 32 × 32 pixel images. We found that there was a gradual detector-induced rise of up to 10% in the signal measured in individual pixels over time. This rise is illumination-dependent; pixels with high levels of illumination (greater than 250 MJy sr −1 ) converge to a constant value within the first two hours of observations and lower-flux pixels increase linearly over time. We characterize this effect by producing a time series of the signal in a series of annuli of increasing radius centered on the star (masking out a 5-pixel-wide box centered on HD 189733's smaller, fainter M dwarf companion 10 ). This set of curves describes the behavior of the ramp for different illumination levels.To correct our images, we estimate the median illumination for each pixel in the array, and interpolate over our base set of curves (scaling as the natural log of the illumination) to calculate a curve describing the behavior of that pixel. We correct for this instrumental effect by dividing the flux in each pixel in a given image by the value of the interpolated curve. Pixels with illumi...

We present near-and mid-infrared photometry obtained with the Spitzer Space Telescope of $300 known members of the IC 348 cluster. We merge this photometry with existing ground-based optical and near-infrared photometry in order to construct optical-infrared spectral energy distributions (SEDs) for all the cluster members and present a complete atlas of these SEDs. We employ these observations to investigate both the frequency and nature of the circumstellar disk population in the cluster. The Spitzer observations span a wavelength range between 3.6 and 24 m, corresponding to disk radii of $0.1-5 AU from the central star. The observations are sufficiently sensitive to enable the first detailed measurement of the disk frequency for very low mass stars at the peak of the stellar initial mass function. Using measurements of infrared excess between 3.6 and 8.0 m, we find the total frequency of diskbearing stars in the cluster to be 50% AE 6%. However, only 30% AE 4% of the member stars are surrounded by optically thick, primordial disks, while the remaining disk-bearing stars are surrounded by what appear to be optically thin, anemic disks. Both these values are below previous estimates for this cluster. The disk fraction appears to be a function of spectral type and stellar mass. The fraction of stars with optically thick disks ranges from 11% AE 8% for stars earlier than K6 to 47% AE 12% for K6-M2 stars to 28% AE 5% for M2-M6 stars. The disk longevity and thus conditions for planet formation appear to be most favorable for the K6-M2 stars, which are objects of comparable mass to the Sun for the age of this cluster. The optically thick disks around later type (>M4) stars appear to be less flared than the disks around earlier type stars. This may indicate a greater degree of dust settling and a more advanced evolutionary state for the late M disk population. Finally, we find that the presence of an optically thick dust disk is correlated with gaseous accretion, as measured by the strength of H emission. A large fraction of stars classified as classical T Tauri stars possess robust, optically thick disks, and very few such stars are found to be diskless. The majority (64%) of stars classified as weak-lined T Tauri stars are found to be diskless. However, a significant fraction (12%) of these stars are found to be surrounded by thick, primordial disks. These results suggest that it is more likely for dust disks to persist in the absence of active gaseous accretion than for active accretion to persist in the absence of dusty disks.

We present Spitzer Space Telescope infrared photometric time series of the transiting extrasolar planet system TrES-1. The data span a predicted time of secondary eclipse, corresponding to the passage of the planet behind the star. In both bands of our observations, we detect a flux decrement with a timing, amplitude, and duration as predicted by published parameters of the system. This signal represents the first direct detection of (i.e. the observation of photons emitted by) a planet orbiting another star. The observed eclipse depths (in units of relative flux) are 0.00066 +/- 0.00013 at 4.5um and 0.00225 +/- 0.00036 at 8.0um. These estimates provide the first observational constraints on models of the thermal emission of hot Jupiters. Assuming that the planet emits as a blackbody, we estimate an effective temperature of T_p=1060 +/- 50 K. Under the additional assumptions that the planet is in thermal equilibrium with the radiation from the star and emits isotropically, we find a Bond albedo of A = 0.31 +/- 0.14. This would imply that the planet absorbs the majority of stellar radiation incident upon it, a conclusion of significant impact to atmospheric models of these objects. We compare our data to a previously-published model of the planetary thermal emission, which predicts prominent spectral features in our observational bands due to water and carbon monoxide. Based on the time of secondary eclipse, we present an upper limit on the orbital eccentricity that is sufficiently small that we conclude that tidal dissipation is unlikely to provide a significant source of energy interior to the planet.(abridged)Comment: 20 pages, 4 figures, to appear in the Astrophysical Journal, 20 June 200

We estimate the strength of the bandpass-integrated thermal emission from the extrasolar planet HD 209458b at 3.6, 4.5, 5.8, and 8.0 µm using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We observe a single secondary eclipse simultaneously in all four bandpasses and find relative eclipse depths of 0.00094 ± 0.00009, 0.00213 ± 0.00015, 0.00301 ± 0.00043, and 0.00240 ± 0.00026, respectively. These eclipse depths reveal that the shape of the inferred emission spectrum for the planet differs significantly from the predictions of standard atmosphere models; instead the most plausible explanation would require the presence of an inversion layer high in the atmosphere leading to significant water emission in the 4.5 and 5.8 µm bandpasses. This is the first clear indication of such a temperature inversion in the atmosphere of a hot Jupiter, as previous observations of other planets appeared to be in reasonably good agreement with the predictions of models without such an inversion layer.

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.

Element-by-element we have combined the optical components in the three 2MASS cameras, and incorporated detector quantum efficiency curves and site-specific atmospheric transmissions, to create three relative spectral response curves (RSRs). We provide the absolute 2MASS attributes associated with "zero magnitude" in the JHK s bands so that these RSRs may be used for synthetic photometry. The RSRs tie 2MASS to the "Cohen-Walker-Witteborn" framework of absolute photometry and stellar spectra for the purpose of using 2MASS data to support the development of absolute calibrators for IRAC and pairwise cross-calibrators between all three SIRTF instruments. We examine the robustness of these RSRs to changes in water vapor within a night. We compare the observed 2MASS magnitudes of thirty three stars (converted from the precision optical calibrators of Landolt and Carter-Meadows into absolute infrared (IR) calibrators from 1.2-35 µm) with our predictions, thereby deriving 2MASS "zero point offsets" from the ensemble. These offsets are the final ingredients essential to merge 2MASS JHK s data with our other absolutely calibrated bands and stellar spectra, and to support the creation of faint calibration stars for SIRTF.

We present a comprehensive analysis of structure in the young, embedded cluster, NGC 1333 using members identified with Spitzer and 2MASS photometry based on their IR-excess emission. A total of 137 members are identified in this way, composed of 39 protostars and 98 more evolved pre-main-sequence stars with disks. Of the latter class, four are transition /debris disk candidates. The fraction of exposed pre-main-sequence stars with disks is 83% AE 11%, showing that there is a measurable diskless pre-main-sequence population. The sources in each of the Class I and II evolutionary states are shown to have very different spatial distributions relative to the distribution of the dense gas in their natal cloud. However, the distribution of nearest neighbor spacings among these two groups of sources are found to be quite similar, with a strong peak at spacings of 0.045 pc. Radial and azimuthal density profiles and surface density maps computed from the identified YSOs show that NGC 1333 is elongated and not strongly centrally concentrated, confirming previous claims in the literature. We interpret these new results as signs of a low velocity dispersion, extremely young cluster that is not in virial equilibrium.

We report new Spitzer Space Telescope observations from the IRAC and MIPS instruments of the young (∼ 3 Myr) σ Orionis cluster. The populous nature of this cluster makes it a good target for statistically-significant studies of disk emission as a function of mass. We identify 336 stars as members of the cluster using optical and near-infrared color magnitude diagrams. Using the spectral energy distribution (SED) slopes in the IRAC spectral range, we place objects in several classes: non-excess stars, stars with optically thick disks (like classical T Tauri stars), class I (protostellar) candidates, and stars with "evolved disks"; the last exhibit smaller IRAC excesses than optically thick disk systems. In general, this classification agrees with the location expected in IRAC-MIPS color-color diagrams for these objects. We find that the evolved disk systems are mostly a combination of objects with optically thick but non-flared disks, suggesting grain growth and/or settling, and transition disks, systems in which the inner disk is partially or fully cleared of small dust. In all, we identify 7 transition disk candidates and 3 possible debris disk systems. There appears to be a spatial extension of infrared excess sources to the north-east, which may be associated with the young (< 1 Myr) embedded cluster NGC 2024. As in other young stellar populations, the fraction of disks depends on the stellar mass, ranging from ∼10%