16375981
DUST PROCESSING AND GRAIN GROWTH IN PROTOPLANETARY DISKS IN THE TAURUS-AURIGA STAR-FORMING REGION
Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using populations of optically thin warm and cool grains to probe the radial variation in dust composition in the uppermost layers of protoplanetary disks. Most spectra with narrow emission features associated with crystalline silicates require Mg-rich minerals and silica, while a few spectra with these features suggest the presence of other components. IRS spectra indicating the presence of large amounts of warm enstatite of ~400-500 K require crystalline silicates (enstatite or forsterite) at temperatures lower than the median temperature of the cool dust in the models, ~127 K; spectra showing a high abundance of other crystalline silicates (forsterite or silica) typically do not. A few spectra show 10 ?m complexes of very small equivalent width. They are fit well using abundant crystalline silicates but very few large grains, inconsistent with the expectation that a low peak-to-continuum ratio of the 10 ?m complex always indicates grain growth. Most of the spectra in our sample are fit well without using the opacities of large crystalline silicate grains. If large grains grow by agglomeration of submicron grains of all dust types, the amorphous silicate components of these aggregates must typically be more abundant than the crystalline silicate components. We also find that the more there is of one crystalline dust species, the more there is of the others. This could suggest that crystalline silicates are processed directly from amorphous silicates, whether through evaporation of the amorphous grains and condensation in chemical equilibrium or by annealing of the amorphous precursors. Alternatively, if one kind of crystalline silicate transforms into another kind, it suggests that the intermediate species transforms into the end-product species at a slower rate than the precursor transforms into the intermediate species; otherwise, there would be a negligible abundance of intermediate species. It is also found that the crystalline silicate abundance is correlated tightly with disk geometry, in the sense of higher crystallinity accompanying more-settled disks, which are commonly associated with growth and settling of grains. The abundance of large grains is also correlated with disks that are more highly settled, but with a wide range of large grain abundance for a given degree of settling. We interpret this range as that the settling of large grains is sensitive to individual disk properties. We also find that lower-mass stars have higher abundances of large grains in their inner regions.
195323488
Spitzer IRS Observations of Class I/II Objects in Taurus: Composition, Temperature 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 micron 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, ∼4% 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 profile shapes vary from source to source, and it is shown to be likely that even combinations of the most common candidates (NH4 and CH3OH) are inadequate to explain the feature fully. We discuss correlations among SED spectral indices, abundance ratios, and thermallyprocessed ice fractions and their implications for CO2 formation and evolution. Comparison of our spectral fits to cold molecular cloud sight-lines indicate 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. Subject headings: stars: pre-main sequence — ISM: abundances — ISM: molecules — infrared: ISM — astrochemistry
119302415
SPITZER SPECTROSCOPY OF THE CIRCUMPRIMARY DISK IN THE BINARY BROWN DWARF 2MASS J04414489+2301513*
Using the Spitzer Infrared Spectrograph, we have performed mid-infrared spectroscopy on the young binary brown dwarf 2MASS J04414489+2301513 (15 AU) in the Taurus star-forming region. The spectrum exhibits excess continuum emission that likely arises from a circumstellar disk around the primary. Silicate emission is not detected in these data, indicating the presence of significant grain growth. This is one of the few brown dwarf disks at such a young age (~1 Myr) that has been found to lack silicate emission. To quantitatively constrain the properties of the disk, we have compared the spectral energy distribution of 2MASS J04414489+2301513 to the predictions of our vertical structure codes for irradiated accretion disks. Our models suggest that the remaining atmospheric grains of moderately depleted layers may have grown to a size of 5 μm. In addition, our model fits indicate an outer radius of 0.2-0.3 AU for the disk. The small size of this circumprimary disk could be due to truncation by the secondary. The absence of an outer disk containing a reservoir of small, primordial grains, combined with a weak turbulent mechanism, may be responsible for the advanced grain growth in this disk.
117890277
Properties and evolution of circumstellar disks and envelopes around young low-mass stars in the Taurus star-forming region
118514242
EMISSION FROM WATER VAPOR AND ABSORPTION FROM OTHER GASES AT 5-7.5 μm IN SPITZER-IRS SPECTRA OF PROTOPLANETARY DISKS
We present spectra of 13 T Tauri stars in the Taurus-Auriga star-forming region showing emission in Spitzer Space Telescope Infrared Spectrograph 5-7.5 μm spectra from water vapor and absorption from other gases in these stars' protoplanetary disks. Seven stars' spectra show an emission feature at 6.6 μm due to the ν{sub 2} = 1-0 bending mode of water vapor, with the shape of the spectrum suggesting water vapor temperatures >500 K, though some of these spectra also show indications of an absorption band, likely from another molecule. This water vapor emission contrasts with the absorption from warm water vapor seen in the spectrum of the FU Orionis star V1057 Cyg. The other 6 of the 13 stars have spectra showing a strong absorption band, peaking in strength at 5.6-5.7 μm, which for some is consistent with gaseous formaldehyde (H{sub 2}CO) and for others is consistent with gaseous formic acid (HCOOH). There are indications that some of these six stars may also have weak water vapor emission. Modeling of these stars' spectra suggests these gases are present in the inner few AU of their host disks, consistent with recent studies of infrared spectra showing gas in protoplanetary disks.
16416157
Radiative Transfer Modeling of Passive Circumstellar Disks: Application to HR4796A
We present a radiative transfer model which computes the spectral energy distribution of a passive, irradiated, circumstellar disk, assuming the grains are in radiative equilibrium. Dependence on radial density profile, grain temperature estimation, and optical depth profiles on the output SED are discussed. The bist fit model for HR4796A has a minimum and maximum spherical grain size of 2.2 and 1000 \mu$m respectively, a size distribution slightly steeper than the "classical" -3.5 MRN power law, grains composed of silicates, trolite, ice, and organics and a peak radial density of 1.0 x 10^-17 g cm^-2 at 70 AU, yielding a disk mass of roughly 2 M_{oplus}$.
118582742
Discovery potential of top-partners in a realistic composite Higgs model with early LHC data
Composite Higgs models provide a natural, non-supersymmetric solution to the hierarchy problem. In these models, one or more sets of heavy top-partners are typically introduced. Some of these new quarks can be relatively light, with a mass of a few hundred GeV, and could be observed with the early LHC collision data expected to be collected during 2010. We analyse in detail the collider signatures that these new quarks can produce. We show that final states with two (same-sign) or three leptons are the most promising discovery channels. They can yield a 5σ excess over the Standard Model expectation already with the 2010 LHC collision data. Exotic quarks of charge 5/3 are a distinctive feature of this model. We present a new method to reconstruct their masses from their leptonic decay without relying on jets in the final state.
119258843
Realistic composite Higgs models
We study the role of fermionic resonances in realistic composite Higgs models. We consider the low energy effective description of a model in which the Higgs arises as the pseudo-Goldstone boson of an SO(5)/SO(4) global symmetry breaking pattern. Assuming that only fermionic resonances are present below the cutoff of our effective theory, we perform a detailed analysis of the electroweak constraints on such a model. This includes the exact one-loop calculation of the T parameter and the anomalous Zb{sub L}b{sub L} coupling for arbitrary new fermions and couplings. Other relevant observables like b{yields}s{gamma} and {delta}B=2 processes have also been examined. We find that, while minimal models are difficult to make compatible with electroweak precision tests, models with several fermionic resonances, such as the ones that appear in the spectrum of viable composite Higgs models from warped extra dimensions, are fully realistic in a large region of parameter space. These fermionic resonances could be the first observable signature of the model at the CERN LHC.
118631641
Higgs production cross-section in a Standard Model with four generations at the LHC
We present theoretical predictions for the Higgs boson production cross-section via gluon fusion at the LHC in a Standard Model with four generations. We include QCD corrections through NLO retaining the full dependence on the quark masses, and the NNLO corrections in the heavy quark effective theory approximation. We also include electroweak corrections through three loops. Electroweak and bottom-quark contributions are suppressed in comparison to the Standard Model with three generations.
2799090
The NNLO gluon fusion Higgs production cross-section with many heavy quarks
We consider extensions of the Standard Model with a number of additional heavy quarks which couple to the Higgs boson via top-like Yukawa interactions. We construct an effective theory valid for a Higgs boson mass which is lighter than twice the lightest heavy quark mass and compute the corresponding Wilson coefficient through NNLO. We present numerical results for the gluon fusion cross-section at the Tevatron for an extension of the Standard Model with a fourth generation of heavy quarks. The gluon fusion cross-section at NLO and in the approximation of infinitely heavy quarks is enhanced by a factor of 9 with respect to the Standard Model value. Tevatron experimental data can place stringent exclusion limits for the Higgs mass in this model. We find that the relative magnitude of the higher order QCD corrections with respect to the LO is similar to the Standard Model. Our result serves to carry out an exclusion analysis for a four-generation Standard Model with the same accuracy as in the Standard Model analysis.
120005192
Phenomenology of composite Higgs models
The long-standing quest for a simple and elegant description of the fundamental components of the world that surrounds us and of their interactions led so far to the construction of a theory describing gravity, the theory of relativity, and of a model for the strong and electroweak interactions, the Standard Model. At the energy scales that we can probe with current collider experiments gravity is too weak to play any role, and is usually ignored. In Chapter 1 we review what is currently considered as the “Standard Model” of particle physics, including the Higgs sector. This sector is introduced to account for the low-energy breaking of the SU(2)L × U(1)Y electroweak gauge symmetry to the electromagnetic symmetry U(1)em. Its most important prediction is the existence of a new particle, the Higgs boson. Such particle has not been observed yet, but from indirect experimental evidence we expect it to be relatively light. A light fundamental scalar is not natural, though. Radiative corrections are expected to drive its mass close to the Planck scale (or to the scale of onset of some new physics). Many extensions of the Standard Model have been introduced to try and address this and other shortcomings of the Standard Model. These new theories need to be consistent with the large amount of data that have been collected so far, and are usually in very good agreement with the Standard Model. We conclude Chapter 1 introducing an effective parametrization of new physics scenarios that allows for an easy comparison with the current electroweak constraints from collider experiments. An elegant way to explain the lightness of a particle is to assume that some symmetry protects its mass from large radiative corrections. The most famous example is supersymmetry, that exploits the cancellation between the contributions given by fermions and by bosons to the Higgs self-energy. This is not the only solution. Composite Higgs