4,197 research outputs found
Empirical logic of finite automata: microstatements versus macrostatements
We compare the two approaches to the empirical logic of automata. The first,
called partition logic (logic of microstatements), refers to experiments on
individual automata. The second one, the logic of simulation (logic of
macrostatements), deals with ensembles of automata.Comment: late
Quantifying the impact of emission outbursts and non-stationary flow on eddy covariance CH<sub>4</sub> flux measurements using wavelet techniques
Methane flux measurements by the eddy-covariance technique are subject to large uncertainties, particularly linked to the partly highly intermittent nature of methane emissions. Outbursts of high methane emissions, termed event fluxes, hold the potential to introduce systematic biases into derived methane budgets, since under such conditions the assumption of stationarity of the flow is violated. In this study, we investigate the net impact of this effect by comparing eddy-covariance fluxes against a wavelet-derived reference that is not negatively influenced by non-stationarity. Our results demonstrate that methane emission events influenced 3–4 % of the flux measurements, and did not lead to systematic biases in methane budgets for the analyzed summer season; however, the presence of events substantially increased uncertainties in short-term flux rates. The wavelet results provided an excellent reference to evaluate the performance of three different gapfilling approaches for eddy-covariance methane fluxes, and we show that none of them could reproduce the range of observed flux rates. The integrated performance of the gapfilling methods for the longer-term dataset varied between the two eddy-covariance towers involved in this study, and we show that gapfilling remains a large source of uncertainty linked to limited insights into the mechanisms governing the short-term variability in methane emissions. With the capability to broaden our observational methane flux database to a wider range of conditions, including the direct resolution of short term variability at the order of minutes, wavelet-derived fluxes hold the potential to generate new insight into methane exchange processes with the atmosphere, and therefore also improve our understanding of the underlying processes
On Iron Enrichment, Star Formation, and Type Ia Supernovae in Galaxy Clusters
The nature of star formation and Type Ia supernovae (SNIa) in galaxies in the
field and in rich galaxy clusters are contrasted by juxtaposing the build-up of
heavy metals in the universe inferred from observed star formation and
supernovae rate histories with data on the evolution of Fe abundances in the
intracluster medium (ICM). Models for the chemical evolution of Fe in these
environments are constructed, subject to observational constraints, for this
purpose. While models with a mean delay for SNIa of 3 Gyr and standard initial
mass function (IMF) are consistent with observations in the field, cluster Fe
enrichment immediately tracks a rapid, top-heavy phase of star formation --
although transport of Fe into the ICM may be more prolonged and star formation
likely continues to redshifts <1. The source of this prompt enrichment is Type
II supernovae (SNII) yielding at least 0.1 solar masses per explosion (if the
SNIa rate normalization is scaled down from its value in the field according to
the relative number of candidate progenitor stars in the 3-8 solar mass range)
and/or SNIa explosions with short delay times associated with the rapid star
formation mode. Star formation is >3 times more efficient in rich clusters than
in the field, mitigating the overcooling problem in numerical cluster
simulations. Both the fraction of baryons cycled through stars, and the
fraction of the total present-day stellar mass in the form of stellar remnants,
are substantially greater in clusters than in the field.Comment: 51 pages including 26 figures and 2 tables, accepted for publication
in ApJ 5/4/0
Conventional and organic-related farming systems research: an assessment of USDA and state research projects
The assessment of research made in this study provides the best estimate available of expenditures by the USD A and state agricultural experiment stations for research related to farming systems in general and to farming systems of an organic nature. Information for the study was provided by the Current Research Information Systems (CRIS), USDA. It was based on fiscal year 1982 information because, at the time of the study, complete data were available only for that period. The procedure developed, however, can be used to update assessments for later years.https://lib.dr.iastate.edu/specialreports/1087/thumbnail.jp
Life Products of Stars
We attempt to document complete energetic transactions of stars in their
life. We calculate photon and neutrino energies that are produced from stars in
their each phase of evolution from 1 to 8 M_sun, using the state-of-the-art
stellar evolution code, tracing the evolution continuously from pre-main
sequence gravitational contraction to white dwarfs. We also catalogue
gravitational and thermal energies and helium, and heavier elements that are
stored in stars and those ejected into interstellar space in each evolutionary
phase.Comment: 26 pages, including 8 figures and 3 tables. Submitted to ApJ
Arctic soil CO<sub>2</sub> release during freeze-thaw cycles modulated by silicon and calcium
Arctic soils are the largest pool of soil organic carbon worldwide. Temperatures in the Arctic have risen faster than the global average during the last decades, decreasing annual freezing days and increasing the number of freeze-thaw cycles (temperature oscillations passing through zero degrees) per year as the temperature is expected to fluctuate more around 0 °C. At the same time, proceeding deepening of seasonal thaw may increase silicon (Si) and calcium (Ca) concentrations in the active layer of Arctic soils as the concentrations in the thawing permafrost layer might be higher depending on location. We analyzed the importance of freeze-thaw cycles for Arctic soil CO2 fluxes. Furthermore, we tested how Si (mobilizing organic C) and Ca (immobilizing organic C) interfere with the soil CO2 fluxes in the context of freeze-thaw cycles. Our results show that with each freeze-thaw cycle the CO2 fluxes from the Arctic soils decreased. Our data revealed a considerable CO2 emission below 0 °C. We also show that pronounced differences emerge in Arctic soil CO2 fluxes with Si increasing and Ca decreasing CO2 fluxes. Furthermore, we show that both Si and Ca concentrations in Arctic soils are central controls on Arctic soil CO2 release, with Si increasing Arctic soil CO2 release especially when temperatures are just below 0 °C. Our findings could provide an important constraint on soil CO2 emissions upon soil thaw, as well as on the greenhouse gas budget of high latitudes. Thus we call for work improving understanding of freeze-thaw cycles as well as the effect of Ca and Si on carbon fluxes, as well as for increased consideration of those factors in wide-scale assessments of carbon fluxes in the high latitudes
B stars as a diagnostic of star-formation at low and high redshift
We have extended the evolutionary synthesis models by Leitherer et al.
(1999b) by including a new library of B stars generated from the IUE
high-dispersion spectra archive. We present the library and show how the
stellar spectral properties vary according to luminosity classes and spectral
types. We have generated synthetic UV spectra for prototypical young stellar
populations varying the IMF and the star formation law. Clear signs of age
effects are seen in all models. The contribution of B stars in the UV line
spectrum is clearly detected, in particular for greater ages when O stars have
evolved. With the addition of the new library we are able to investigate the
fraction of stellar and interstellar contributions and the variation in the
spectral shapes of intense lines. We have used our models to date the spectrum
of the local super star cluster NGC1705-1. Photospheric lines of CIII1247,
SiIII1417, and SV1502 were used as diagnostics to date the burst of NGC 1705-1
at 10 Myr. We have selected the star-forming galaxy 1512-cB58 as a first
application of the new models to high-z galaxies. This galaxy is at z=2.723, it
is gravitationally lensed, and its high signal-to-noise Keck spectrum show
features typical of local starburst galaxies, such as NGC 1705-1. Models with
continuous star formation were found to be more adequate for 1512-cB58 since
there are spectral features typical of a composite stellar population of O and
B stars. A model with Z =0.4Z_solar and an IMF with alpha=2.8 reproduces the
stellar features of the 1512-cB58 spectrum.Comment: 23 pages with figures, see
http://sol.stsci.edu/~demello/welcomeb.htm
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