With the goal to study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low luminosity YSOs (L ∼ 0.1− 10 L⊙) using 5–38 μm Spitzer Space Telescope and 3–4 μm ground-based spectra. The sample is complemented with previously published Spitzer spectra of background stars and with ISO spectra of well studied massive YSOs (L ∼ 10 L⊙). This paper focuses on the origin of the prominent absorption features in the 5-8 μm spectral region. The long-known 6.0 and 6.85 μm bands are detected toward all sources, with the Class 0-type low mass YSOs showing the deepest bands ever observed. In almost all sources the 6.0 μm band is deeper, by up to a factor of 3, than expected from the bending mode of pure solid H2O, based on the optical depths of the 3.0 μm stretching and 13 μm libration modes. The depth and shape variations of the remaining 5–7 μm absorption indicate that it consists of 5 independent components, which, by comparison to laboratory studies, must be from at least 8 different carriers. Together with information from the 3-4 μm spectra and the additionally detected weak 7.25, 7.40, 9.0, and 9.7 μm features it is argued that overlapping bands of simple species are responsible for much of the absorption in the 5-7 μm region, at abundances of 1-30% for CH3OH, 3-8% for NH3, 1-5% for HCOOH, ∼6% for H2CO, and ∼0.3% for HCOO − with respect to solid H2O. The 6.85 μm band likely consists of one or two carriers, of which one is less volatile than H2O because its abundance relative to H2O is enhanced at lower H2O/τ9.7 ratios. It does not survive in the diffuse interstellar medium (ISM), however. The similarity of the 6.85 μm bands for YSOs and background stars indicates that its carrier(s) must be formed early in the molecular cloud evolution. If an NH4 salt is the carrier its abundance with respect to solid H2O is typically 7%, and low temperature acid-base chemistry or cosmic ray induced reactions must have been involved in its formation. Possible origins are discussed for the carrier of an enigmatic, very broad absorption between 5 and 8 μm. It shows large depth variations toward both lowand high-mass YSOs. Weak evidence is found that it correlates with temperature tracers. Finally, all the phenomena observed for ices toward massive YSOs are also observed toward low mass YSOs, indicating that processing of the ices by internal ultraviolet radiation fields is a minor factor in the early chemical evolution of the ices. Subject headings: infrared: ISM — ISM: molecules — ISM: abundances — stars: formation — infrared: stars— astrochemistry 1 Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. 2 The VLT/ISAAC spectra were obtained at the European Southern Observatory, Paranal, Chile, within the observing programs 164.I-0605, 69.C-0441, and 272.C-5008 3 Division of PMA, Mail Code 105-24, California Institute of Technology, Pasadena, CA 91125, USA 4 AURA/NOAO-South, Gemini Science Center, Casilla 603, La Serena, Chile 5 current address: IPAC, NASA Herschel Science Center, Mail Code 100-22, California Institute of Technology, Pasadena, CA 91125, USA (email: aboogert@ipac.caltech.edu) 6 Division of GPS, Mail Code 150-21, California Institute of Technology, Pasadena, CA 91125, USA 7 Hubble Fellow 8 Department of Astronomy, University of Maryland, College Park, MD 20742, USA 9 Leiden Observatory, PO Box 9513, 2300 RA Leiden, the Netherlands 10 SRON, PO Box 800, 9700 AV Groningen, the Netherlands 11 Department of Astronomy, University of Texas at Austin, 1

Dust grains in the planet forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks. As part of the Cores to Disks Legacy Program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars. More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. Observational properties of the crystalline features seen at lambda > 20 mu correlate with each other, while they are largely uncorrelated with the properties of the amorphous silicate 10 mu feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (< 1 AU) emitting at lambda ~ 10 mu and a much colder region emitting at lambda > 20 mu (< 10 AU). We identify a crystallinity paradox, as the long-wavelength crystalline silicate features are 3.5 times more frequently detected (~55 % vs. ~15%) than the crystalline features arising from much warmer disk regions. This suggests that the disk has an inhomogeneous dust composition within ~10 AU. The abundant crystalline silicates found far from their presumed formation regions suggests efficient outward radial transport mechanisms in the disks. The analysis of the shape and strength of both the amorphous 10 mu feature and the crystalline feature around 23 mu provides evidence for the prevalence of micron-sized grains in upper layers of disks. Their presence in disk atmospheres suggests efficient vertical diffusion, likely accompanied by grain-grain fragmentation to balance the efficient growth expected. Finally, the depletion of submicron-sized grains points toward removal mechanisms such as stellar winds or radiation pressure.

Aims. We present a panchromatic study, involving a multiple technique approach, of the circumstellar disc surrounding the T Tauri star IM Lupi (Sz 82).Methods. We have undertaken a comprehensive observational study of IM Lupi using photometry, spectroscopy, millimetre interferometry and multi-wavelength imaging. For the first time, the disc is resolved from optical and near-infrared wavelengths in scattered light, to the millimetre regime in thermal emission. Our data-set, in conjunction with existing photometric data, provides an extensive coverage of the spectral energy distribution, including a detailed spectrum of the silicate emission bands. We have performed a simultaneous modelling of the various observations, using the radiative transfer code MCFOST, and analysed a grid of models over a large fraction of the parameter space via Bayesian inference.Results. We have constructed a model that can reproduce all of the observations of the disc. Our analysis illustrates the importance of combining a wide range of observations in order to fully constrain the disc model, with each observation providing a strong constraint only on some aspects of the disc structure and dust content. Quantitative evidence of dust evolution in the disc is obtained: grain growth up to millimetre-sized particles, vertical stratification of dust grains with micrometric grains close to the disc surface and larger grains which have settled towards the disc midplane, and possibly the formation of fluffy aggregates and/ or ice mantles around grains.

To study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low-luminosity YSOs (L similar to 0.1-10 L-circle dot) using 3-38 mu m Spitzer and ground-based spectra. The sample is complemented with previously published Spitzer spectra of background stars and with ISO spectra of well-studied massive YSOs (L similar to 10(5) L-circle dot). The long-known 6.0 and 6.85 mu m bands are detected toward all sources, with the Class 0-type YSOs showing the deepest bands ever observed. The 6.0 mu m band is often deeper than expected from the bending mode of pure solid H2O. The additional 5-7 mu m absorption consists of five independent components, which, by comparison to laboratory studies, must be from at least eight different carriers. Much of this absorption is due to simple species likely formed by grain surface chemistry, at abundances of 1%-30% for CH3OH, 3%-8% for NH3, 1%-5% for HCOOH, similar to 6% for H2CO, and similar to 0.3% for HCOO- relative to solid H2O. The 6.85 mu m band has one or two carriers, of which one may be less volatile than H2O. Its carrier(s) formed early in the molecular cloud evolution and do not survive in the diffuse ISM. If an NH4+- containing salt is the carrier, its abundance relative to solid H2O is similar to 7%, demonstrating the efficiency of low-temperature acid-base chemistry or cosmic-ray-induced reactions. Possible origins are discussed for enigmatic, very broad absorption between 5 and 8 mu m. Finally, the same ices are observed toward massive and low-mass YSOs, indicating that processing by internal UV radiation fields is a minor factor in their early chemical evolution.

Aims. We present a panchromatic study, involving a multiple technique approach, of the circumstellar disc surrounding the T Tauri star IM Lupi (Sz 82).

We present a survey of mid-infrared gas-phase lines toward a sample of 76 circumstellar disks around low-mass pre-main-sequence stars from the Spitzer "Cores to Disks" legacy program. We report the first detections of [Ne II] and [Fe I] toward classical T Tauri stars in ~20% and ~9% of our sources, respectively. The observed [Ne II] line fluxes and upper limits are consistent with [Ne II] excitation in an X-ray irradiated disk around stars with X-ray luminosities LX = 1029-1031 erg s-1. [Fe I] is detected at ~10-5 to 10-4 L☉, but no [S I] or [Fe II] is detected down to ~10-6 L☉. The [Fe I] detections indicate the presence of gas-rich disks with masses of ≳0.1 MJ. No H2 0-0 S(0) and S(1) disk emission is detected, except for S(1) toward one source. These data give upper limits on the warm (T ~ 100-200 K) gas mass of a few Jovian masses, consistent with recent T Tauri disk models that include gas heating by stellar radiation. Compact disk emission of hot (T ≳ 500 K) gas is observed through the H2 0-0 S(2) and/or S(3) lines toward ~8% of our sources. The line fluxes are, however, higher by more than an order of magnitude than those predicted by recent disk models, even when X-ray and excess UV radiation are included. The [Ne II]/H2 0-0 S(2) ratios for these sources are similarly lower than predicted, consistent with the presence of an additional hot molecular gas component not included in current disk models. Oblique shocks of stellar winds interacting with the disk can explain many aspects of the hot gas emission but are inconsistent with the nondetection of [S I] and [Fe II] lines.

We have identified four circumstellar disks with a deficit of dust emission from their inner 15-50 AU. All four stars have F-G spectral type and were uncovered as part of the Spitzer Space Telescope "Cores to Disks" Legacy Program Infrared Spectrograph (IRS) first-look survey of similar to 100 pre - main- sequence stars. Modeling of the spectral energy distributions indicates a reduction in dust density by factors of 100-1000 from disk radii between similar to 0.4 and 15-50 AU but with massive gas- rich disks at larger radii. This large contrast between the inner and outer disk has led us to use the term "cold disks" to distinguish these unusual systems. However, hot dust [(0.02-0.2)M-moon] is still present close to the central star (R <= 0.8 AU). We introduce the 30 mu m/13 mu m flux density ratio as a new diagnostic for identifying cold disks. The mechanisms for dust clearing over such large gaps are discussed. Although rare, cold disks are likely in transition from an optically thick to an optically thin state and so offer excellent laboratories for the study of planet formation.

We announce the discovery of SST-Lup3-1, a very low mass star close to the brown dwarf boundary in Lupus III with a circum(sub) stellar disk, discovered by the "Cores to Disks'' Spitzer Legacy Program from mid-infrared, with very conspicuous crystalline silicate features in its spectrum. It is the first of such objects with a full 5-35 mu m spectrum taken with the IRS, and it shows strong 10 and 20 mu m silicate features with high feature-to-continuum ratios and clear crystalline features out to 33 mu m. The dust in the disk upper layer has a crystalline silicate grain fraction between 15% and 33%, depending on the assumed dust continuum. The availability of the full Spitzer infrared spectrum allows an analysis of the dust composition as a function of temperature and position in the disk. The hot ( similar to 300K) dust responsible for the 10 mu m feature consists of a roughly equal mix of small (similar to 0.1 mu m) and large (mu 1.5 mu m) grains, whereas the cold (similar to 70 K) dust responsible for the longer wavelength silicate features contains primarily large grains (> 1 mu m). Since the cold dust emission arises from deeper layers in the inner (< 3 AU) disk as well as from the surface layers of the outer (3-5 AU) disk, this provides direct evidence for combined grain growth and settling in the disk. The inferred crystalline mass fractions in the two components are comparable. Since only the inner 0.02 AU of the disk is warm enough to anneal the amorphous silicate grains, even the lowest fraction of 15% of crystalline material requires either very efficient mixing or other formation mechanisms.

We present mid-infrared Spitzer IRS spectra of the UX Orionis star VV Ser, combined with interferometric and spectroscopic data from the literature covering UV to submillimeter wavelengths. The full set of data are modeled by an axisymmetric Monte Carlo radiative transfer code to test the prediction of Dullemond et al. that disks around UX Orionis stars are self-shadowed and seen nearly edge-on. Our model is consistent with all the available observational constraints, providing strong support for this interpretation. The mid-infrared SED is declining and exhibits weak silicate emission features, consistent with a self-shadowed geometry. MIPS imaging shows that the disk has a small grain dust mass as low as 0.8 × 10-7 M☉, which may be due to strong grain growth and settling. The grains in the upper layers of the puffed-up inner rim must be small (0.01-0.4 μm) to reproduce the colors (RV ~ 3.6) of the optical light curve, while the silicate emission features indicate that grains in the outer disk (>1-2 AU) are somewhat larger (0.3-3.0 μm). If grains in the inner disk are small, the location of the puffed-up inner rim is estimated to be at 0.7-0.8 AU. This is almost twice the rim radius estimated from near-infrared interferometry. Since larger (more gray) grains are able to penetrate closer to the star for the same dust sublimation temperature, we suggest a model in which large grains in the disk midplane reach to within 0.25 AU of the star, while small grains in the disk surface create a puffed-up rim at ~0.7-0.8 AU.

Recent results indicate that the grain size and crystallinity inferred from observations of silicate features may be correlated with the spectral type of the central star and/or disk geometry. In this paper, we show that grain size, as probed by the 10 μm silicate feature peak-to-continuum and 11.3 to 9.8 μm flux ratios, is inversely proportional to log L⋆. These trends can be understood using a simple two-layer disk model for passive irradiated flaring disks, CGPLUS. We find that the radius, R10, of the 10 μm silicate emission zone in the disk goes as (L⋆/L☉)0.56, with slight variations depending on disk geometry (flaring angle and inner disk radius). The observed correlations, combined with simulated emission spectra of olivine and pyroxene mixtures, imply a dependence of grain size on luminosity. Combined with the fact that R10 is smaller for less luminous stars, this implies that the apparent grain size of the emitting dust is larger for low-luminosity sources. In contrast, our models suggest that the crystallinity is only marginally affected, because for increasing luminosity, the zone for thermal annealing (assumed to be at T > 800 K) is enlarged by roughly the same factor as the silicate emission zone. The observed crystallinity is affected by disk geometry, however, with increased crystallinity in flat disks. The apparent crystallinity may also increase with grain growth due to a corresponding increase in contrast between crystalline and amorphous silicate emission bands.

We present Spitzer observations and McDonald Observatory Smith Telescope and Anglo-Australian Telescope high spectral resolution optical observations of 4 nearby stars with variable or anomalous optical absorption, likely caused by circumstellar material. The optical observations of CaII and NaI cover a 2.8 year baseline, and extend the long term monitoring of these systems by previous researchers. In addition, mini-surveys of the local interstellar medium (LISM) around our primary targets provide a reconstruction of the intervening LISM along the line of sight. We confirm that the anomalous absorption detected toward alpha Oph is not due to circumstellar material, but to a small filamentary cloud <14.3 pc from the Sun. The three other primary targets, beta Car, HD85905, and HR10 show both short and long term variability, and little of the observed absorption can be attributed to the LISM along the line of sight. The Spitzer observations did not detect infrared excesses. We are able to place upper limits on any possible fractional infrared luminosity, which range from L_IR/L_star < 2-5 10^-6, for our three disk stars. No stable gas absorption component centered at the radial velocity of the star is detected for any of our targets. Based on simple assumptions of the variable gas absorption component, we estimate limits on the circumstellar gas mass causing the variable absorption, which range from 0.4-20 10^-8 M_Earth. These multiwavelength observations place strong limits on any possible circumstellar dust, while confirming variable circumstellar gas absorption, and therefore are interesting targets to explore the origins and evolution of variable circumstellar gas. (abridged)

We have used TEXES, the Texas Echelon-cross-Echelle Spectrograph on the NASA Infrared Telescope Facility (IRTF) to search for pure rotational H-2 emission from young stars with disks. From the ground, three pure rotational transitions of molecular hydrogen are accessible: J = 6-4 (gimel = 8.025 mu m), J = 4-2 (gimel = 12.27 mu m), and J = 3-1 (gimel = 17.035 mu m). Thus far, studies of gas in protoplanetary disks have focused on either small radii using near-infrared CO observations as a probe or on large radii with observations in the millimeter wavelength range. Observations of molecular emission in the mid-infrared offer the potential to study gas in disks at intermediate radii (1-10 AU). Molecular hydrogen can be a useful probe since it is the dominant constituent in disks and therefore allows for mass determinations which avoid CO/H-2 conversion factors. When coupled with knowledge of the stellar mass and inclination, high resolution observations (R approximate to 60,000 for J = 3-1 and R approximate to 80,000 for J = 4-2 and J = 6-4) may allow us to study line profiles and determine the radial location of the emission. In the case of AB Aurigae, we have detected J = 4-2 emission with FWHM of 7 km s(-1). Assuming an inclination of 20 degrees and a stellar mass of 2.5 M-circle dot centers the emission at approximate to 16 AU in the disk.

Spitzer Space Telescope spectra of the low mass young stellar object (YSO) IRS 46 (L_bol 0.6 L_sun) in Ophiuchus reveal strong vibration-rotation absorption bands of gaseous C2H2, HCN, and CO2. This is the only source out of a sample of 100 YSO's that shows these features and the first time they are seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined with Keck L- and M-band spectra gives excitation temperatures of > 350 K and abundances of 10(-6)-10(-5) with respect to H2, orders of magnitude higher than those found in cold clouds. In spite of this high abundance, the HCN J=4-3 line is barely detected with the James Clerk Maxwell Telescope, indicating a source diameter less than 13 AU. The (sub)millimeter continuum emission and the absence of scattered light in near-infrared images limits the mass and temperature of any remnant collapse envelope to less than 0.01 M_sun and 100 K, respectively. This excludes a hot-core type region as found in high-mass YSO's. The most plausible origin of this hot gas rich in organic molecules is in the inner (<6 AU radius) region of the disk around IRS 46, either the disk itself or a disk wind. A nearly edge-on 2-D disk model fits the spectral energy distribution (SED) and gives a column of dense warm gas along the line of sight that is consistent with the absorption data. These data illustrate the unique potential of high-resolution infrared spectroscopy to probe organic chemistry, gas temperatures and kinematics in the planet-forming zones close to a young star.

Aims. We present a compositional analysis of 8-13 mu m spectra of 32 young stellar objects (YSOs). Our sample consists of 5 intermediate-mass stars and 27 low-mass stars. Although some previous studies give reasons for the similarity between the dust in circumstellar disks of T Tauri stars and Herbig Ae/ Be stars, a quantitative comparison has been lacking so far. Therefore, we include a discussion of the results of the 10 mu m spectroscopic survey of van Boekel et al. (2005, A& A, 437, 189), who focus on Herbig Ae/Be stars, the higher mass counterparts of T Tauri stars, and draw comparisons to this and other studies.Methods. While the spectra of our 32 objects and first scientific results have already been published elsewhere we perform a more detailed analysis of the similar to 10 mu m silicate feature. In our analysis we assume that this emission feature can be represented by a linear superposition of the wavelength-dependent opacity kappa(abs)(lambda) describing the optical properties of silicate grains with different chemical composition, structure, and grain size. Determining an adequate fitting equation is another goal of this study. Using a restricted number of fitting parameters, we investigate which silicate species are necessary for the compositional fitting. Particles, with radii of 0.1 mu m- and 1.5 mu m and consisting of amorphous olivine and pyroxene, forsterite, enstatite, and quartz are considered. Only compact, homogeneous dust grains are used in the presented fitting procedures. In this context we show that acceptable fitting results can also be achieved if emission properties of porous silicate grains are considered instead.Results. Our analysis shows -in terms of the properties of the circumstellar dust-like crystallinity - T Tauri systems are a continuation of HAeBe systems at their lower mass end. However, a weak correlation between grain growth and stellar luminosity could be found, in contrast to HAeBe systems.