3,456 research outputs found
Ergodic to Non-ergodic Behavior Transitions and Hysteresis in Ecosystem Models
A widely used concept in natural sciences is the ergodic principle stating that the temporal average state of system equals the average of single states of an ensemble of the system. Originally formulated by Boltzmann to describe the physics of an ideal gas, the ergodic principle was and still is applied in, for example, the assessment of developmental aspects of individuals, but also in the growth series concept of whole ecosystems.
Hysteresis on the other hand describes the observable contrary of the ergodic principle, that is, that the current state of a system strictly depends on the individual temporal development steps, or that individual history is unequivocally important.
This work will provide evidence for ergodic to non-ergodic transitions in the application of biogeochemical ecosystem models using the showcase of Congo Basin rainforests. Using a climate gradient from west to east, ergodic model behavior is shown for a virgin forest refuge, non-ergodic behavior for a current forest savannah mosaic and ergodic behavior again for large open savannahs. Additionally, the occurrence of hysteresis related to the prevailing initial vegetation will be demonstrated, whereby the non-ergodic phase along the climate gradient is shown to be more extensive if rainforest was the original vegetation, as opposed to rainforest establishing on non-forest sites.
The combination of ergodic to non-ergodic transitions—sometimes referred to as catastrophic shifts in ecosystems—will be put into context with the changing distribution patterns of rainforest and savannah over the course of the Holocene
Investigation of the Influence of Plastic Evaporation on Pressure Rise due to Fault Arcs in Electrical Installations
Internal arcs in electrical installations cause a sudden pressure rise due to heating of the gas surroundingthe arc. The pressure rise may endanger personnel, the electrical installation and even the switchgearbuilding. Based on the need to design switchgear more compact, plastic materials are used. The interactionof the arc with these materials releases gases by evaporation and thereby influences the pressure risewithin the installation as well as the energy balance of the arc. In order to analyse these effects in detail,some commonly used plastics are exposed to a high current arc. In this contribution the pressure developmentin a closed vessel equipped with different plastic materials, their mass losses and the portion ofelectrical energy causing overpressure, known as thermal transfer coefficient or kp–factor, are determinedat three values of the filling pressure of the vessel. The plastic materials under investigation are epoxy,polyamide 6.6, polybutyleneterephtalate, polycarbonate and silicone rubber. With the information provided,the influence of the different plastics on pressure development can be predicted
Swift J053041.9-665426, a new Be/X-ray binary pulsar in the Large Magellanic Cloud
We observed the newly discovered X-ray source Swift J053041.9-665426 in the
X-ray and optical regime to confirm its proposed nature as a high mass X-ray
binary. We obtained XMM-Newton and Swift X-ray data, along with optical
observations with the ESO Faint Object Spectrograph, to investigate the
spectral and temporal characteristics of Swift J053041.9-665426. The XMM-Newton
data show coherent X-ray pulsations with a period of 28.77521(10) s (1 sigma).
The X-ray spectrum can be modelled by an absorbed power law with photon index
within the range 0.76 to 0.87. The addition of a black body component increases
the quality of the fit but also leads to strong dependences of the photon
index, black-body temperature and absorption column density. We identified the
only optical counterpart within the error circle of XMM-Newton at an angular
distance of ~0.8 arcsec, which is 2MASS J05304215-6654303. We performed optical
spectroscopy from which we classify the companion as a B0-1.5Ve star. The X-ray
pulsations and long-term variability, as well as the properties of the optical
counterpart, confirm that Swift J053041.9-665426 is a new Be/X-ray binary
pulsar in the Large Magellanic Cloud.Comment: 10 pages, 8 figures, accepted for publication in A&
Science with the EXTraS Project: Exploring the X-ray Transient and variable Sky
The EXTraS project (Exploring the X-ray Transient and variable Sky) will
characterise the temporal behaviour of the largest ever sample of objects in
the soft X-ray range (0.1-12 keV) with a complex, systematic and consistent
analysis of all data collected by the European Photon Imaging Camera (EPIC)
instrument onboard the ESA XMM-Newton X-ray observatory since its launch. We
will search for, and characterize variability (both periodic and aperiodic) in
hundreds of thousands of sources spanning more than nine orders of magnitude in
time scale and six orders of magnitude in flux. We will also search for fast
transients, missed by standard image analysis. Our analysis will be completed
by multiwavelength characterization of new discoveries and phenomenological
classification of variable sources. All results and products will be made
available to the community in a public archive, serving as a reference for a
broad range of astrophysical investigations.Comment: 4 pages, 1 figure. Refereed Proceeding of "The Universe of Digital
Sky Surveys" conference held at the INAF - Observatory of Capodimonte,
Naples, on 25th-28th November 2014, to be published in the Astrophysics and
Space Science Proceedings, edited by Longo, Napolitano, Marconi, Paolillo,
Iodic
A new super-soft X-ray source in the Small Magellanic Cloud: Discovery of the first Be/white dwarf system in the SMC?
The Small Magellanic Cloud (SMC) hosts a large number of Be/X-ray binaries,
however no Be/white dwarf system is known so far, although population synthesis
calculations predict that they might be more frequent than Be/neutron star
systems. XMMUJ010147.5-715550 was found as a new faint super-soft X-ray source
(SSS) with a likely Be star optical counterpart. We investigate the nature of
this system and search for further high-absorbed candidates in the SMC. We
analysed the XMM-Newton X-ray spectrum and light curve, optical photometry, and
the I-band OGLE III light curve. The X-ray spectrum is well represented by
black-body and white dwarf atmosphere models with highly model-dependent
temperature between 20 and 100 eV. The likely optical counterpart AzV 281
showed low near infrared emission during X-ray activity, followed by a
brightening in the I-band afterwards. We find further candidates for
high-absorbed SSSs with a blue star as counterpart. We discuss
XMMUJ010147.5-715550 as the first candidate for a Be/white dwarf binary system
in the SMC.Comment: 6 pages, 4 figures, accepted by A&
Probabilistic Spatial and Temporal Resilience Landscapes for the Congo Basin
Recent research by Hirota et al. (2011) introduced the concept of resilience landscapes for tropical forests and savannahs. Basically, the approach statistically relates the probability of current forest/savannah occurrence with the concept of tipping points, at which the ecosystem has no other choice except to switch from on stable state (e.g., forest) to its alternative stable state (e.g., savannah) or vice versa.
This work will use a biogeochemical modelling approach to establish such probabilistic resilience landscapes for the Congo Basin rainforest biome. In a first step, the occurrence of tipping points will be related to climate features like annual precipitation, dry season length, occurrence of startiform non-precipitating cloud cover and the inter-annual variation in precipitation. In the second, spatial resilience landscapes for the Congo Basin will be provided using present climate conditions. Their relation to current forest/savannah distribution will be assessed and evident congruencies and discrepancies will be discussed. In a third step, the concept of temporal resilience landscapes will be developed along the patch-level life cycle dynamics of the Congo Basin rainforest biome. In a final step, the implications of results for ecosystem management decision will be assessed and possible implications on policy and land-use decisions will be presented
Ferromagnetism or slow paramagnetic relaxation in Fe-doped LiN?
We report on isothermal magnetization, M\"ossbauer spectroscopy, and
magnetostriction as well as temperature-dependent alternating-current (ac)
susceptibility, specific heat, and thermal expansion of single crystalline and
polycrstalline Li(LiFe)N with and .
Magnetic hysteresis emerges at temperatures below K with
coercivity fields of up to T at K and magnetic
anisotropy energies of K (meV). The ac susceptibility is strongly
frequency dependent (--Hz) and reveals an effective energy
barrier for spin reversal of K. The relaxation times
follow Arrhenius behavior for K. For K, however, the
relaxation times of s are only weakly
temperature-dependent indicating the relevance of a quantum tunneling process
instead of thermal excitations. The magnetic entropy amounts to more than
J molK which significantly exceeds ln2, the
value expected for the entropy of a ground state doublet. Thermal expansion and
magnetostriction indicate a weak magneto-elastic coupling in accordance with
slow relaxation of the magnetization. The classification of
Li(LiFe)N as ferromagnet is stressed and contrasted with highly
anisotropic and slowly relaxing paramagnetic behavior.Comment: 12 pages, 10 figure
Discovery of pulsations from the Be/X-ray binary RX J0101.3-7211 in the SMC by XMM-Newton
We report pulsations in the X-ray flux of RX J0101.3-7211 in the Small
Magellanic Cloud (SMC) with a period of 455+/-2 s in XMM-Newton EPIC-PN data.
The X-ray spectrum can be described by a power-law with a photon index of
0.6+/-0.1. Timing analysis of ROSAT PSPC and HRI archival data confirms the
pulsations and indicates a period increase of ~5 s since 1993. RX J0101.3-7211
varied in brightness during the ROSAT observations with timescales of years
with a maximum unabsorbed flux of 6 x 10^-13 erg cm^-2 s^-1 (0.1 - 2.4 keV).
The flux during the XMM-Newton observation in the ROSAT band was lower than
during the faintest ROSAT detection. The unabsorbed luminosity derived from the
EPIC-PN spectrum is 2 x 10^35 erg s^-1 (0.2 - 10.0 keV) assuming a distance of
60 kpc. Optical spectra of the proposed counterpart taken at the 2.3 m
telescope of MSSSO in Australia in August 2000 show strong Halpha emission and
indicate a Be star. The X-ray and optical data confirm RX J0101.3-7211 as a
Be/X-ray binary pulsar in the SMC.Comment: letter accepted by A&
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