5,377 research outputs found
Accretion Disks Around Young Objects. III. Grain Growth
We present detailed models of irradiated T Tauri disks including dust grain
growth with power-law size distributions. The models assume complete mixing
between dust and gas and solve for the vertical disk structure
self-consistentlyincluding the heating effects of stellar irradiation as well
as local viscous heating. For a given total dust mass, grain growth is found to
decrease the vertical height of the surface where the optical depth to the
stellar radiation becomes unit and thus the local irradiation heating, while
increasing the disk emission at mm and sub-mm wavelengths. The resulting disk
models are less geometrically thick than our previous models assuming
interstellar medium dust, and agree better with observed spectral energy
distributions and images of edge-on disks, like HK Tau/c and HH 30. The
implications of models with grain growth for determining disk masses from
long-wavelength emission are considered.Comment: 29 pages, including 11 figures and 1 table, APJ accepte
Accretion Disks Around Young Objects. II. Tests of Well-Mixed Models with Ism Dust
We construct detailed vertical structure models of irradiated accretion disks
around T Tauri stars with interstellar medium dust uniformly mixed with gas.
The dependence of the structure and emission properties on mass accretion rate,
viscosity parameter, and disk radius is explored using these models. The
theoretical spectral energy distributions (SEDs) and images for all
inclinations are compared with observations of the entire population of
Classical T Tauri stars (CTTS) and Class I objects in Taurus. In particular, we
find that the median near-infrared fluxes can be explained within the errors
with the most recent values for the median accretion rates for CTTS. We further
show that the majority of the Class I sources in Taurus cannot be Class II
sources viewed edge-on because they are too luminous and their colors would be
consistent with disks seen only in a narrow range of inclinations. Our models
appear to be too geometrically thick at large radii, as suggested by: (a)
larger far-infrared disk emission than in the typical SEDs of T Tauri stars;
(b) wider dark dust lanes in the model images than in the images of HH30 and HK
Tau/c; and (c) larger predicted number of stars extincted by edge-on disks than
consistent with current surveys. The large thickness of the model is a
consequence of the assumption that dust and gas are well-mixed, suggesting that
some degree of dust settling may be required to explain the observations.Comment: 41 pages, 13 figures, accepted in Ap
Spitzer observations of the Orion OB1 association: disk census in the low mass stars
We present new Spitzer Space Telescope observations of two fields in the
Orion OB1 association. We report here IRAC/MIPS observations for 115 confirmed
members and 41 photometric candidates of the ~10 Myr 25 Orionis aggregate in
the OB1a subassociation, and 106 confirmed members and 65 photometric
candidates of the 5 Myr region located in the OB1b subassociation. The 25
Orionis aggregate shows a disk frequency of 6% while the field in the OB1b
subassociation shows a disk frequency of 13%. Combining IRAC, MIPS and 2MASS
photometry we place stars bearing disks in several classes: stars with
optically thick disks (class II systems), stars with an inner transitional
disks (transitional disk candidates) and stars with "evolved disks"; the last
exhibit smaller IRAC/MIPS excesses than class II systems. In all, we identify 1
transitional disk candidate in the 25 Orionis aggregate and 3 in the OB1b
field; this represents ~10% of the disk bearing stars, indicating that the
transitional disk phase can be relatively fast. We find that the frequency of
disks is a function of the stellar mass, suggesting a maximum around stars with
spectral type M0. Comparing the infrared excess in the IRAC bands among several
stellar groups we find that inner disk emission decays with stellar age,
showing a correlation with the respective disk frequencies. The disk emission
at the IRAC and MIPS bands in several stellar groups indicates that disk
dissipation takes place faster in the inner region of the disks. Comparison
with models of irradiated accretion disks, computed with several degrees of
settling, suggests that the decrease in the overall accretion rate observed in
young stellar groups is not sufficient to explain the weak disk emission
observed in the IRAC bands for disk bearing stars with ages 5 Myr or older.Comment: Accepted in the Astrophysical Journa
Structure and Composition of Two Transitional Circumstellar Disks in Corona Australis
The late stages of evolution of the primordial circumstellar disks
surrounding young stars are poorly understood, yet vital to constrain theories
of planet formation. We consider basic structural models for the disks around
two ~10 Myr-old members of the nearby RCrA association, RX J1842.9-3532 and RX
J1852.3-3700. We present new arcsecond-resolution maps of their 230 GHz
continuum emission from the Submillimeter Array and unresolved CO(3-2) spectra
from the Atacama Submillimeter Telescope Experiment. By combining these data
with broadband fluxes from the literature and infrared fluxes and spectra from
the catalog of the Formation and Evolution of Planetary Systems (FEPS) Legacy
program on the Spitzer Space Telescope, we assemble a multiwavelength data set
probing the gas and dust disks. Using the Monte Carlo radiative transfer code
RADMC to model simultaneously the SED and millimeter continuum visibilities, we
derive basic dust disk properties and identify an inner cavity of radius 16 AU
in the disk around RX J1852.3-3700. We also identify an optically thin 5 AU
cavity in the disk around RX J1842.9-3532, with a small amount of optically
thick material close to the star. The molecular line observations suggest an
intermediate disk inclination in RX J1842.9-3532, consistent with the continuum
emission. In combination with the dust models, the molecular data allow us to
derive a lower CO content than expected, suggesting that the process of gas
clearing is likely underway in both systems, perhaps simultaneously with planet
formation.Comment: 11 pages, 5 figures, accepted for publication in A
Seismic scattering and absorption mapping of debris flows, feeding paths, and tectonic units at Mount St. Helens volcano
Acknowledgments We thank Edoardo Del Pezzo, Ludovic Margerin, Haruo Sato, Mare Yamamoto, Tatsuhiko Saito, Malcolm Hole, and Seth Moran for the valuable suggestions regarding the methodology and interpretation. Greg Waite provided the P wave velocity model of MSH. An important revision of the methods was done after two blind reviews performed before submission. The suggestions of two anonymous reviewers greatly enhanced our ability of imaging structures, interpreting our results, and testing their reliability. The facilities of the IRIS Data Management System, and specifically the IRIS Data Management Center, were used for access to waveform and metadata required in this study, and provided by the Cascades Volcano Observatory â USGS. Interaction with geologists and geographers part of the Landscape Dynamics Theme of the Scottish Alliance for Geoscience, Environment and Society (SAGES) has been important for the interpretation of the results.Peer reviewedPostprin
Crystallization and Melting Behavior of Cocoa Butter in Lipid Bodies of Fresh Cacao Beans
The present study aims at observing the crystallization and polymorphic behavior of cocoa butter (CB) in fresh cacao beans with DSC and X-ray diffraction techniques. Underlying idea of this study was to relate the necessary conditions of germination of cacao beans to the crystallization of CB, which are present in oil-in-water emulsion droplets (lipid bodies) with diameters of 1-2 Όm. Different cooling and heating conditions, with rates of 15, 2, 0.5, and 0.1 °C/min, were applied to fresh cacao beans and compared to bulk CB. The results showed that the crystallization temperatures (Tc) of CB in fresh cacao beans were lower than those of bulk CB at all the experimental conditions. In addition, polymorphic behavior of the occurrence of metastable and stable forms of CB was also different between the fresh cacao beans and the bulk state. The thermal behavior of fresh and dried cacao beans that have different geographical origins was also analyzed. This study indicates that the germination conditions of cacao beans are extended to low-temperature environmental areas by reducing the Tc value of CB in fresh beans than of the bulk oil. The results were consistent with the germination experiments of fresh cacao beans at different temperatures
Crescimento de plantas de feijão de corda submetidas à irrigação com ågua salina ao longo do desenvolvimento.
The Truncated Disk of CoKu Tau/4
We present a model of a dusty disk with an inner hole which accounts for the
Spitzer Space Telescope Infrared Spectrograph observations of the low-mass
pre-main sequence star CoKu Tau/4. We have modeled the mid-IR spectrum (between
8 and 25 mic) as arising from the inner wall of a disk. Our model disk has an
evacuated inner zone of radius ~ 10 AU, with a dusty inner ``wall'', of
half-height ~ 2 AU, that is illuminated at normal incidence by the central
star. The radiative equilibrium temperature decreases from the inner disk edge
outward through the optically-thick disk; this temperature gradient is
responsible for the emission of the silicate bands at 10 and 20 mic. The
observed spectrum is consistent with being produced by Fe-Mg amorphous glassy
olivine and/or pyroxene, with no evidence of a crystalline component. The
mid-infrared spectrum of CoKu Tau/4 is reminiscent of that of the much older
star TW Hya, where it has been suggested that the significant clearing of its
inner disk is due to planet formation. However, no inner disk remains in CoKu
Tau/4, consistent with the star being a weak-emission (non-accreting) T Tauri
star. The relative youth of CoKu Tau/4 (~ 1 Myr) may indicate much more rapid
planet formation than typically assumed.Comment: 32 pages, 9 figures, accepted in Ap
Predicted Colors and Flux Densities of Protostars in the Herschel PACS and SPIRE Filters
Upcoming surveys with the Herschel Space Observatory will yield far-IR
photometry of large samples of young stellar objects, which will require
careful interpretation. We investigate the color and luminosity diagnostics
based on Herschel broad-band filters to identify and discern the properties of
low-mass protostars. We compute a grid of 2,016 protostars in various physical
congurations, present the expected flux densities and flux density ratios for
this grid of protostars, and compare Herschel observations of three protostars
to the model results. These provide useful constraints on the range of colors
and fluxes of protostar in the Herschel filters. We find that Herschel data
alone is likely a useful diagnostic of the envelope properties of young starsComment: Part of HOPS KP papers to the Herschel special A&A issu
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