22,712 research outputs found
Structural Conservation Practices in U.S. Corn Production: Evidence on Environmental Stewardship by Program Participants and Non-Participants
This study used the 2005 ERS CEAP-ARMS data for corn production to first compare key operator, field, farm, economic, and environmental characteristics of conservation program participants with non-participants, by farm-size class. We then estimate a cost-function based technology adoption model of producer decisions regarding the allocation of field-level acres between corn production and infield and perimeter-field conservation structures to examine how these conservation choices differ between program participants and non-participants, while accounting for differences in other field, farm, and environmental factors. Our null hypothesis is that the average conservation structural practice acres across U.S. corn acres supplied by growers participating in a conservation program are not different from non-participants. Infield conservation structures include terraces, grassed waterways, vegetative buffers, contour buffers, filter strips, and grade stabilization structures. Perimeter-field conservation structures include hedgerow plantings, stream-side forest and herbaceous buffers, windbreaks and herbaceous wind barriers, field borders, and critical area plantings. Because the dependent variable in this analysis is continuous, we use a Generalized Estimating Equations (GEE) procedure to estimate two models. The GEE estimation procedure (Liang and Zeger, 1986) accounts for correlation between adoption decisions measured as a continuous variable while maintaining the theoretical integrity of a multinomial discrete-choice model typically used in technology adoption studies. The cost-function models estimate field-level, producer acreage allocation decisions for corn, first, as a function of normalized production input costs (prices) and structural technology class and installation time-period attributes (Model 1), and second, as a function of Model 1 variables plus socio-environmental variables reflecting the potential influence of a variety of field, farm, and environmental characteristics (Model 2). Evidence indicates significant characteristic differences exist between conservation program participants and non-participants across U.S. corn production, that non-program factors do heavily influence producer conservation practice decisions, and that farm-size matters. In addition, results suggest that program non-participants tend to adopt infield conservation structures much more intensively while program participants emphasize the adoption of perimeter-field conservation structures. Finally, these results seem to suggest that because perimeter-field structural practices can involve differential productivity/cost effects and off-site benefits, program incentives may need to play a greater role in encouraging their adoption than they do for infield structural practices.Crop Production/Industries, Environmental Economics and Policy,
O/Fe in metal-poor main sequence and subgiant stars
A study of the O/Fe ratio in metal-poor main sequence and subgiant stars is
presented using the [OI] 6300A line, the OI 7774A triplet, and a selection of
weak FeII lines observed on high-resolution spectra acquired with the VLT UVES
spectrograph. The [OI] line is detected in the spectra of 18 stars with -0.5 <
[Fe/H] < -2.4, and the triplet is observed for 15 stars with [Fe/H] ranging
from -1.0 to -2.7. The abundance analysis was made first using standard model
atmospheres taking into account non-LTE effects on the triplet: the [OI] line
and the triplet give consistent results with [O/Fe] increasing quasi-linearly
with decreasing [Fe/H] reaching [O/Fe] ~ +0.7 at [Fe/H] = -2.5. When
hydrodynamical model atmospheres representing stellar granulation in dwarf and
subgiant stars replace standard models, the [O/Fe] from the [OI] and FeII lines
is decreased by an amount which increases with decreasing [Fe/H]. The [O/Fe] vs
[Fe/H] relation remains quasi-linear extending to [O/Fe] ~ +0.5 at [Fe/H] =
-2.5, but with a tendency of a plateau with [O/Fe] ~ +0.3 for -2.0 < [Fe/H] <
-1.0, and a hint of cosmic scatter in [O/Fe] at [Fe/H] ~ -1.0. Use of the
hydrodynamical models disturbs the broad agreement between the oxygen
abundances from the [OI], OI, and OH lines, but 3D non-LTE effects may serve to
erase these differences.Comment: ps file, 18 pages (including 10 figures) - Accepted for publication
in A&
Cost effectiveness analysis of different approaches of screening for familial hypercholesterolaemia
Objectives To assess the cost effectiveness of
strategies to screen for and treat familial
hypercholesterolaemia.
Design Cost effectiveness analysis. A care pathway for
each patient was delineated and the associated
probabilities, benefits, and costs were calculated.
Participants Simulated population aged 16Â54 years
in England and Wales.
Interventions Identification and treatment of patients
with familial hypercholesterolaemia by universal
screening, opportunistic screening in primary care,
screening of people admitted to hospital with
premature myocardial infarction, or tracing family
members of affected patients.
Main outcome measure Cost effectiveness calculated
as cost per life year gained (extension of life
expectancy resulting from intervention) including
estimated costs of screening and treatment.
Results Tracing of family members was the most cost
effective strategy (£3097 (&5066, $4479) per life year
gained) as 2.6 individuals need to be screened to
identify one case at a cost of £133 per case detected. If
the genetic mutation was known within the family
then the cost per life year gained (£4914) was only
slightly increased by genetic confirmation of the
diagnosis. Universal population screening was least
cost effective (£13 029 per life year gained) as 1365
individuals need to be screened at a cost of £9754 per
case detected. For each strategy it was more cost
effective to screen younger people and women.
Targeted strategies were more expensive per person
screened, but the cost per case detected was lower.
Population screening of 16 year olds only was as cost
effective as family tracing (£2777 with a clinical
confirmation).
Conclusions Screening family members of people
with familial hypercholesterolaemia is the most cost
effective option for detecting cases across the whole
population
Stability analysis of the Witten black hole (cigar soliton) under world-sheet RG flow
We analyze the stability of the Euclidean Witten black hole (the cigar
soliton in mathematics literature) under first-order RG (Ricci) flow of the
world-sheet sigma model. This analysis is from the target space point of view.
We find that the Witten black hole has no unstable normalizable perturbative
modes in a linearized mode analysis in which we consider circularly symmetric
perturbations. Finally, we discuss a result from mathematics that implies the
existence of a non-normalizable mode of the Witten black hole under which the
geometry flows to the sausage solution studied by Fateev, Onofri and
Zamolodchikov.Comment: 17 pages, version to appear in Physical Review D, and now has
complete proof of stability for circularly symmetric perturbations, in
response to referee comment
The Interstellar Rubidium Isotope Ratio toward Rho Ophiuchi A
The isotope ratio, 85Rb/87Rb, places constraints on models of the
nucleosynthesis of heavy elements, but there is no precise determination of the
ratio for material beyond the Solar System. We report the first measurement of
the interstellar Rb isotope ratio. Our measurement of the Rb I line at 7800 A
for the diffuse gas toward rho Oph A yields a value of 1.21 +/- 0.30 (1-sigma)
that differs significantly from the meteoritic value of 2.59. The Rb/K
elemental abundance ratio for the cloud also is lower than that seen in
meteorites. Comparison of the 85Rb/K and 87Rb/K ratios with meteoritic values
indicates that the interstellar 85Rb abundance in this direction is lower than
the Solar System abundance. We attribute the lower abundance to a reduced
contribution from the r-process. Interstellar abundances for Kr, Cd, and Sn are
consistent with much less r-process synthesis for the solar neighborhood
compared to the amount inferred for the Solar System.Comment: 12 pages with 2 figures and 1 table; will appear in ApJ Letter
Granulation across the HR diagram
We have obtained ultra-high quality spectra (R=180,000; S/N>300) with
unprecedented wavelength coverage (4400 to 7400 A) for a number of stars
covering most of the HR diagram in order to test the predictions of models of
stellar surface convection. Line bisectors and core wavelength shifts are both
measured and modeled, allowing us to validate and/or reveal the limitations of
state-of-the-art hydrodynamic model atmospheres of different stellar
parameters. We show the status of our project and preliminary results.Comment: 4 pages, 3 figures; proceedings article for Joint Discussion 10 at
the IAU General Assembly, Rio de Janeiro, Brazil, August 200
Nucleosynthesis and mixing on the Asymptotic Giant Branch. III. Predicted and observed s-process abundances
We present the results of s-process nucleosynthesis calculations for AGB
stars of different metallicities and initial masses. The computations were
based on previously published stellar evolutionary models that account for the
III dredge up phenomenon occurring late on the AGB. Neutron production is
driven by the 13C(alpha,n)16O reaction during the interpulse periods in a tiny
layer in radiative equilibrium at the top of the He- and C-rich shell. The
s-enriched material is subsequently mixed with the envelope by the III dredge
up, and the envelope composition is computed after each thermal pulse. We
follow the changes in the photospheric abundance of the Ba-peak elements (heavy
s, or `hs') and that of the Zr-peak ones (light s, or `ls'), whose logarithmic
ratio [hs/ls] has often been adopted as an indicator of the s-process
efficiency. The theoretical predictions are compared with published abundances
of s elements for Galactic AGB giants of classes MS, S, SC, post-AGB
supergiants, and for various classes of binary stars. The observations in
general confirm the complex dependence of n captures on metallicity. They
suggest that a moderate spread exists in the abundance of 13C that is burnt in
different stars. Although additional observations are needed, a good
understanding has been achieved of s-process operation in AGB. The detailed
abundance distribution including the light elements (CNO) of a few s-enriched
stars at different metallicity are examined.Comment: Accepted for ApJ, 59 pages, 19 figures, 5 table
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