3,827 research outputs found
Submillimetre dust polarisation and opacity in the HD163296 protoplanetary ring system
We present ALMA images of the sub-mm continuum polarisation and spectral
index of the protoplanetary ringed disk HD163296. The polarisation fraction at
870{\mu}m is measured to be ~0.9% in the central core and generally increases
with radius along the disk major axis. It peaks in the gaps between the dust
rings, and the largest value (~4%) is found between rings 1 and 2. The
polarisation vectors are aligned with the disk minor axis in the central core,
but become more azimuthal in the gaps, twisting by up to +/-9degrees in the gap
between rings 1 and 2. These general characteristics are consistent with a
model of self-scattered radiation in the ringed structure, without requiring an
additional dust alignment mechanism. The 870/1300{\mu}m dust spectral index
exhibits minima in the centre and the inner rings, suggesting these regions
have high optical depths. However, further refinement of the dust or the disk
model at higher resolution is needed to reproduce simultaneously the observed
degree of polarisation and the low spectral index.Comment: 5 pages +2 pages supplemental data. v2 - revised figures and final
values; conclusions unchange
Gravitational Stirring in Planetary Debris Disks
We describe gravitational stirring models of planetary debris disks using a
new multi-annulus planetesimal evolution code. The current code includes
gravitational stirring and dynamical friction; future studies will include
coagulation, fragmentation, Poynting-Robertson drag, and other physical
processes. We use the results of our calculations to investigate the physical
conditions required for small bodies in a planetesimal disk to reach the
shattering velocity and begin a collisional cascade. Our results demonstrate
that disks composed primarily of bodies with a single size will not undergo a
collisional cascade which produces small dust grains at 30-150 AU on timescales
of 1 Gyr or smaller. Disks with a size distribution of bodies reach conditions
necessary for a collisional cascade in 10 Myr to 1 Gyr if the disk is at least
as massive as a minimum mass solar nebula and if the disk contains objects with
radii of 500 km or larger. The estimated 500 Myr survival time for these disks
is close to the median age of roughly 400 Myr derived for nearby stars with
dusty disks.Comment: 23 pages of text + 16 Figures; to appear in the Astronomical Journal,
January 200
Investigation of the fundamental constants stability based on the reactor Oklo burn-up analysis
The burn-up for SC56-1472 sample of the natural Oklo reactor zone 3 was
calculated using the modern Monte Carlo codes. We reconstructed the neutron
spectrum in the core by means of the isotope ratios: Sm/Sm and
Lu/Lu. These ratios unambiguously determine the spectrum index
and core temperature. The effective neutron absorption cross section of
Sm calculated using this spectrum was compared with experimental one.
The disagreement between these two values allows to limit a possible shift of
the low laying resonance of Sm even more . Then, these limits were
converted to the limits for the change of the fine structure constant .
We found that for the rate of change the inequality is fulfilled, which is of the next
higher order than our previous limit.Comment: 16 pages, 7 figure
We Could, but Should We? Ethical Considerations for Providing Access to GeoCities and Other Historical Digital Collections
We live in an era in which the ways that we can make sense of our past are evolving as more artifacts from that past become digital. At the same time, the responsibilities of traditional gatekeepers who have negotiated the ethics of historical data collection and use, such as librarians and archivists, are increasingly being sidelined by the system builders who decide whether and how to provide access to historical digital collections, often without sufficient reflection on the ethical issues at hand. It is our aim to better prepare system builders to grapple with these issues. This paper focuses discussions around one such digital collection from the dawn of the web, asking what sorts of analyses can and should be conducted on archival copies of the GeoCities web hosting platform that dates to 1994.This research was supported by the Natural Sciences and Engineering Research Council of Canada, the Social Sciences and Humanities Research Council of Canada, the US National Science Foundation (grants 1618695 and 1704369), the Andrew W. Mellon Foundation, Start Smart Labs, and Compute Canada
Cosmological perturbations in f(T) gravity
We investigate the cosmological perturbations in f(T) gravity. Examining the
pure gravitational perturbations in the scalar sector using a diagonal
vierbien, we extract the corresponding dispersion relation, which provides a
constraint on the f(T) ansatzes that lead to a theory free of instabilities.
Additionally, upon inclusion of the matter perturbations, we derive the fully
perturbed equations of motion, and we study the growth of matter overdensities.
We show that f(T) gravity with f(T) constant coincides with General Relativity,
both at the background as well as at the first-order perturbation level.
Applying our formalism to the power-law model we find that on large subhorizon
scales (O(100 Mpc) or larger), the evolution of matter overdensity will differ
from LCDM cosmology. Finally, examining the linear perturbations of the vector
and tensor sectors, we find that (for the standard choice of vierbein) f(T)
gravity is free of massive gravitons.Comment: 11 pages, 4 figures. Analysis of the vector and tensor sectors adde
An Absolute Flux Density Measurement of the Supernova Remnant Casseopia A at 32 GHz
We report 32 GHz absolute flux density measurements of the supernova remnant
Cas A, with an accuracy of 2.5%. The measurements were made with the 1.5-meter
telescope at the Owens Valley Radio Observatory. The antenna gain had been
measured by NIST in May 1990 to be .
Our observations of Cas A in May 1998 yield . We also report absolute flux density measurements of 3C48, 3C147, 3C286,
Jupiter, Saturn and Mars.Comment: 30 pages, 4 figures; accepted for publication by AJ. Revised
systematic error budget, corrected typos, and added reference
Combined assessment (aspiration cytology and mammography) of clinically suspicious breast masses
We examined the safety and utility of the combined assessment of aspiration cytology and mammography in 705 women who had clinically suspicious or malignant palpable breast masses. Histological assessment confirmed 176 benign and 529 malignant lesions. There were no incorrect (false positive) diagnoses made in the 176 benign masses when combined assessment was used (specificity 1,0; predictive value 0,86); in isolation, however, there was a false positive cytological diagnosis («papillary carcinoma») and 3 false positive mammographic diagnoses. Benign disease (false negative) was incorrectly diagnosed by combined assessment in 4 of the 529 malignant masses (sensitivity 0,99; predictive value 0,98): cytological diagnoses were of fat necrosis (2) and benign cells on cytospin (1) and aspiration biopsy (1); mammographic diagnoses were of benign disease (2) and normality (2). Indeterminate («atypical», «suspicious») diagnoses were problematic and frequent (overall 223 (31,6%), malignant masses 137 (25,9%), benign masses 86 (48,9%); cytology 117 (16,6%), mammography 141 (20%). Thus, with the combined assessment of mammography and cytology in clinically suspicious breast masses, a decisive diagnosis was made in about two-thirds of cases allowing the safe commencement of therapy; the balance of patients required cone or excision biopsy
Combined assessment (aspiration cytology and mammography) of clinically suspicious breast masses
We examined the safety and utility of the combined assessment of aspiration cytology and mammography in 705 women who had clinically suspicious or malignant palpable breast masses. Histological assessment confirmed 176 benign and 529 malignant lesions. There were no incorrect (false positive) diagnoses made in the 176 benign masses when combined assessment was used (specificity 1,0; predictive value 0,86); in isolation, however, there was a false positive cytological diagnosis ('papillary carcinoma') and 3 false positive mammographic diagnoses. Benign disease (false negative) was incorrectly diagnosed by combined assessment in 4 of the 529 malignant masses (sensitivity 0,99; predictive value 0,98): cytological diagnoses were of fat necrosis (2) and benign cells on cytospin (1) and aspiration biopsy (1); mammographic diagnoses were of benign disease (2) and normality (2). Indeterminate ('atypical', 'suspicious') diagnoses were problematic and frequent (overall 223 (31,6%), malignant masses 137 (25,9%), benign masses 86 (48,9%); cytology 117 (16,6%), mammography 141 (20%). Thus, with the combined assessment of mammography and cytology in clinically suspicious breast masses, a decisive diagnosis was made in about two-thirds of cases allowing the safe commencement of therapy; the balance of patients required core or excision biopsy
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