The c2d Spitzer Legacy project obtained images and photometry with both IRAC and MIPS instruments for five large, nearby molecular clouds. Three of the clouds were also mapped in dust continuum emission at 1.1 mm, and optical spectroscopy has been obtained for some clouds. This paper combines information drawn from studies of individual clouds into a combined and updated statistical analysis of star formation rates and efficiencies, numbers and lifetimes for SED classes, and clustering properties. Current star formation efficiencies range from 3% to 6%; if star formation continues at current rates for 10 Myr, efficiencies could reach 15% to 30%. Star formation rates and rates per unit area vary from cloud to cloud; taken together, the five clouds are producing about 260 M ⊙ of stars per Myr. The star formation surface density is more than an order of magnitude larger than would be predicted from the Kennicutt relation used in extragalactic studies, reflecting the fact that those relations apply to larger scales, where more diffuse matter is included in the gas surface density. Measured against the dense gas probed by the maps of dust continuum emission, the efficiencies are much higher, with stellar masses similar to masses of dense gas, and the current stock of dense cores would be exhausted in 1.8 Myr on average. Nonetheless, star formation is still slow compared to that expected in a free fall time, even in the dense cores. The derived lifetime for the Class I phase is 0.54 Myr, considerably longer than some estimates. Similarly, the lifetime for the Class 0 SED class, 0.16 Myr, with the notable exception of the Ophiuchus cloud, is longer than early estimates. If photometry is corrected for estimated extinction before calculating class indicators, the lifetimes drop to 0.44 Myr for Class I and to 0.10 for Class 0. These lifetimes assume a continuous flow through the Class II phase and should be considered median lifetimes or half-lives. Star formation is highly concentrated to regions of high extinction, and the youngest objects are very strongly associated with dense cores. The great majority (90%) of young stars lie within loose clusters with at least 35 members and a stellar density of 1 M ⊙ pc −3 . Accretion at the sound speed from an isothermal sphere over the lifetime derived for the Class I phase could build a star of about 0.25 M ⊙ , given an efficiency of 0.3. Building larger mass stars by using higher mass accretion rates could be problematic, as our data confirm and aggravate the "luminosity problem" for protostars. At a given T bol , the values for L bol are mostly less than predicted by standard infall models and scatter over several orders of magnitude. These results strongly suggest that accretion is time variable, with prolonged periods of very low accretion. Based on a very simple model and this sample of sources, half the mass of a star would be accreted during only 7% of the Class I lifetime, as represented by the eight most luminous objects.

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"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 investigate the relation between star formation rate (SFR) and gas surface densities in Galactic star forming regions using a sample of young stellar objects (YSOs) and massive dense clumps. Our YSO sample consists of objects located in 20 large molecular clouds from the Spitzer cores to disks (c2d) and Gould's Belt (GB) surveys. These data allow us to probe the regime of low-mass star formation essentially invisible to tracers of high-mass star formation used to establish extragalactic SFR-gas relations. We estimate the gas surface density (Σ gas ) from extinction (A V ) maps and YSO SFR surface densities (Σ SFR ) from the number of YSOs, assuming a mean mass and lifetime. We also divide the clouds into evenly spaced contour levels of A V , counting only Class I and Flat SED YSOs, which have not yet migrated from their birthplace. For a sample of massive star forming clumps, we derive SFRs from the total infrared luminosity and use HCN gas maps to estimate gas surface densities. We find that c2d and GB clouds lie above the extragalactic SFR-gas relations (e.g., Kennicutt-Schmidt Law) by factors up to 17. Cloud regions with high Σ gas lie above extragalactic relations up to a factor of 54 and overlap with high-mass star forming regions. We use 12 CO and 13 CO gas maps of the Perseus and Ophiuchus clouds from the COMPLETE survey to estimate gas surface densities and compare to measurements from A V maps. We find that 13 CO, with the standard conversions to total gas, underestimates the A Vbased mass by factors of ∼4-5. 12 CO may underestimate the total gas mass at Σ gas 200 M ⊙ pc −2 by 30%; however, this small difference in mass estimates does not explain the large discrepancy between Galactic and extragalactic relations. We find evidence for a threshold of star formation (Σ th ) at 129±14 M ⊙ pc −2 . At Σ gas > Σ th , the Galactic SFR-gas relation is linear. A possible reason for the difference between Galactic and extragalactic relations is that much of Σ gas is below Σ th in extragalactic studies, which detect all the CO-emitting gas. If the Kennicutt-Schmidt relation (Σ SFR ∝ Σ 1.4 gas ) and a linear relation between dense gas and star formation is assumed, the fraction of dense star forming gas (f dense ) increases as ∼ Σ 0.4 gas . When Σ gas reaches ∼300Σ th , the fraction of dense gas is ∼1, creating a maximal starburst.