108 research outputs found
ISO LWS Spectra of T Tauri and Herbig AeBe stars
We present an analysis of ISO-LWS spectra of eight T Tauri and Herbig AeBe young stellar objects.
Some of the objects are in the embedded phase of star-formation, whereas others have cleared their environs
but are still surrounded by a circumstellar disk. Fine-structure lines of [OI] and [CII] are most likely excited by
far-ultraviolet photons in the circumstellar environment rather than high-velocity outflows, based on comparisons
of observed line strengths with predictions of photon-dominated and shock chemistry models. A subset of our
stars and their ISO spectra are adequately explained by models constructed by Chiang & Goldreich (1997) and
Chiang et al. (2001) of isolated, passively heated, flared circumstellar disks. For these sources, the bulk of the
LWS flux at wavelengths longward of 55 ”m arises from the disk interior which is heated diffusively by reprocessed
radiation from the disk surface. At 45 ”m, water ice emission bands appear in spectra of two of the coolest stars,
and are thought to arise from icy grains irradiated by central starlight in optically thin disk surface layers
Spectral Energy Distributions of T Tauri and Herbig Ae Disks: Grain Mineralogy, Parameter Dependences, and Comparison with ISO LWS Observations
We improve upon the radiative, hydrostatic equilibrium models of passive
circumstellar disks constructed by Chiang & Goldreich (1997). New features
include (1) account for a range of particle sizes, (2) employment of
laboratory-based optical constants of representative grain materials, and (3)
numerical solution of the equations of radiative and hydrostatic equilibrium
within the original 2-layer (disk surface + disk interior) approximation. We
explore how the spectral energy distribution (SED) of a face-on disk depends on
grain size distributions, disk geometries and surface densities, and stellar
photospheric temperatures. Observed SEDs of 3 Herbig Ae and 2 T Tauri stars,
including spectra from the Long Wavelength Spectrometer (LWS) aboard the
Infrared Space Observatory (ISO), are fitted with our models. Silicate emission
bands from optically thin, superheated disk surface layers appear in nearly all
systems. Water ice emission bands appear in LWS spectra of 2 of the coolest
stars. Infrared excesses in several sources are consistent with vertical
settling of photospheric grains. While this work furnishes further evidence
that passive reprocessing of starlight by flared disks adequately explains the
origin of infrared-to-millimeter wavelength excesses of young stars, we
emphasize how the SED alone does not provide sufficient information to
constrain particle sizes and disk masses uniquely.Comment: Accepted to ApJ, 35 pages inc. 14 figures, AAS preprin
The cool atmospheres of the binary brown dwarf eps Indi B
We have imaged Indi B, the closest brown dwarf binary known, with
VISIR at the VLT in three narrow-band mid-infrared bandpasses located around
8.6m, 10.5m and 11.3m. We are able to spatially resolve both
components, and determine accurate mid-infrared photometry for both components
independently. In particular, our VISIR observations probe the NH feature
in the atmospheres of the cooler and warmer brown dwarfs. For the first time,
we can disentangle the contributions of the two components, and find that % our
photometry of IndiBb is in good agreement with recent ``cloud-free''
atmosphere models having an effective temperature of K.
With an assumed age of 1 Gyr for the Indi system, component Ba
agrees more with K rather than with
K, as suggested by SPITZER spectroscopic observations of
the combined Indi B system (Roellig et al., 2004). Even higher
effective temperatures appear inconsistent with our absolute photometry, as
they would imply an unphysical small size of the brown dwarf IndiBa.Comment: 4 pages, 2 figure
GREAT [CII] and CO observations of the BD+40{\deg}4124 region
The BD+40\degree4124 region was observed with high angular and spectral
resolution with the German heterodyne instrument GREAT in CO J = 13 \rightarrow
12 and [CII] on SOFIA. These observations show that the [CII] emission is very
strong in the reflection nebula surrounding the young Herbig Ae/Be star
BD+40\degree4124. A strip map over the nebula shows that the [CII] emission
approximately coincides with the optical nebulosity. The strongest [CII]
emission is centered on the B2 star and a deep spectrum shows that it has faint
wings, which suggests that the ionized gas is expanding. We also see faint CO J
= 13 \rightarrow 12 at the position of BD+40\degree4124, which suggests that
the star may still be surrounded by an accretion disk.We also detected [CII]
emission and strong CO J = 13 \rightarrow 12 toward V1318 Cyg. Here the [CII]
emission is fainter than in BD+40\degree4124 and appears to come from the
outflow, since it shows red and blue wings with very little emission at the
systemic velocity, where the CO emission is quite strong. It therefore appears
that in the broad ISO beam the [CII] emission was dominated by the reflection
nebula surrounding BD+40\degree4124, while the high J CO lines originated from
the adjacent younger and more deeply embedded binary system V1318 Cyg
Infrared interferometric observations of young stellar objects
We present infrared observations of four young stellar objects using the
Palomar Testbed Interferometer (PTI). For three of the sources, T Tau, MWC 147
and SU Aur, the 2.2 micron emission is resolved at PTI's nominal fringe spacing
of 4 milliarcsec (mas), while the emission region of AB Aur is over-resolved on
this scale. We fit the observations with simple circumstellar material
distributions and compare our data to the predictions of accretion disk models
inferred from spectral energy distributions. We find that the infrared emission
region is tenths of AU in size for T Tau and SU Aur and ~1 AU for MWC 147.Comment: 11 pages, 3 figures, to appear in the Astrophysical Journa
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Prediction of heat capacities of solid inorganic salts from group contributions
A group contribution technique is proposed to predict the coefficients in the heat capacity correlation, C{sub p} = a + bT + c/T{sup 2} + dT{sup 2}, for solid inorganic salts. The results from this work are compared with fits to experimental data from the literature. It is shown to give good predictions for both simple and complex solid inorganic salts. Literature heat capacities for a large number (664) of solid inorganic salts covering a broad range of cations (129), anions (17) and ligands (2) have been used in regressions to obtain group contributions for the parameters in the heat capacity temperature function. A mean error of 3.18% is found when predicted values are compared with literature values for heat capacity at 298{degrees} K. Estimates of the error standard deviation from the regression for each additivity constant are also determined
Survival of icy grains in debris discs. The role of photosputtering
We put theoretical constraints on the presence and survival of icy grains in
debris discs. Particular attention is paid to UV sputtering of water ice, which
has so far not been studied in detail in this context. We present a
photosputtering model based on available experimental and theoretical studies.
We quantitatively estimate the erosion rate of icy and ice-silicate grains,
under the influence of both sublimation and photosputtering, as a function of
grain size, composition and distance from the star. The effect of erosion on
the grain's location is investigated through numerical simulations coupling the
grain size to its dynamical evolution. Our model predicts that photodesorption
efficiently destroy ice in optically thin discs, even far beyond the
sublimation snow line. For the reference case of beta Pictoris, we find that
only > 5mm grains can keep their icy component for the age of the system in the
50-150AU region. When taking into account the collisional reprocessing of
grains, we show that the water ice survival on grains improves (grains down to
~ 20 um might be partially icy). However, estimates of the amount of gas
photosputtering would produce on such a hypothetical population of big icy
grains lead to values for the OI column density that strongly exceed
observational constraints for beta Pic, thus ruling out the presence of a
significant amount of icy grains in this system. Erosion rates and icy grains
survival timescales are also given for a set of 11 other debris disc systems.
We show that, with the possible exception of M stars, photosputtering cannot be
neglected in calculations of icy grain lifetimes.Comment: 12 pages, 9 figures. accepted by A&
Multiwavelength Study of Pulsation and Dust Production in Mira Variables Using Optical Interferometry for Constraints
Optical interferometry is a technique by which the diameters and indeed the direct pulsations of stars are routinely being measured. As a follow-on to a 7 year interferometric campaign to measure the pulsations of over 100 mira variables, our team has been using the Spitzer Space Telescope to obtain 95 mid-infrared spectra of 25 miras during their pulsations over one year while simultaneously ascertaining their near-infrared diameters using the Palomar Testbed Interferometer. These data will then be combined with modeling from NLTE and radiative transfer codes to place hard constraints on our understanding of these stars and their circumstellar environments. We present some initial results from this work and discuss the next steps toward fully characterizing the atmosphere, molecular photosphere and dust production in mira variables.Some of the work on this project was supported through a NASA grant to the PI and team associated with Spitzer program GO50717
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