616 research outputs found
A study of participation in the Eastern Massachusetts Chapter of National Association of Social Workers
Thesis (M.S.)--Boston Universit
Modeling the HD32297 Debris Disk with Far-IR Herschel Data
HD32297 is a young A-star (~30 Myr) 112 pc away with a bright edge-on debris
disk that has been resolved in scattered light. We observed the HD32297 debris
disk in the far-infrared and sub-millimeter with the Herschel Space Observatory
PACS and SPIRE instruments, populating the spectral energy distribution (SED)
from 63 to 500{\mu}m. We aimed to determine the composition of dust grains in
the HD32297 disk through SED modeling, using geometrical constraints from the
resolved imaging to break degeneracies inherent in SED modeling. We found the
best fitting SED model has 2 components: an outer ring centered around 110 AU,
seen in the scattered light images, and an inner disk near the habitable zone
of the star. The outer disk appears to be composed of grains > 2{\mu}m
consisting of silicates, carbonaceous material, and water ice with an abundance
ratio of 1:2:3 respectively and 90% porosity. These grains appear consistent
with cometary grains, implying the underlying planetesimal population is
dominated by comet-like bodies. We also discuss the 3.7{\sigma} detection of [C
II] emission at 158{\mu}m with the Herschel PACS Spectrometer, making HD32297
one of only a handful of debris disks with circumstellar gas detected.Comment: 11 pages, 4 figures, accepted for publication in The Astrophysical
Journa
CGIAR citations in IPCC reports: a summary report
In March 2015, Science-Metrix was contracted by CGIAR to evaluate the presence of CGIAR publications in the IPCC assessment reports (ARs). The study was carried through the analysis of the IPCC ARs' references
Probing the role of Nd3+ ions in the weak multiferroic character of NdMn2O5 by optical spectroscopies
Raman and infrared spectroscopies are used as local probes to study the
dynamics of the Nd-O bonds in the weakly multiferroic NdMn2O5 system. The
temperature dependence of selected Raman excitations reveals the splitting of
the Nd-O bonds in NdMn2O5. The Nd3+ ion crystal field (CF) excitations in
NdMn2O5 single crystals are studied by infrared transmission as a function of
temperature, in the 1800-8000 cm-1 range, and under an applied magnetic field
up to 11 T. The frequencies of all 4Ij crystal-field levels of Nd3+ are
determined. We find that the degeneracy of the ground-state Kramers doublet is
lifted ({\Delta}0 ~7.5 cm-1) due to the Nd3+-Mn3+ interaction in the
ferroelectric phase, below TC ~ 28 K. The Nd3+ magnetic moment mNd(T) and its
contribution to the magnetic susceptibility and the specific heat are evaluated
from {\Delta}0(T) indicating that the Nd3+ ions are involved in the magnetic
and the ferroelectric ordering observed below ~ 28 K. The Zeeman splitting of
the excited crystal field levels of the Nd3+ ions at low temperature is also
analyzed.Comment: This paper is accepted for publication as a Regular Article in
Physical Review
HST-STIS Spatially Resolved Spectroscopy and Coronagraphic Imaging of the TW Hydrae Circumstellar Disk
We present the first spatially resolved spectrum of scattered light from the
TW Hydrae protoplanetary disk. This nearly face-on disk is optically thick,
surrounding a classical T Tauri star in the nearby 10 Myr old TW Hya
association. The spectrum was taken with the HST-STIS CCD, providing resolution
R ~ 360 over the wavelength range 5250 - 10300 A. Spatially resolved
spectroscopy of circumstellar disks is difficult due to the high contrast ratio
between the bright star and faint disk. Our novel observations provide optical
spectra of scattered light from the disk between 40 AU and 155 AU from the
star. The scattered light has the same color as the star (gray scattering) at
all radii, except the innermost region. This likely indicates that the
scattering dust grains are larger than about 1 micron all the way out to large
radii. From the spectroscopic data, we also obtained radial profiles of the
integrated disk brightness at two position angles, over almost the same region
as previously observed in HST-WFPC2 and NICMOS coronagraphic images (35 AU to
173 AU from the star). The profiles have the same shape as the earlier ones,
but show a small azimuthal asymmetry in the disk not previously noted. Our STIS
broad-band coronagraphic images of TW Hya confirm the reality of this
asymmetry, and show that the disk surface brightness interior to 140 AU has a
sinusoidal dependence on azimuthal angle. The maximum brightness occurs at a
position angle of 233.6 +/- 5.7 degrees East of North. This might be caused by
the combination of forward-scattering and an increase in inclination in the
inner region of the disk, suggesting that the TW Hya disk has a warp like that
seen in the Beta Pictoris debris disk.Comment: 18 pages, 9 figures (some in color). First version submitted to ApJ
2004-08-29, posted to astro-ph 2004-10-10. Revised version accepted for
publication in ApJ 2004-12-07, posted to astro-ph 2004-12-0
Can gas in young debris disks be constrained by their radial brightness profiles?
Disks around young stars are known to evolve from optically thick,
gas-dominated protoplanetary disks to optically thin, almost gas-free debris
disks. It is thought that the primordial gas is largely removed at ages of ~10
Myr, but it is difficult to discern the true gas densities from gas
observations. This suggests using observations of dust: it has been argued that
gas, if present with higher densities, would lead to flatter radial profiles of
the dust density and surface brightness than those actually observed. However,
here we show that these profiles are surprisingly insensitive to variation of
the parameters of a central star, location of the dust-producing planetesimal
belt, dustiness of the disk and - most importantly - the parameters of the
ambient gas. This result holds for a wide range of gas densities (three orders
of magnitude), for different radial distributions of the gas temperature, and
different gas compositions. The brightness profile slopes of -3...-4 we find
are the same that were theoretically found for gas-free debris disks, and they
are the same as actually retrieved from observations of many debris disks. Our
specific results for three young (10-30 Myr old), spatially resolved, edge-on
debris disks (beta Pic, HD 32297, and AU Mic) show that the observed radial
profiles of the surface brightness do not pose any stringent constraints on the
gas component of the disk. We cannot exclude that outer parts of the systems
may have retained substantial amounts of primordial gas which is not evident in
the gas observations (e.g. as much as 50 Earth masses for beta Pic). However,
the possibility that gas, most likely secondary, is only present in little to
moderate amounts, as deduced from gas detections (e.g. ~0.05 Earth masses in
the beta Pic disk), remains open, too.Comment: Accepted for publication in Astronomy and Astrophysic
Fermion Back-Reaction and the Sphaleron
Using a simple model, a new sphaleron solution which incorporates finite
fermionic density effects is obtained. The main result is that the height of
the potential barrier (sphaleron energy) decreases as the fermion density
increases. This suggests that the rate of sphaleron-induced transitions
increases when the fermionic density increases. However the rate increase is
not expected to change significantly the predictions from the standard
sphaleron-induced baryogenesis scenarios.Comment: 11 pages, Revtex (2 figures available upon request), to appear in
Phys. Rev. D (Rapid Communication
Modeling the HD 32297 Debris Disk With Far-Infrared Herschel Data
HD 32297 is a young A-star (approx. 30 Myr) 112 pc away with a bright edge-on debris disk that has been resolved in scattered light. We observed the HD 32297 debris disk in the far-infrared and sub-millimeter with the Herschel Space Observatory PACS and SPIRE instruments, populating the spectral energy distribution (SED) from 63 to 500 micron..We aimed to determine the composition of dust grains in the HD 32297 disk through SED modeling, using geometrical constraints from the resolved imaging to break the degeneracies inherent in SED modeling. We found the best fitting SED model has two components: an outer ring centered around 110 AU, seen in the scattered light images, and an inner disk near the habitable zone of the star. The outer disk appears to be composed of grains>2 micron consisting of silicates, carbonaceous material, and water ice with an abundance ratio of 1:2:3 respectively and 90% porosity. These grains appear consistent with cometary grains, implying the underlying planetesimal population is dominated by comet-like bodies. We also discuss the 3.7 sigma detection of [C ii] emission at 158 micron with the Herschel PACS instrument, making HD 32297 one of only a handful of debris disks with circumstellar gas detecte
Interstellar Polarization in the Taurus Dark Clouds, Wavelength Dependent Position Angles and Cloud Structure Near TMC-1
We use polarimetric observations of two stars (HD29647, HD283809) in the
general direction of TMC-1 in the Taurus Dark Cloud to investigate grain
properties and cloud structure in this region. We show the data to be
consistent with a simple two-component model, in which general interstellar
polarization in the Taurus Cloud is produced by a widely distributed cloud
component with relatively uniform magnetic field orientation; the light from
stars close to TMC-1 suffers additional polarization arising in one (or more)
subcloud(s) with larger average grain size and different magnetic field
directions compared with the general trend. Towards HD29647, in particular, we
show that the unusually low degree of visual polarization relative to
extinction is due to the presence of distinct cloud components in the line of
sight with markedly different magnetic field orientations. Stokes parameter
calculations allow us to separate out the polarization characteristics of the
individual components. Results are fit with the Serkowski empirical formula to
determine the degree and wavelength of maximum polarization. Whereas lambda_max
values in the widely distributed material are similar to the average (0.55um)
for the diffuse interstellar medium, the subcloud in line of sight to
HD~283809, the most heavily reddened star in our study, has lambda_max approx.
0.73um, indicating the presence of grains about 30% larger than this average.
Our model also predicts detectable levels of circular polarization toward both
HD~29647 and HD~283809.Comment: 17 pages including 6 figures, LaTeX, to appear in the Astrophysical
Journal, vol 48
Herschel PACS Observations and Modeling of Debris Disks in the Tucana-Horologium Association
We present Herschel PACS photometry of seventeen B- to M-type stars in the 30
Myr-old Tucana-Horologium Association. This work is part of the Herschel Open
Time Key Programme "Gas in Protoplanetary Systems" (GASPS). Six of the
seventeen targets were found to have infrared excesses significantly greater
than the expected stellar IR fluxes, including a previously unknown disk around
HD30051. These six debris disks were fitted with single-temperature blackbody
models to estimate the temperatures and abundances of the dust in the systems.
For the five stars that show excess emission in the Herschel PACS photometry
and also have Spitzer IRS spectra, we fit the data with models of optically
thin debris disks with realistic grain properties in order to better estimate
the disk parameters. The model is determined by a set of six parameters:
surface density index, grain size distribution index, minimum and maximum grain
sizes, and the inner and outer radii of the disk. The best fitting parameters
give us constraints on the geometry of the dust in these systems, as well as
lower limits to the total dust masses. The HD105 disk was further constrained
by fitting marginally resolved PACS 70 micron imaging.Comment: 15 pages, 7 figures, Accepted to Ap
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