2,556 research outputs found
A Market-Based Approach for Valuing Ecosystem Services on Coastal Properties
As part of ongoing efforts to assess ecosystem services provided by wetlands, this research focuses on estimating the private monetary value of wetland services to residential property owners in Louisiana and Alabama using a hedonic price model. Understanding that tradeoffs must be made with limited resources, valuing ecosystem services is important for policy and decision making purposes, such as determining how much public and private financial resources should be put towards wetland maintenance and restoration. Data on property transactions, wetland coverage, and built infrastructure were collected for the analysis. Based on theory and results from a Box-Cox model, the log-linear functional form was the best fit for the data. Marginal implicit prices of services from wetland ecosystems were estimated using proximity variables. Most wetland proximity variables were statistically significant and indicative of preferences for different wetland types. Based on coefficient estimates, freshwater ponds were the only preferred wetland ecosystems to live in closer proximity to in Louisiana, while freshwater emergent wetlands were the only preferred wetland ecosystems in Alabama. Homeowners preferred living further from rivers in both Louisiana and Alabama. Results are consistent with the wetland service valuation literature from across the United States which used the property-price hedonic approach
Causal connection in parsec-scale relativistic jets: results from the MOJAVE VLBI survey
We report that active galactic nucleus (AGN) jets are causally connected on
parsec scales, based on 15 GHz Very Long Baseline Array (VLBA) data from a
sample of 133 AGN jets. This result is achieved through a new method for
measuring the product of the jet Lorentz factor and the intrinsic opening angle
Gamma*theta_j from measured apparent opening angles in flux density limited
samples of AGN jets. The Gamma*theta_j parameter is important for jet physics
because it is related to the jet-frame sidewise expansion speed and causal
connection between the jet edges and its symmetry axis. Most importantly, the
standard model of jet production requires that the jet be causally connected
with its symmetry axis, implying that Gamma*theta_j < 1. When we apply our
method to the MOJAVE flux density limited sample of radio loud objects, we find
Gamma*theta_j = 0.2, implying that AGN jets are causally connected. We also
find evidence that AGN jets viewed very close to the line of sight effectively
have smaller intrinsic opening angles compared with jets viewed more off-axis,
which is consistent with Doppler beaming and a fast inner spine/slow outer
sheath velocity field. Notably, gamma-ray burst (GRB) jets have a typical
Gamma*theta_j that is two orders of magnitude higher, suggesting that different
physical mechanisms are at work in GRB jets compared to AGN jets. A useful
application of our result is that a jet's beaming parameters can be derived.
Assuming Gamma*theta_j is approximately constant in the AGN jet population, an
individual jet's Doppler factor and Lorentz factor (and therefore also its
viewing angle) can be determined using two observable quantities: apparent jet
opening angle and the apparent speed of jet components.Comment: 9 pages, 4 figure
Multi-frequency VLBA study of the blazar S5 0716+714 during the active state in 2004: I. Inner jet kinematics
We observed the blazar \object{0716+714} with the
VLBA during its active state in 2003-2004. In this paper we discuss
multi-frequency analysis of the inner jet (first 1 mas) kinematics. The
unprecedentedly dense time sampling allows us to trace jet components without
misidentification and to calculate the component speeds with good accuracy. In
the smooth superluminal jet we were able to identify and track three components
over time moving outwards with relatively high apparent superluminal speeds
(8.5-19.4 ), which contradicts the hypothesis of a stationary oscillating
jet in this source. Component ejections occur at a relatively high rate (once
in two months), and they are accompanied by mm-continuum outbursts.
Superluminal jet components move along wiggling trajectories, which is an
indication of actual helical motion. Fast proper motion and rapid decay of the
components suggest that this source should be observed with the VLBI at a rate
of at least once in one or two months in order to trace superluminal jet
components without confusion.Comment: 4 pages, 3 figures, Astronomy & Astrophysics Letter, language
corrections adde
The connection between the radio jet and the gamma-ray emission in the radio galaxy 3C 120
We present the analysis of the radio jet evolution of the radio galaxy 3C 120
during a period of prolonged gamma-ray activity detected by the Fermi satellite
between December 2012 and October 2014. We find a clear connection between the
gamma-ray and radio emission, such that every period of gamma-ray activity is
accompanied by the flaring of the mm-VLBI core and subsequent ejection of a new
superluminal component. However, not all ejections of components are associated
with gamma-ray events detectable by Fermi. Clear gamma-ray detections are
obtained only when components are moving in a direction closer to our line of
sight.This suggests that the observed gamma-ray emission depends not only on
the interaction of moving components with the mm-VLBI core, but also on their
orientation with respect to the observer. Timing of the gamma-ray detections
and ejection of superluminal components locate the gamma-ray production to
within almost 0.13 pc from the mm-VLBI core, which was previously estimated to
lie about 0.24 pc from the central black hole. This corresponds to about twice
the estimated extension of the broad line region, limiting the external photon
field and therefore suggesting synchrotron self Compton as the most probable
mechanism for the production of the gamma-ray emission. Alternatively, the
interaction of components with the jet sheath can provide the necessary photon
field to produced the observed gamma-rays by Compton scattering.Comment: Already accepted for publication in The Astrophysical Journa
Mechanical analysis of cranial distractor attachment with three different resorbable fixation systems
Distraction osteogenesis (DO) has become increasingly popular to correct craniosynostosis. Disadvantages of DO include the secondary operation needed for device removal and titanium screw related dura injury. To reduce invasiveness of the secondary device removal operation and to overcome titanium-related problems, fixation of the cranial distractor with resorbable materials is a potential alternative. New resorbable fixation methods, such as ultrasound-activated pins (UAPs) or heat-activated pins (HAPs), allow faster attachment on thinner bone than conventional resorbable screws (CRSs) since tapping is not required. However, resorbable materials are designed to be attached with a resorbable plate, not with a titanium distractor. We evaluated the suitability of CRSs, HAPs and UAPs for the cranial distractor fixation in a laboratory setting with a mechanical testing machine. Fracture tests were conducted in two directions with respect to the longitudinal axis; vertical i.e. axial pull-out strength, and horizontal i.e. shear strength. Mean maximum pull-out strength for CRS, HAP and UAP was 48.9 N, 32.5 N and 14.7 N, respectively. Mean maximum shear strength for CRS, HAP and UAP was 40.8 N, 77.9 N and 38.9 N, respectively. According to our in vitro tests, the cranial distractor attachment with four CRSs or six HAPs per footplate would provide sufficient fixation stability. (C) 2018 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.Peer reviewe
Catching the Radio Flare in CTA 102 III. Core-Shift and Spectral Analysis
The temporal and spatial spectral evolution of the jets of AGN can be studied
with multi-frequency, multi-epoch VLBI observations. The combination of both,
morphological and spectral parameters can be used to derive source intrinsic
physical properties such as the magnetic field and the non-thermal particle
density. In the first two papers of this series, we analyzed the single-dish
light curves and the VLBI kinematics of the blazar CTA 102 and suggested a
shock-shock interaction between a traveling and a standing shock wave as a
possible scenario to explain the observed evolution of the component associated
to the 2006 flare. In this paper we investigate the core-shift and spectral
evolution to test our hypothesis of a shock-shock interaction. We used 8
multi-frequency VLBA observations to analyze the temporal and spatial evolution
of the spectral parameters during the flare. We observed CTA 102 between May
2005 and April 2007 using the VLBA at six different frequencies spanning from 2
- 86 GHz. After the calibrated VLBA images were corrected for opacity, we
performed a detailed spectral analysis. From the derived values we estimated
the magnetic field and the density of the relativistic particles. The detailed
analysis of the opacity shift reveals that the position of the jet core is
proportional to nu^-1 with some temporal variations. The value suggests
possible equipartition between magnetic field energy and particle kinetic
energy densities at the most compact regions. From the variation of the
physical parameters we deduced that the 2006 flare in CTA 102 is connected to
the ejection of a new traveling feature (t=2005.9) and the interaction between
this shock wave and a stationary structure around 0.1 mas from the core. The
source kinematics together with the spectral and structural variations can be
described by helical motions in an over-pressured jet.Comment: 35 pages, 46 figure
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