G. C. Sloan

The SMART Data Analysis Package for the Infrared Spectrograph* on the Spitzer Space Telescope **

SMART is a software package written in IDL to reduce and analyze Spitzer data from all four modules of the Infrared Spectrograph, including the peak-up arrays. The software is designed to make full use of the ancillary files generated in the Spitzer Science Center pipeline so that it can either remove or flag artifacts and corrupted data and maximize the signal-to-noise in the extraction routines. It may be run in both interactive and batch mode. The software and Users Guide will be available for public release in December 2004. We briefly describe some of the main features of SMART including: visualization tools for assessing the data quality, basic arithmetic operations for either 2-d images or 1-d spectra, extraction of both point and extended sources and a suite of spectral analysis tools. Subject headings: methods: data analysis — techniques: spectroscopic — telescopes: Spitzer Space Telescope

Observations of Ultraluminous Infrared Galaxies with the Infrared Spectrograph on the Spitzer Space Telescope. II. The IRAS Bright Galaxy Sample

We present spectra taken with the Infrared Spectrograph on Spitzer covering the 5-38 μm region of the 10 ultraluminous infrared galaxies (ULIRGs) found in the IRAS Bright Galaxy Sample (BGS). There is a factor of 50 spread in the rest-frame 5.5-60 μm spectral slopes, and the 9.7 μm silicate optical depths range from at least τ_(9.7) ≤ 0.4 (A_V ~ 8) to τ_(9.7) ≥ 4.2 (A_V ≥ 78). There is evidence for water ice and hydrocarbon absorption and C_2H_2 and HCN absorption features in 4 and possibly 6 of the 10 BGS ULIRGs, indicating shielded molecular clouds and a warm, dense ISM. We have detected [Ne V] emission in 3 of the 10 BGS ULIRGs, at flux levels of 5-18 × 10^(-14) ergs cm^(-2) s^(-1) and [Ne V] 14.3/[Ne II] 12.8 line flux ratios of 0.12-0.85. The remaining BGS ULIRGs have limits on their [Ne V]/[Ne II]line flux ratios, which range from ≤0.15 to ≤0.01. Among the BGS ULIRGs, the AGN fractions implied by either the [Ne V]/[Ne II] or [O IV]/[Ne II] line flux ratios (or their upper limits) are significantly lower than implied by the MIR slope or strength of the 6.2 μm PAH EQW feature. There is evidence for hot (T > 300 K) dust in five of the BGS ULIRGs, with the fraction of hot dust to total dust luminosity ranging from ~1% to 23%, before correcting for extinction. When integrated over the IRAC-8, IRS blue peak-up, and MIPS-24 filter bandpasses, the IRS spectra imply very blue colors for some ULIRGs at z ~ 1.3. The large range in diagnostic parameters among the nearest ULIRGs suggests that matching survey results to a small number of templates may lead to biased results about the fraction of luminous dusty starbursts and AGNs at high z.

A Uniform Database of 2.4-45.4 Micron Spectra from the Infrared Space Observatory Short Wavelength Spectrometer*

We present a complete set of all valid SWS full-scan 2.4-45.4 μm spectra processed and renormalized in as uniform a manner as possible. The processing produces a single spectrum for each observation from the 288 individual spectral segments, which are the most processed form available from the ISO archive. The spectra, and the programs used to create them, are available to the community on-line.

CASSIS: THE CORNELL ATLAS OF SPITZER/INFRARED SPECTROGRAPH SOURCES

We present the spectral atlas of sources observed in low resolution with the Infrared Spectrograph on board the Spitzer Space Telescope. More than 11,000 distinct sources were extracted using a dedicated algorithm based on the SMART software with an optimal extraction (AdOpt package). These correspond to all 13,000 low-resolution observations of fixed objects (both single source and cluster observations). The pipeline includes image cleaning, individual exposure combination, and background subtraction. Particular attention is given to bad pixel and outlier rejection at the image and spectra levels. Most sources are spatially unresolved so that optimal extraction reaches the highest possible signal-to-noise ratio. For all sources, an alternative extraction is also provided that accounts for all of the source flux within the aperture. CASSIS provides publishable quality spectra through an online database together with several important diagnostics, such as the source spatial extent and a quantitative measure of detection level. Ancillary data such as available spectroscopic redshifts are also provided. The database interface will eventually provide various ways to interact with the spectra, such as on-the-fly measurements of spectral features or comparisons among spectra.

The global gas and dust budget of the Large Magellanic Cloud: AGB stars and supernovae, and the impact on the ISM evolution

We report on an analysis of the gas and dust budget in the 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 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 star formation rate 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 star formation rate 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 life time (400–800Myrs) is significant less than the dust mass in the ISM. Another dust source is required, possibly related to star-forming regions.

The Detection of Silicate Emission from Quasars at 10 and 18 Microns

We report the spectroscopic detection of silicate emission at 10 and 18 mm in five PG quasars, the first detection of these two features in galaxies outside the Local Group. This finding is consistent with the unification model for active galactic nuclei (AGNs), which predicts that an AGN torus seen pole-on should show a silicate emission feature in the mid-infrared. The strengths of the detected silicate emission features range from 0.12 to 1.25 times the continuum at 10 mu m and from 0.20 to 0.79 times the continuum at 18 mu m. The silicate grain temperatures inferred from the ratio of 18 mu m to 10 mm silicate features under the assumption of optically thin emission range from 140 to 220 K.

The Unusual Hydrocarbon Emission from the Early Carbon Star HD 100764: The Connection between Aromatics and Aliphatics

We have used the Infrared Spectrograph (IRS) on the Spitzer Space Telescope to obtain spectra of HD 100764, an apparently single carbon star with a circumstellar disk. The spectrum shows emission features from polycyclic aromatic hydrocarbons (PAHs) that are shifted to longer wavelengths than normally seen, a characteristic of ‘‘class C’’ systems in the classification scheme of Peeters et al. All seven of the known class C PAH sources are illuminated by radiation fields that are cooler than those which typically excite PAH emission features. The observed wavelength shifts are consistent with hydrocarbon mixtures containing both aromatic and aliphatic bonds. We proposethat the class C PAH spectra are distinctive because the carbonaceous material has not been subjected to a strong ultraviolet radiation field, allowing relatively fragile aliphatic materials to survive. Subject headingg circumstellar matter — stars: carbon Online material: color figures

Crystalline silicates and dust processing in the protoplanetary disks of the taurus young cluster

We characterize the crystalline-silicate content and spatial distribution of small dust grains in a large sample of protoplanetary disks in the Taurus-Auriga young cluster, using the Spitzer Space Telescope mid-IR spectra. In turn we use the results to analyze the evolution of structure and composition of these 1-2 Myr old disks around Solar- and later-type young stars, and test the standard models of dust processing which result in the conversion of originally amorphous dust into minerals. We find strong evidence of evolution of the dust-crystalline mass fraction in parallel with that of the structure of the disks, in the sense that increasing crystalline mass fraction is strongly linked to dust settling to the disk midplane. We also confirm that the crystalline silicates are confined to small radii, r 10 AU. However, we see no significant correlation of crystalline mass fraction with stellar mass or luminosity, stellar-accretion rate, disk mass, or disk/star mass ratio, as would be expected in the standard models of dust processing based upon photoevaporation and condensation close to the central star, accretion-heating-driven annealing at r 1 AU, or spiral-shock heating at r 10 AU, with or without effective large-scale radial mixing mechanisms. Either another grain-crystallizing mechanism dominates over these, or another process must be at work within the disks to erase the correlations they produce. We propose one of each sort that seems to be worth further investigation, namely X-ray heating and annealing of dust grains, and modulation of disk structure by giant-planetary formation and migration.

Advanced Optimal Extraction for the Spitzer/IRS

We present new advances in the spectral extraction of point-like sources adapted to the Infrared Spectrograph onboard the Spitzer Space Telescope. For the first time, we created a super-sampled point spread function of the low-resolution modules. We describe how to use the point spread function to perform optimal extraction of a single source and of multiple sources within the slit. We also examine the case of the optimal extraction of one or several sources with a complex background. The new algorithms are gathered in a plugin called Adopt which is part of the SMART data analysis software.

A Spitzer mid-infrared spectral survey of mass-losing carbon stars in the Large Magellanic Cloud

We present a Spitzer Space Telescopespectroscopic survey of mass-losing carbon stars (and one oxygen-rich star) in the Large Magellanic Cloud. The stars represent the superwind phase on the Asymptotic Giant Branch, which forms a major source of dust for the interstellar medium in galaxies. The spectra cover the wavelength range 5‐38� m. They show varying combinations of dust continuum, dust emission features (SiC, MgS) and molecular absorption bands (C2H2, HCN). A set of four narrow bands, dubbed the Manchester system, is used to define the infrared continuum for dusty carbon stars. The r elations between the continuum colours and the strength of the dust and molecular features are studied, and are compared to Galactic stars of similar colours. The circumstellar 7-� m C2H2 band is found to be stronger at lower metallicity, from a comparison of stars in the Galaxy, the LMC and the SMC. This is explained by dredge-up of carbon, causing higher C/O ratios at low metallicity (less O). A possible 10-� m absorption feature seen in our spectra may be due to C3. This band has also been identified with interstellar silicate or silicon-nitr ite dust. We investigate the strength and central wavelength of the SiC and MgS dust bands as function of colour and metallicity. The line-to-continuum ratio of these bands shows some indication of being lower at low metallicity. The MgS band is only seen at dust temperatures below 600 K. We discuss the selection of carbon versus oxygen-rich AGB stars using the J K vs. K A colours, and show that these colours are relatively insensitive to chemical type. Metal -poor carbon stars form amorphous carbon dust from self-produced carbon. This type of dust forms more readily in the presence of a higher C/O ratio. Low metallicity carbon dust may contain a smaller fraction of SiC and MgS constituents, which do depend on metallicity. The formation efficiency of oxygen-rich dust depends more strongly on metallicity. We suggest that in lower-metallicity environments, the dust input into the Interstellar Medium by AGB stars is efficient but may be strongly biassed towards carbonaceous dust, as compared to the Galaxy.