Joel D. Green

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

A Survey and Analysis of Spitzer Infrared Spectrograph Spectra of T Tauri Stars in Taurus

We present mid-infrared spectra of T Tauri stars in the Taurus star-forming region obtained with the Spitzer Infrared Spectrograph (IRS). For the first time, the 5–36 � m spectra of a large sample of T Tauri stars belonging to the same star-forming region is studied, revealing details of the midinfrared excess due to dust in circumstellar disks. We analyze common features and differences in the mid-IR spectra based on disk structure, dust grain properties, and the presence of companions. Our analysis encompasses spectral energy distributions from the optical to the far-infrared, a morphological sequence based on the IRS spectra, and spectral indices in IRS wave bands representative of continuum emission. By comparing the observed spectra to a grid of accretion disk models, we infer some basic disk properties for our sample of T Tauri stars, and find additional evidence for dust settling. Subject headings: circumstellar matter — planetary systems: protoplanetary disks — stars: pre-main sequence — infrared: stars

Disks in Transition in the Taurus Population: Spitzer IRS Spectra of GM Aurigae and DM Tauri

We present Spitzer Infrared Spectrograph (IRS) observations of two objects of the Taurus population that show unambiguous signs of clearing in their inner disks. In one of the objects, DM Tau, the outer disk is truncated at 3 AU; this object is akin to another recently reported in Taurus, CoKu Tau/4, in that the inner disk region is free of small dust. Unlike CoKu Tau/4, however, this star is still accreting, so optically thin gas should still remain in the inner disk region. The other object, GM Aur, also accreting, has ~0.02 lunar masses of small dust in the inner disk region within ~5 AU, consistent with previous reports. However, the IRS spectrum clearly shows that the optically thick outer disk has an inner truncation at a much larger radius than previously suggested, ~24 AU. These observations provide strong evidence for the presence of gaps in protoplanetary disks.

An Old Disk Still Capable of Forming a Planetary System

From the masses of the planets orbiting the Sun, and the abundance of elements relative to hydrogen, it is estimated that when the Solar System formed, the circumstellar disk must have had a minimum mass of around 0.01 solar masses within about 100 astronomical units of the star. (One astronomical unit is the Earth–Sun distance.) The main constituent of the disk, gaseous molecular hydrogen, does not efficiently emit radiation from the disk mass reservoir, and so the most common measure of the disk mass is dust thermal emission and lines of gaseous carbon monoxide. Carbon monoxide emission generally indicates properties of the disk surface, and the conversion from dust emission to gas mass requires knowledge of the grain properties and the gas-to-dust mass ratio, which probably differ from their interstellar values. As a result, mass estimates vary by orders of magnitude, as exemplified by the relatively old (3–10 million years) star TW Hydrae, for which the range is 0.0005–0.06 solar masses. Here we report the detection of the fundamental rotational transition of hydrogen deuteride from the direction of TW Hydrae. Hydrogen deuteride is a good tracer of disk gas because it follows the distribution of molecular hydrogen and its emission is sensitive to the total mass. The detection of hydrogen deuteride, combined with existing observations and detailed models, implies a disk mass of more than 0.05 solar masses, which is enough to form a planetary system like our own.

Spitzer Observations of HH 54 and HH 7-11: Mapping the H2 Ortho-to-Para Ratio in Shocked Molecular Gas

We report the results of spectroscopic mapping observations carried out toward the Herbig-Haro objects HH 7-11 and HH 54 over the 5.2-37 μm region using the Infrared Spectrograph on the Spitzer Space Telescope. These observations have led to the detection and mapping of the S(0)-S(7) pure rotational lines of molecular hydrogen, together with emissions in fine-structure transitions of Ne+, Si+, S, and Fe+. The H2 rotational emissions indicate the presence of warm gas with a mixture of temperatures in the range 400-1200 K—consistent with the expected temperature behind nondissociative shocks of velocity ~10-20 km s-1—while the fine-structure emissions originate in faster shocks of velocity ~35-90 km s-1 that are dissociative and ionizing. The H2 ortho-to-para ratio is quite variable, typically falling substantially below the equilibrium value of 3 attained at the measured gas temperatures. The nonequilibrium ortho-to-para ratios are characteristic of temperatures as low as ~50 K, and are a remnant of an earlier epoch, before the gas temperature was elevated by the passage of a shock. Correlations between the gas temperature and H2 ortho-to-para ratio show that ortho-to-para ratios <0.8 are attained only at gas temperatures below ~900 K; this behavior is consistent with theoretical models in which the conversion of para- to ortho-H2 behind the shock is driven by reactive collisions with atomic hydrogen, a process that possesses a substantial activation energy barrier (EA/k ~ 4000 K) and is therefore very inefficient at low temperature. The lowest observed ortho-to-para ratios of only ~0.25 suggest that the shocks in HH 54 and HH 7 are propagating into cold clouds of temperature 50 K in which the H2 ortho-to-para ratio is close to equilibrium.

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.

Mid-Infrared Spectra of Polycyclic Aromatic Hydrocarbon Emission in Herbig Ae/Be Stars

We present spectra of four Herbig Ae/Be stars obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. All four of the sources show strong emission from polycyclic aromatic hydrocarbons (PAHs), with the 6.2 μm emission feature shifted to 6.3 μm and the strongest C–C skeletal-mode feature occurring at 7.9 μm instead of at 7.7 μm, as is often seen. Remarkably, none of the four stars has silicate emission. The strength of the 7.9 μm feature varies with respect to the 11.3 μm feature among the sources, indicating that we have observed PAHs with a range of ionization fractions. The ionization fraction is higher for systems with hotter and brighter central stars. Two sources, HD 34282 and HD 169142, show emission features from aliphatic hydrocarbons at 6.85 and 7.25 μm. The spectrum of HD 141569 shows a previously undetected emission feature at 12.4 μm that may be related to the 12.7 μm PAH feature. The spectrum of HD 135344, the coolest star in our sample, shows an unusual profile in the 7-9 μm region, with the peak emission to the red of 8.0 μm and no 8.6 μm PAH feature.We present spectra of four Herbig AeBe stars obtained with the Infrared Spectrograph (IRS) 1 on the Spitzer Space Telescope. All four of the sources show strong emission from polycyclic aromatic hydrocarbons (PAHs), with the 6.2 µm emission feature shifted to 6.3 µm and the strongest C C skeletal-mode feature occuring at 7.9 µm instead of at 7.7 µm as is often seen. Remarkably, none of the four stars have silicate emission. The strength of the 7.9 µm feature varies with respect to the 11.3 µm feature among the sources, indicating that we have observed PAHs with a range of ionization fractions. The ionization fraction is higher for systems with hotter and brighter central stars. Two sources, HD 34282 and HD 169142, show emission features from aliphatic hydrocarbons at 6.85 and 7.25 µm. The spectrum of HD 141569 shows a previously undetected emission feature at 12.4 µm which may be related to the 12.7 µm PAH feature. The spectrum of HD 135344, the coolest star in our sample, shows an unusual profile in the 7–9 µm region, with the peak emission to the red of 8.0 µm and no 8.6 µm PAH feature. Subject headings: stars: chemically peculiar — infrared: stars

THE SPITZER INFRARED SPECTROGRAPH SURVEY OF T TAURI STARS IN TAURUS

We present 161 Spitzer Infrared Spectrograph (IRS) spectra of T Tauri stars and young brown dwarfs in the Taurus star-forming region. All of the targets were selected based on their infrared excess and are therefore surrounded by protoplanetary disks; they form the complete sample of all available IRS spectra of T Tauri stars with infrared excesses in Taurus. We also present the IRS spectra of seven Class 0/I objects in Taurus to complete the sample of available IRS spectra of protostars in Taurus. We use spectral indices that are not significantly affected by extinction to distinguish between envelope- and disk-dominated objects. Together with data from the literature, we construct spectral energy distributions for all objects in our sample. With spectral indices derived from the IRS spectra we infer disk properties such as dust settling and the presence of inner disk holes and gaps. We find a transitional disk frequency, which is based on objects with unusually large 13–31 μm spectral indices indicative of a wall surrounding an inner disk hole, of about 3%, and a frequency of about 20% for objects with unusually large 10 μm features, which could indicate disk gaps. The shape and strength of the 10 μm silicate emission feature suggests weaker 10 μm emission and more processed dust for very low mass objects and brown dwarfs (spectral types M6–M9). These objects also display weaker infrared excess emission from their disks, but do not appear to have more settled disks than their higher-mass counterparts. We find no difference for the spectral indices and properties of the dust between single and multiple systems.

SPITZER INFRARED SPECTROGRAPH OBSERVATIONS OF CLASS I/II OBJECTS IN TAURUS: COMPOSITION AND THERMAL HISTORY OF THE CIRCUMSTELLAR ICES

We present observations of Taurus-Auriga Class I/II protostars obtained with the Spitzer InfraRed Spectrograph. Detailed spectral fits to the 6 and 15.2 micron ice features are made, using publicly available laboratory data, to constrain the molecular composition, abundances, and levels of thermal processing along the lines of sight. We provide an inventory of the molecular environments observed, which have an average composition dominated by water-ice with ~12% CO2 (abundance relative to H2O), 2%-9% CH3OH, ~14% NH3, ~3% CH4, ~2% H2CO, ~0.6% HCOOH, and ~0.5% SO2. We find CO2/H2O ratios nearly equivalent to those observed in cold clouds and lines of sight toward the galactic center. The unidentified 6.8 micron profiles vary from source to source, and it is shown to be likely that even combinations of the most common candidates (NH+ 4 and CH3OH) are inadequate to explain the feature fully. We discuss correlations among SED spectral indices, abundance ratios, and thermally processed ice fractions and their implications for CO2 formation and evolution. Comparison of our spectral fits with cold molecular cloud sight lines indicates abundant prestellar ice environments made even richer by the radiative effects of protostars. Our results add additional constraints and a finer level of detail to current full-scale models of protostellar and protoplanetary systems.

Spitzer IRS Spectra and Envelope Models of Class I Protostars in Taurus

We present Spitzer Infrared Spectrograph (IRS) spectra of 28 Class I protostars in the Taurus star-forming region. The 5-36 μm spectra reveal excess emission from the inner regions of the envelope and accretion disk surrounding these predecessors of low-mass stars, as well as absorption features due to silicates and ices. Together with shorter and longer wavelength data from the literature, we construct spectral energy distributions and fit envelope models to 22 protostars of our sample, most of which are well constrained due to the availability of the IRS spectra. We infer that the envelopes of the Class I objects in our sample cover a wide range in parameter space, particularly in density and centrifugal radius, implying different initial conditions for the collapse of protostellar cores.