862 research outputs found
Cross-Disciplinary Analysis of the On-Farm Transition from Conventional to Organic Vegetable Production
This farm-scale analysis of the three-year transition to organic from conventional vegetable production tracked the changes in crop, soil, pest and management on two ranches (40 and 47 ha) in the Salinas Valley, California. Many small plantings of a diverse set of cash crop and cover crop species were used, as compared to only a few species in large monocultures in conventional production. The general trends with time were: increase in soil biological indicators, low soil nitrate pools, adequate crop nutrients, minor disease and weed problems, and sporadic mild insect damage. Some crops and cultivars consistently produced higher yields than others, relative to the maximum yield for a given crop. Differences in insect and disease damage were also observed. These results support the value of initially using a biodiverse set of taxa to reduce risk, then later choosing the best-suited varieties for optimal production. The grower used some principles of organic farming (e.g., crop diversity, crop rotation, and organic matter management), but also relied on substitution-based management, such as fertigation with soluble nutrients, initially heavy applications of organic pesticides, and use of inputs derived from off-farm sources. The organic transition was conducive to both production goals and environmental quality
The Evolutionary Status of Isolated Dwarf Irregular Galaxies II. Star Formation Histories and Gas Depletion
The results of UBV and H alpha imaging of a large sample of isolated dwarf
irregular galaxies are interpreted in the context of composite stellar
population models. The observed optical colors are best fit by composite
stellar populations which have had approximately constant star formation rates
for at least 10 Gyr. The galaxies span a range of central surface brightness,
from 20.5 to 25.0 mag arcsec^{-2}; there is no correlation between surface
brightness and star formation history. Although the current star formation
rates are low, it is possible to reproduce the observed luminosities without a
major starburst episode. The derived gas depletion timescales are long,
typically ~20 Gyr. These results indicate that dwarf irregular galaxies will be
able to continue with their slow, but constant, star formation activity for at
least another Hubble time.
The sample of isolated dIs is compared to a sample of star bursting dwarf
galaxies taken from the literature. The star bursting dwarf galaxies have many
similar properties; the main difference between these two types of gas-rich
dwarf galaxies is that the current star formation is concentrated in the center
of the star bursting systems while it is much more distributed in the quiescent
dIs. This results in pronounced color gradients for the starbursting dwarf
galaxies, while the majority of the quiescent dwarf irregular galaxies have
minor or non-existent color gradients. Thus, the combination of low current
star formation rates, blue colors, and the lack of significant color gradients
indicates that star formation percolates slowly across the disk of normal dwarf
galaxies in a quasi-continuous manner.Comment: 16 pages, uses emulateapj, to appear in The Astronomical Journal
(April 2001
Timescale of Stellar Feedback-Driven Turbulence in the ISM: A Deep Dive into UGC 4305
Understanding the interplay of stellar feedback and turbulence in the
interstellar medium (ISM) is essential to modeling the evolution of galaxies.
To determine the timescales over which stellar feedback drives turbulence in
the ISM, we performed a spatially resolved, multi-wavelength study of the
nearby star-forming dwarf galaxy UGC 4305 (aka Holmberg II). As indicators of
turbulence on local scales (400 pc), we utilized ionized gas velocity
dispersion derived from IFU H observations and atomic gas velocity
dispersion and energy surface densities derived from HI synthesis observations
with the Very Large Array. These indicators of turbulence were tested against
star formation histories over the past 560 Myr derived from Color-Magnitude
Diagrams (CMD) using Spearman's rank correlation coefficient. The strongest
correlation identified at the 400 pc scale is between measures of HI turbulence
and star formation 70-140 Myr ago. We repeated our analysis of UGC 4305's
current turbulence and past star formation activity on multiple physical scales
(560, and 800 pc) to determine if there are indications of changes in the
correlation timescale with changes to the physical scale. No notable
correlations were found at larger physical scales emphasizing the importance of
analyzing star formation driven turbulence as a local phenomenon.Comment: 17 pages, 9 figure, accepted to A
Does the availability of snack foods in supermarkets vary internationally?
BackgroundCross-country differences in dietary behaviours and obesity rates have been previously reported. Consumption of energy-dense snack foods and soft drinks are implicated as contributing to weight gain, however little is known about how the availability of these items within supermarkets varies internationally. This study assessed variations in the display of snack foods and soft drinks within a sample of supermarkets across eight countries.MethodsWithin-store audits were used to evaluate and compare the availability of potato chips (crisps), chocolate, confectionery and soft drinks. Displays measured included shelf length and the proportion of checkouts and end-of-aisle displays containing these products. Audits were conducted in a convenience sample of 170 supermarkets across eight developed nations (Australia, Canada, Denmark, Netherlands, New Zealand, Sweden, United Kingdom (UK), and United States of America (US)).ResultsThe mean total aisle length of snack foods (adjusted for store size) was greatest in supermarkets from the UK (56.4 m) and lowest in New Zealand (21.7 m). When assessed by individual item, the greatest aisle length devoted to chips, chocolate and confectionery was found in UK supermarkets while the greatest aisle length dedicated to soft drinks was in Australian supermarkets. Only stores from the Netherlands (41%) had less than 70% of checkouts featuring displays of snack foods or soft drinks.ConclusionWhilst between-country variations were observed, overall results indicate high levels of snack food and soft drinks displays within supermarkets across the eight countries. Exposure to snack foods is largely unavoidable within supermarkets, increasing the likelihood of purchases and particularly those made impulsively.<br /
Markarian 421's Unusual Satellite Galaxy
We present Hubble Space Telescope (HST) imagery and photometry of the active
galaxy Markarian 421 and its companion galaxy 14 arcsec to the ENE. The HST
images indicate that the companion is a morphological spiral rather than
elliptical as previous ground--based imaging has concluded. The companion has a
bright, compact nucleus, appearing unresolved in the HST images. This is
suggestive of Seyfert activity, or possibly a highly luminous compact star
cluster. We also report the results of high dynamic range long-slit
spectroscopy with the slit placed to extend across both galaxies and nuclei. We
detect no emission lines in the companion nucleus, though there is evidence for
recent star formation. Velocities derived from a number of absorption lines
visible in both galaxies indicate that the two systems are probably tidally
bound and thus in close physical proximity. Using the measured relative
velocities, we derive a lower limit on the MKN 421 mass within the companion
orbit (R \sim 10 kpc) of 5.9 \times 10^{11} solar masses, and a mass-to-light
ratio of >= 17. Our spectroscopy also shows for the first time the presence of
H\alpha and [NII] emission lines from the nucleus of MKN 421, providing another
example of the appearance of new emission features in the previously
featureless spectrum of a classical BL Lac object. We see both broad and narrow
line emission, with a velocity dispersion of several thousand km s^{-1} evident
in the broad lines.Comment: LaTeX (aaspp4 style), 28 pages, 8 figures, to appear in AJ. Revised
text from ref. comments; new & modified figures; new photometry included;
minor corrections of typos. Color version of Fig. 1 to appear in Feb. 2000
Sky & Telescop
Departures From Axisymmetric Morphology and Dynamics in Spiral Galaxies
New HI synthesis data have been obtained for six face-on galaxies with the
Very Large Array. These data and reanalyses of three additional data sets make
up a sample of nine face-on galaxies analyzed for deviations from axisymmetry
in morphology and dynamics. This sample represents a subsample of galaxies
already analyzed for morphological symmetry properties in the R-band. Four
quantitative measures of dynamical nonaxisymmetry are compared to one another
and to the quantitative measures of morphological asymmetry in HI and R-band to
investigate the relationships between nonaxisymmetric morphology and dynamics.
We find no significant relationship between asymmetric morphology and most of
the dynamical measures in our sample. A possible relationship is found,
however, between morphology and dynamical position angle differences between
approaching and receding sides of the galaxy.Comment: 24 pages, 19 figures, AASTeX, accepted for publication in AJ,
postscript figures available at
ftp://culebra.tn.cornell.edu/pub/david/figures.tar.g
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High Efficiency Approaches to Coal Syngas Use in Fuel Cell Systems with CO2 Isolation
An evaluation of edge effects in nutritional accessibility and availability measures: a simulation study
<p>Abstract</p> <p>Background</p> <p>This paper addresses the statistical use of accessibility and availability indices and the effect of study boundaries on these measures. The measures are evaluated via an extensive simulation based on cluster models for local outlet density. We define outlet to mean either food retail store (convenience store, supermarket, gas station) or restaurant (limited service or full service restaurants). We designed a simulation whereby a cluster outlet model is assumed in a large study window and an internal subset of that window is constructed. We performed simulations on various criteria including one scenario representing an urban area with 2000 outlets as well as a non-urban area simulated with only 300 outlets. A comparison is made between estimates obtained with the full study area and estimates using only the subset area. This allows the study of the effect of edge censoring on accessibility measures.</p> <p>Results</p> <p>The results suggest that considerable bias is found at the edges of study regions in particular for accessibility measures. Edge effects are smaller for availability measures (when not smoothed) and also for short range accessibility</p> <p>Conclusions</p> <p>It is recommended that any study utilizing these measures should correct for edge effects. The use of edge correction via guard areas is recommended and the avoidance of large range distance-based accessibility measures is also proposed.</p
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Calculating the Energy Cost of CO2 Removal in a Coal Based Gas Turbine Fuel Cell Hybrid Power Generation System with an Isolated Anode Stream
In recent years there has been significant interest in identifying carbon capturing technologies that can be applied to fossil fuel power generation plants.CO2 capture technologies seek to reduce the amount of CO2 that would normally be emitted into the atmosphere from the daily operation of these plants. In terms of system efficiency and operating costs, this carbon capture is expensive. Further, the additional equipment that would be used to capture CO2 emissions greatly adds to the complexity of the system. There has also been significant interest in coal based gas turbine fuel cell hybrid power plants. A hybrid power plant can have much greater system efficiency than a normal gas turbine power plant because the heat that is normally unused in a standalone solid oxide fuel cell (SOFC) is recovered and used to drive a power producing turbine. It is thought that the increased system efficiency of the hybrid system might compensate for the increased expense of performing carbon capture. In order to provide some analytical insight on this tradeoff we present a 100 MW class coal fired gas turbine SOFC hybrid power generation system. The hybrid system operates at a pressure ratio of 6, and uses heat recuperation and cathode air recirculation to control the SOFC inlet temperature and the temperature change across the SOFC. A carbon capture scheme is added to this system in order to calculate the relative energy cost in terms of system efficiency due to CO2 ompression. The carbon capture is performed by burning the unused fuel from the SOFC in an isolated anode stream using pure O2 injection. The resulting heat that is generated from this process is then used to drive a secondary turbine that is placed in the anode exhaust stream where more work is extracted. With an isolated anode stream, the products of combustion from this secondary combustion process are mostly water and carbon dioxide. The water by-product is then condensed out of the stream leaving a relatively high concentration of CO2. This is then compressed, and removed from the system. In this study we present power plant efficiency calculations for the performance of the hybrid system with the carbon capturing loop. Our results show the effects on system performance that result from a changing fuel utilization factor
Oxygen and Nitrogen in Leo A and GR 8
We present elemental abundances for multiple HII regions in Leo A and GR 8
obtained from long slit optical spectroscopy of these two nearby low luminosity
dwarf irregular galaxies. As expected from their luminosities, and in agreement
with previous observations, the derived oxygen abundances are extremely low in
both galaxies. High signal-to-noise ratio observations of a planetary nebula in
Leo A yield 12 + log(O/H) = 7.30 +/- 0.05; "semi-empirical" calculations of the
oxygen abundance in four HII regions in Leo A indicate 12 + log(O/H) = 7.38 +/-
0.10. These results confirm that Leo A has one of the lowest ISM metal
abundances of known nearby galaxies. Based on results from two HII regions with
high signal-to-noise measurements of the weak [O III] 4363 line, the mean
oxygen abundance of GR 8 is 12 + log(O/H) = 7.65 +/- 0.06; using "empirical"
and "semi-empirical" methods, similar abundances are derived for 6 other GR 8
HII regions. Similar to previous results in other low metallicity galaxies, the
mean log(N/O) = -1.53 +/- 0.09 for Leo A and -1.51 +/- 0.07 for GR 8. There is
no evidence of significant variations in either O/H or N/O in the HII regions.
The metallicity-luminosity relation for nearby (D < 5 Mpc) dwarf irregular
galaxies with measured oxygen abundances has a mean correlation of 12 +
log(O/H) = 5.67 - 0.151 M_B with a dispersion in oxygen about the relationship
of 0.21. These observations confirm that gas-rich low luminosity galaxies have
extremely low elemental abundances in the ionized gas-phase of their
interstellar media. Although Leo A has one of the lowest metal abundances of
known nearby galaxies, detection of tracers of an older stellar population
indicate that it is not a newly formed galaxy as has been proposed for some
other similarly low metallicity star forming galaxies.Comment: 36 pages, 7 figures. Accepted to Ap
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