9,813 research outputs found
Second-Order Nonlinear Mixing of Two Modes in a Planar Photonic Crystal Microcavity
Polarization-resolved second-harmonic spectra are obtained from the resonant
modes of a two-dimensional planar photonic crystal microcavity patterned in a
free-standing InP slab. The photonic crystal microcavity is comprised of a
single missing-hole defect in a hexagonal photonic crystal host formed with
elliptically-shaped holes. The cavity supports two orthogonally-polarized
resonant modes split by 60 wavenumbers. Sum-frequency data are reported from
the nonlinear interaction of the two coherently excited modes, and the
polarization dependence is explained in terms of the nonlinear susceptibility
tensor of the host InP.Comment: 7 pages, 8 Postscript figures, to be presented at Photonics West Jan.
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New insights into the nature of transition disks from a complete disk survey of the Lupus star forming region
Transition disks with large dust cavities around young stars are promising
targets for studying planet formation. Previous studies have revealed the
presence of gas cavities inside the dust cavities hinting at recently formed,
giant planets. However, many of these studies are biased towards the brightest
disks in the nearby star forming regions, and it is not possible to derive
reliable statistics that can be compared with exoplanet populations. We present
the analysis of 11 transition disks with large cavities (>20 AU radius) from a
complete disk survey of the Lupus star forming region, using ALMA Band 7
observations at 0.3" (22-30 AU radius) resolution of the 345 GHz continuum,
13CO and C18O 3-2 observations and the Spectral Energy Distribution of each
source. Gas and dust surface density profiles are derived using the
physical-chemical modeling code DALI. This is the first study of transition
disks of large cavities within a complete disk survey within a star forming
region. The dust cavity sizes range from 20-90 AU radius and in three cases, a
gas cavity is resolved as well. The deep drops in gas density and large dust
cavity sizes are consistent with clearing by giant planets. The fraction of
transition disks with large cavities in Lupus is ~11%, which is inconsistent
with exoplanet population studies of giant planets at wide orbits. Furthermore,
we present a hypothesis of an evolutionary path for large massive disks
evolving into transition disks with large cavities.Comment: 29 pages, 15 figures, Accepted by Ap
Quantum description of the orientational degrees of freedom in a biaxial nematic liquid
The quantum mechanical version of a classical model for studying the
orientational degrees of freedom corresponding to a nematic liquid composed of
biaxial molecules is presented. The effective degrees of freedom are described
by operators carrying an SU(3) representation, which allows the explicit
calculation of the partition function in the mean field approximation. The
algebraic consistency conditions are solved numerically and the equilibrium
phases of the system are determined. In particular, the entropy, the specific
heat and the order parameters are presented for different choices of the
constituent biaxial molecules. Our results reproduce the classical calculation
in the limit of high temperatures and high quantum numbers.Comment: 33 pages, Latex, 11 figure
Sources of Airborne Endotoxins in Ambient Air and Exposure of Nearby Communities—A Review
Endotoxin is a bioaerosol component that is known to cause respiratory effects in exposed populations. To date, most research focused on occupational exposure, whilst much less is known about the impact of emissions from industrial operations on downwind endotoxin concentrations. A review of the literature was undertaken, identifying studies that reported endotoxin concentrations in both ambient environments and around sources with high endotoxin emissions. Ambient endotoxin concentrations in both rural and urban areas are generally below 10 endotoxin units (EU) m−3; however, around significant sources such as compost facilities, farms, and wastewater treatment plants, endotoxin concentrations regularly exceeded 100 EU m−3. However, this is affected by a range of factors including sampling approach, equipment, and duration. Reported downwind measurements of endotoxin demonstrate that endotoxin concentrations can remain above upwind concentrations. The evaluation of reported data is complicated due to a wide range of different parameters including sampling approaches, temperature, and site activity, demonstrating the need for a standardised methodology and improved guidance. Thorough characterisation of ambient endotoxin levels and modelling of endotoxin from pollution sources is needed to help inform future policy and support a robust health-based risk assessment process
The ACS Nearby Galaxy Survey Treasury IV. The Star Formation History of NGC 2976
We present resolved stellar photometry of NGC 2976 obtained with the Advanced
Camera for Surveys (ACS) as part of the ACS Nearby Galaxy Survey Treasury
(ANGST) program. The data cover the radial extent of the major axis of the disk
out to 6 kpc, or ~6 scale lengths. The outer disk was imaged to a depth of
M_F606W ~ 1, and an inner field was imaged to the crowding limit at a depth of
M_F606W ~ -1. Through detailed analysis and modeling of these CMDs we have
reconstructed the star formation history of the stellar populations currently
residing in these portions of the galaxy, finding similar ancient populations
at all radii but significantly different young populations at increasing radii.
In particular, outside of the well-measured break in the disk surface
brightness profile, the age of the youngest population increases with distance
from the galaxy center, suggesting that star formation is shutting down from
the outside-in. We use our measured star formation history, along with H I
surface density measurements, to reconstruct the surface density profile of the
disk during previous epochs. Comparisons between the recovered star formation
rates and reconstructed gas densities at previous epochs are consistent with
star formation following the Schmidt law during the past 0.5 Gyrs, but with a
drop in star formation efficiency at low gas densities, as seen in local
galaxies at the present day. The current rate and gas density suggest that
rapid star formation in NGC 2976 is currently in the process of ceasing from
the outside-in due to gas depletion. This process of outer disk gas depletion
and inner disk star formation was likely triggered by an interaction with the
core of the M81 group >~1 Gyr ago that stripped the gas from the galaxy halo
and/or triggered gas inflow from the outer disk toward the galaxy center.Comment: 22 pages, 14 figures, 2 tables, accepted for publication by Ap
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