Variations of the 10 um Silicate Features in the Actively Accreting T Tauri Stars: DG Tau and XZ Tau

Using the Infrared Spectrograph aboard the Spitzer Space Telescope, we observed multiple epochs of 11 actively accreting T Tauri stars in the nearby Taurus-Auriga star forming region. In total, 88 low-resolution mid-infrared spectra were collected over 1.5 years in Cycles 2 and 3. The results of this multi-epoch survey show that the 10 um silicate complex in the spectra of two sources - DG Tau and XZ Tau - undergoes significant variations with the silicate feature growing both weaker and stronger over month- and year-long timescales. Shorter timescale variations on day- to week-long timescales were not detected within the measured flux errors. The time resolution coverage of this data set is inadequate for determining if the variations are periodic. Pure emission compositional models of the silicate complex in each epoch of the DG Tau and XZ Tau spectra provide poor fits to the observed silicate features. These results agree with those of previous groups that attempted to fit only single-epoch observations of these sources. Simple two-temperature, two-slab models with similar compositions successfully reproduce the observed variations in the silicate features. These models hint at a self-absorption origin of the diminution of the silicate complex instead of a compositional change in the population of emitting dust grains. We discuss several scenarios for producing such variability including disk shadowing, vertical mixing, variations in disk heating, and disk wind events associated with accretion outbursts.Comment: 6 pages, emulate apj format, accepted for publication in ApJ Letter

Evidence for J and H-band excess in classical T Tauri stars and the implications for disk structure and estimated ages

We argue that classical T Tauri stars (cTTs) possess significant non- photospheric excess in the J and H bands. We first show that normalizing the spectral energy distributions (SEDs) of cTTs to the J-band leads to a poor fit of the optical fluxes, while normalizing the SEDs to the Ic-band produces a better fit to the optical bands and in many cases reveals the presence of a considerable excess at J and H. NIR spectroscopic veiling measurements from the literature support this result. We find that J and H-band excesses correlate well with the K-band excess, and that the J-K and H-K colors of the excess emission are consistent with that of a black body at the dust sublimation temperature (~ 1500-2000 K). We propose that this near-IR excess originates at a hot inner rim, analogous to those suggested to explain the near-IR bump in the SEDs of Herbig Ae/Be stars. To test our hypothesis, we use the model presented by Dullemond et al. (2001) to fit the photometry data between 0.5 um and 24 um of 10 cTTs associated with the Chamaeleon II molecular cloud. The models that best fit the data are those where the inner radius of the disk is larger than expected for a rim in thermal equilibrium with the photospheric radiation field alone. In particular, we find that large inner rims are necessary to account for the mid infrared fluxes (3.6-8.0 um) obtained by the Spitzer Space Telescope. Finally, we argue that deriving the stellar luminosities of cTTs by making bolometric corrections to the J-band fluxes systematically overestimates these luminosities. The overestimated luminosities translate into underestimated ages when the stars are placed in the H-R diagram. Thus, the results presented herein have important implications for the dissipation timescale of inner accretion disks.Comment: 45 pages, 13 figure

Modeling Spitzer observations of VV Ser. I. The circumstellar disk of a UX Orionis star

We present mid-infrared Spitzer-IRS spectra of the well-known UX Orionis star VV Ser. We combine the Spitzer data with interferometric and spectroscopic data from the literature covering UV to submillimeter wavelengths. The full set of data are modeled by a two-dimensional axisymmetric Monte Carlo radiative transfer code. The model is used to test the prediction of (Dullemond et al. 2003) that disks around UX Orionis stars must have a self-shadowed shape, and that these disks are seen nearly edge-on, looking just over the edge of a puffed-up inner rim, formed roughly at the dust sublimation radius. We find that a single, relatively simple model is consistent with all the available observational constraints spanning 4 orders of magnitude in wavelength and spatial scales, providing strong support for this interpretation of UX Orionis stars. The grains in the upper layers of the puffed-up inner rim must be small (0.01-0.4 micron) to reproduce the colors (R_V ~ 3.6) of the extinction events, while the shape and strength of the mid-infrared silicate emission features indicate that grains in the outer disk (> 1-2 AU) are somewhat larger (0.3-3.0 micron). From the model fit, the location of the puffed-up inner rim is estimated to be at a dust temperature of 1500 K or at 0.7-0.8 AU for small grains. This is almost twice the rim radius estimated from near-infrared interferometry. A best fitting model for the inner rim in which large grains in the disk mid-plane reach to within 0.25 AU of the star, while small grains in the disk surface create a puffed-up inner rim at ~0.7-0.8 AU, is able to reproduce all the data, including the near-infrared visibilities. [Abstract abridged]Comment: 12 pages, accepted for publication in Ap

Complex Organic Materials in the HR 4796A Disk?

The red spectral shape of the visible to near infrared reflectance spectrum of the sharply-edged ring-like disk around the young main sequence star HR 4796A was recently interpreted as the presence of tholin-like complex organic materials which are seen in the atmosphere and surface of Titan and the surfaces of icy bodies in the solar system. However, we show in this Letter that porous grains comprised of common cosmic dust species (amorphous silicate, amorphous carbon, and water ice) also closely reproduce the observed reflectance spectrum, suggesting that the presence of complex organic materials in the HR 4796 disk is still not definitive.Comment: 6 pages, 3 figures; To be published in The Astrophysical Journal Letter

Hot Organic Molecules Toward a Young Low-Mass Star: A Look at Inner Disk Chemistry

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.Comment: 4 pages, 4 figures; To appear in Astrophysical Journal Letter

On the Evolution of Dust Mineralogy, From Protoplanetary Disks to Planetary Systems

Mineralogical studies of silicate features emitted by dust grains in protoplanetary disks and Solar System bodies can shed light on the progress of planet formation. The significant fraction of crystalline material in comets, chondritic meteorites and interplanetary dust particles indicates a modification of the almost completely amorphous ISM dust from which they formed. The production of crystalline silicates thus must happen in protoplanetary disks, where dust evolves to build planets and planetesimals. Different scenarios have been proposed, but it is still unclear how and when this happens. This paper presents dust grain mineralogy of a complete sample of protoplanetary disks in the young Serpens cluster. These results are compared to those in the young Taurus region and to sources that have retained their protoplanetary disks in the older Upper Scorpius and Eta Chamaeleontis stellar clusters, using the same analysis technique for all samples. This comparison allows an investigation of the grain mineralogy evolution with time for a total sample of 139 disks. The mean cluster age and disk fraction are used as indicators of the evolutionary stage of the different populations. Our results show that the disks in the different regions have similar distributions of mean grain sizes and crystallinity fractions (~10-20%) despite the spread in mean ages. Furthermore, there is no evidence of preferential grain sizes for any given disk geometry, nor for the mean cluster crystallinity fraction to increase with mean age in the 1-8 Myr range. The main implication is that a modest level of crystallinity is established in the disk surface early on (< 1 Myr), reaching a equilibrium that is independent of what may be happening in the disk midplane. These results are discussed in the context of planet formation, in comparison with mineralogical results from small bodies in our Solar System. [Abridged]Comment: Accepted for publication in the Astrophysical Journa

Cold Disks: Spitzer Spectroscopy of Disks around Young Stars with Large Gaps

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 ~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 ~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 Mmoon] is still present close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux density ratio as a new diagnostic for identifying cold disks. The mechanisms for dust clearing over such large gaps are discussed. Though 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.Comment: 13 pages, 3 figures, accepted to ApJ

Protostellar holes: Spitzer Space Telescope observations of the protostellar binary IRAS16293-2422

Mid-infrared (23-35 micron) emission from the deeply embedded "Class 0" protostar IRAS16293-2422 is detected with the Spitzer Space Telescope infrared spectrograph. A detailed radiative transfer model reproducing the full spectral energy distribution (SED) from 23 micron to 1.3 mm requires a large inner cavity of radius 600 AU in the envelope to avoid quenching the emission from the central sources. This is consistent with a previous suggestion based on high angular resolution millimeter interferometric data. An alternative interpretation using a 2D model of the envelope with an outflow cavity can reproduce the SED but not the interferometer visibilities. The cavity size is comparable to the centrifugal radius of the envelope and therefore appears to be a natural consequence of the rotation of the protostellar core, which has also caused the fragmentation leading to the central protostellar binary. With a large cavity such as required by the data, the average temperature at a given radius does not increase above 60-80 K and although hot spots with higher temperatures may be present close to each protostar, these constitute a small fraction of the material in the inner envelope. The proposed cavity will also have consequences for the interpretation of molecular line data, especially of complex species probing high temperatures in the inner regions of the envelope.Comment: Accepted for publication in ApJ Letter

Millimeter Dust Emission in the GQ Lup System

We present Submillimeter Array observations of the GQ Lup system at 1.3 millimeters wavelength with 0\farcs4 ($\sim$60 AU) resolution. Emission is detected from the position of the primary star, GQ Lup A, and is marginally resolved. No emission is detected from the substellar companion, GQ Lup B, 0\farcs7 away. These data, together with models of the spectral energy distribution, suggest a compact disk around GQ Lup A with mass $\sim 3$ M$_{Jup}$, perhaps truncated by tidal forces. There is no evidence for a gap or hole in the disk that might be the signature of an additional inner companion body capable of scattering GQ Lup B out to $\sim100$ AU separation from GQ Lup A. For GQ Lup B to have formed {\it in situ}, the disk would have to have been much more massive and extended.Comment: 16 pages, 2 figures, accepted to A

Hydrolysable tannin-based diet rich in gallotannins has a minimal impact on pig performance but significantly reduces salivary and bulbo-urethral gland size

Tannins have long been considered ‘anti-nutritional’ factors in monogastric nutrition, shown to reduce feed intake and palatability. However, recent studies revealed that compared to condensed tannins, hydrolysable tannins (HT) appear to have far less impact on growth performance but may be inhibitory to the total activity of caecal bacteria. This in turn could reduce microbial synthesis of skatole and indole in the hindgut of entire males (EM). Thus, the objective of this study was to determine the impact of a group of dietary HT on growth performance, carcass traits and boar taint compounds of group housed EM. For the study, 36 Swiss Large White boars were assigned within litter to three treatment groups. Boars were offered ad libitum one of three finisher diets supplemented with 0 (C), 15 (T15) or 30 g/kg (T30) of HT from d 105 to 165 of age. Growth performance, carcass characteristics, boar taint compounds in the adipose tissue and CYP2E1, CYP1A2, and CYP2A19 gene expression in the liver was assessed. Compared to C, feed efficiency but not daily gain and daily feed intake was lower (P < 0.05) in T15 and T30 boars. Except for the percent carcass weight loss during cooling, which tended (P < 0.10) to be greater in T30 than C and T15, carcass characteristics were not affected by the diets. In line with the numerically lower androstenone level, bulbo-urethral and salivary glands of T30 boars were lighter (P < 0.05) than of T15 with intermediate values for C. Indole level was lower (P < 0.05) in the adipose tissue of T30 than C pigs with intermediate levels in T15. Skatole levels tended (P < 0.10) to be lower in T30 and C than T15 pigs. Hepatic gene expression of CYP isoenzymes did not differ between treatment groups but was negatively correlated (P < 0.05) with androstenone (CYP2E1 and CYP1A2), skatole (CYP2E1, CYP2A) and indole (CYP2A) level. In line with the numerically highest androstenone and skatole concentrations, boar taint odour but not flavour was detected by the panelists in loins from T15 compared with loins from C and T30 boars. These results provide evidence that HT affected metabolism of indolic compounds and androstenone and that they affected the development of accessory sex glands. However, the effects were too small to be detected by sensory evaluation