We are performing a uniform and unbiased imaging survey of the Large Magellanic Cloud (LMC; $7 ; 7) using the IRAC (3.6, 4.5, 5.8, and 8 m) and MIPS (24, 70, and 160 m) instruments on board the Spitzer Space Telescope in the Surveying the Agents of a Galaxy's Evolution (SAGE) survey, these agents being the interstellar medium (ISM) and stars in the LMC. This paper provides an overview of the SAGE Legacy project, including observing strategy, data processing, and initial results. Three key science goals determined the coverage and depth of the survey. The detection of diffuse ISM with column densities >1:2 ; 10 21 H cm À2 permits detailed studies of dust processes in the ISM. SAGE's point-source sensitivity enables a complete census of newly formed stars with masses >3 M that will determine the current star formation rate in the LMC. SAGE's detection of evolved stars with mass-loss rates >1 ; 10 À8 M yr À1 will quantify the rate at which evolved stars inject mass into the ISM of the LMC. The observing strategy includes two epochs in 2005, separated by 3 months, that both mitigate instrumental artifacts and constrain source variability. The SAGE data are nonproprietary. The data processing includes IRAC and MIPS pipelines and a database for mining the point-source catalogs, which will be released to the community in support of Spitzer proposal cycles 4 and 5. We present initial results on the epoch 1 data for a region near N79 and N83. The MIPS 70 and 160 m images of the diffuse dust emission of the N79/N83 region reveal a similar distribution to the gas emissions, especially the H i 21 cm emission. The measured point-source sensitivity for the epoch 1 data is consistent with expectations for the survey. The point-source counts are highest for the IRAC 3.6 m band and decrease dramatically toward longer wavelengths, A
We report on an analysis of the gas and dust budget in the interstellar medium (ISM) of the Large Magellanic Cloud (LMC). Recent observations from the Spitzer Space Telescope enable us to study the mid‐infrared dust excess of asymptotic giant branch (AGB) stars in the LMC. This is the first time we can quantitatively assess the gas and dust input from AGB stars over a complete galaxy, fully based on observations. The integrated mass‐loss rate over all intermediate and high mass‐loss rate carbon‐rich AGB candidates in the LMC is 8.5 × 10−3 M⊙ yr−1, up to 2.1 × 10−2 M⊙ yr−1. This number could be increased up to 2.7 × 10−2 M⊙ yr−1 if oxygen‐rich stars are included. This is overall consistent with theoretical expectations, considering the star formation rate (SFR) when these low‐ and intermediate‐mass stars where formed, and the initial mass functions. AGB stars are one of the most important gas sources in the LMC, with supernovae (SNe), which produces about 2–4 × 10−2 M⊙ yr−1. At the moment, the SFR exceeds the gas feedback from AGB stars and SNe in the LMC, and the current star formation depends on gas already present in the ISM. This suggests that as the gas in the ISM is exhausted, the SFR will eventually decline in the LMC, unless gas is supplied externally. Our estimates suggest ‘a missing dust‐mass problem’ in the LMC, which is similarly found in high‐z galaxies: the accumulated dust mass from AGB stars and possibly SNe over the dust lifetime (400–800 Myr) is significant less than the dust mass in the ISM. Another dust source is required, possibly related to star‐forming regions.
IRAS low-resolution spectra were extracted for 11,224 IRAS sources. These spectra were classiÐed into astrophysical classes, based on the presence of emission and absorption features and on the shape of the continuum. Counterparts of these IRAS sources in existing optical and infrared catalogs are identiÐed, and their optical spectral types are listed if they are known. The correlations between the photospheric/ optical and circumstellar/infrared classiÐcation are discussed.
Infrared Space Observatory (ISO) 2È45 km observations of seven protoÈplanetary nebulae (PPNs) and two other carbon-rich objects are presented. The unidentiÐed emission features at 21 and 30 km are detected in six sources, including four new detections of the 30 km feature. This previously unresolved 30 km feature is now resolved and found to consist of a broad feature peaking at 27.2 km (the "" 30 km ÏÏ feature) and a narrower feature at 25.5 km (the "" 26 km ÏÏ feature). This new 26 km feature is detected in eight sources and is particularly strong in IRAS Z02229]6208 and 16594[4656. The unidentiÐed infrared (UIR) emission features at 3.3, 6.2, 7.7, and 11.3 km, which are commonly observed in planetary nebulae and H II regions, are also seen in these PPNs. However, their strengths relative to the continuum plateaus at 8 and 12 km are weaker than in planetary nebulae. The 6.9 km feature, seen almost exclusively in PPNs, is strong. New millimeter CO and HCN observations were made ; they support the carbon-rich nature of the objects and yield the expansion velocities of the gaseous envelopes. The spectral energy distributions of these PPNs were Ðtted with a radiative-transfer model, taking into account the emission features at 21, 26, and 30 km. A signiÐcant fraction of the total energy output is emitted in these features : as high as 20% in the 30 km feature and 8% in the 21 km feature. The fact that so much energy is carried in these features suggests that the material responsible for these features must be made of abundant elements and most likely involves carbon. appears to be ruled out as the emitter of the SiS 2 21 km feature due to the absence of a predicted companion feature.
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