210 research outputs found
On the observability of T Tauri accretion shocks in the X-ray band
Context. High resolution X-ray observations of classical T Tauri stars
(CTTSs) show a soft X-ray excess due to high density plasma (n_e=10^11-10^13
cm^-3). This emission has been attributed to shock-heated accreting material
impacting onto the stellar surface. Aims. We investigate the observability of
the shock-heated accreting material in the X-ray band as a function of the
accretion stream properties (velocity, density, and metal abundance) in the
case of plasma-beta<<1 in the post-shock zone. Methods. We use a 1-D
hydrodynamic model describing the impact of an accretion stream onto the
chromosphere, including the effects of radiative cooling, gravity and thermal
conduction. We explore the space of relevant parameters and synthesize from the
model results the X-ray emission in the [0.5-8.0] keV band and in the resonance
lines of O VII (21.60 Ang) and Ne IX (13.45 Ang), taking into account the
absorption from the chromosphere. Results. The accretion stream properties
influence the temperature and the stand-off height of the shocked slab and its
sinking in the chromosphere, determining the observability of the shocked
plasma. Our model predicts that X-ray observations preferentially detect
emission from low density and high velocity shocked accretion streams due to
the large absorption of dense post-shock plasma. In all the cases examined, the
post-shock zone exhibits quasi-periodic oscillations due to thermal
instabilities, but in the case of inhomogeneous streams and beta<<1, the shock
oscillations are hardly detectable. Conclusions. We suggest that, if accretion
streams are inhomogeneous, the selection effect introduced by the absorption on
observable plasma components may explain the discrepancy between the accretion
rate measured by optical and X-ray data as well as the different densities
measured using different He-like triplets in the X-ray band.Comment: 12 pages, 7 figures. Accepted for publication on A&
Challenging claims in the study of migratory birds and climate change
Recent shifts in phenology in response to climate change are well established but often poorly understood. Many animals integrate climate change across a spatially and temporally dispersed annual life cycle, and effects are modulated by ecological interactions, evolutionary change and endogenous control mechanisms. Here we assess and discuss key statements emerging from the rapidly developing study of changing spring phenology in migratory birds. These well-studied organisms have been instrumental for understanding climate-change effects, but research is developing rapidly and there is a need to attack the big issues rather than risking affirmative science. Although we agree poorly on the support for most claims, agreement regarding the knowledge basis enables consensus regarding broad patterns and likely causes. Empirical data needed for disentangling mechanisms are still scarce, and consequences at a population level and on community composition remain unclear. With increasing knowledge, the overall support (âconsensus viewâ) for a claim increased and between-researcher variability in support (âexpert opinions') decreased, indicating the importance of assessing and communicating the knowledge basis. A proper integration across biological disciplines seems essential for the field's transition from affirming patterns to understanding mechanisms and making robust predictions regarding future consequences of shifting phenologies
Some formal results for the valence bond basis
In a system with an even number of SU(2) spins, there is an overcomplete set
of states--consisting of all possible pairings of the spins into valence
bonds--that spans the S=0 Hilbert subspace. Operator expectation values in this
basis are related to the properties of the closed loops that are formed by the
overlap of valence bond states. We construct a generating function for spin
correlation functions of arbitrary order and show that all nonvanishing
contributions arise from configurations that are topologically irreducible. We
derive explicit formulas for the correlation functions at second, fourth, and
sixth order. We then extend the valence bond basis to include triplet bonds and
discuss how to compute properties that are related to operators acting outside
the singlet sector. These results are relevant to analytical calculations and
to numerical valence bond simulations using quantum Monte Carlo, variational
wavefunctions, or exact diagonalization.Comment: 22 pages, 14 figure
PCBs and dioxins/furans in attic dust collected near former PCB production and secondary copper facilities in Sauget, IL
AbstractSamples of settled attic dust from fourteen buildings located within two miles of the Solutia W.G. Krummrich and Cerro Flow Products facilities in Sauget, Illinois were analyzed for PCBs and dioxins/furans using HRGC/HRMS. The facilities released vast quantities of PCBs and dioxins/furans into the environment over many decades. The concentrations and homologues present in the samples of attic dust and in samples of soil collected by U.S. EPA demonstrate atmospheric transport of PCBs and dioxins/furans from these manufacturing sites and local dumps contaminated with these pollutants. The results demonstrate that attic dust is a useful metric for assessing historical exposure to atmospheric emissions
The History and Future of the Local and Loop I Bubbles
The Local and Loop I superbubbles are the closest and best investigated
supernova (SN) generated bubbles and serve as test laboratories for
observations and theories of the interstellar medium. Since the morphology and
dynamical evolution of bubbles depend on the ambient density and pressure
distributions, a realistic modelling of the galactic environment is crucial for
a detailed comparison with observations. We have performed 3D high resolution
(down to 1.25 pc on a kpc-scale grid) hydrodynamic simulations of the Local
Bubble (LB) and the neighbouring Loop I (L1) superbubble in a realistically
evolving inhomogeneous background ISM, disturbed already by SN explosions at
the Galactic rate for 200 Myr before the LB and L1 are generated. The LB is the
result of 19 SNe occurring in a moving group, which passed through the present
day local HI cavity. We can reproduce (i) the OVI column density in absorption
within the LB in agreement with COPERNICUS and recent FUSE observations, giving
N(OVI) <2 10^{13} cm^-2 and N(OVI)<7 10^{12} cm^-2, respectively, (ii) the
observed sizes of the Local and Loop I superbubbles, (iii) the interaction
shell between LB and L1, discovered with ROSAT, (iv) constrain the age of the
LB to be 14.5+0.7/-0.4 Myr, (v) predict the merging of the two bubbles in about
3 Myr, when the interaction shell starts to fragment, (vi) the generation of
blobs like the Local Cloud as a consequence of a dynamical instability. We find
that evolving superbubbles strongly deviate from idealised self-similar
solutions due to ambient pressure and density gradients, as well as due to
turbulent mixing and mass loading. Hence, at later times the hot interior can
break through the surrounding shell, which may also help to explain the
puzzling energy "deficit" observed in LMC bubbles.Comment: Accepted for publication in Astronomy and Astrophysics Letters. The
paper contains 5 pages and 11 figures. Fig. 1a replaced by correct figur
A new radiative cooling curve based on an up to date plasma emission code
This work presents a new plasma cooling curve that is calculated using the
SPEX package. We compare our cooling rates to those in previous works, and
implement the new cooling function in the grid-adaptive framework `AMRVAC'.
Contributions to the cooling rate by the individual elements are given, to
allow for the creation of cooling curves tailored to specific abundance
requirements. In some situations, it is important to be able to include
radiative losses in the hydrodynamics. The enhanced compression ratio can
trigger instabilities (such as the Vishniac thin-shell instability) that would
otherwise be absent. For gas with temperatures below 10,000 K, the cooling time
becomes very long and does not affect the gas on the timescales that are
generally of interest for hydrodynamical simulations of circumstellar plasmas.
However, above this temperature, a significant fraction of the elements is
ionised, and the cooling rate increases by a factor 1000 relative to lower
temperature plasmas.Comment: 11 pages, 6 figures. Typos fixed to match version on A&A
'forthcoming' website. Tables in text format online available at
http://www.phys.uu.nl/~schure/coolin
Mass distribution in the most X-ray-luminous galaxy cluster RX J1347.5-1145 studied with XMM-Newton
We report on the analysis of XMM-Newton observations of RX J1347.5-1145
(z=0.451), the most X-ray-luminous galaxy cluster. We present a detailed total
and gas mass determination up to large distances (~1.7 Mpc), study the scaling
properties of the cluster, and explore the role of AGN heating in the cluster
cool core. By means of spatially resolved spectroscopy we derive density,
temperature, entropy, and cooling time profiles of the intra-cluster medium. We
compute the total mass profile of the cluster in the assumption of hydrostatic
equilibrium. If the disturbed south-east region of the cluster is excluded from
the analysis, our results on shape, normalization, scaling properties of
density, temperature, entropy, and cooling time profiles are fully consistent
with those of relaxed, cool core clusters. We compare our total and gas mass
estimates with previous X-ray, lensing, dynamical, and SZ studies. We find good
agreement with other X-ray results, dynamical mass measurements, weak lensing
masses and SZ results. We confirm a discrepancy of a factor ~2 between strong
lensing and X-ray mass determinations and find a gross mismatch between our
total mass estimate and the mass reconstructed through the combination of both
strong and weak lensing. We explore the effervescent heating scenario in the
core of RX J1347.5-1145 and find support to the picture that AGN outflows and
heat conduction are able to quenching radiative cooling.Comment: 12 pages, 6 figures, accepted for publication in A&
The effect of feedback on the emission properties of the Warm-Hot Intergalactic Medium
At present, 30-40 per cent of the baryons in the local Universe is still
undetected. According to theoretical predictions, this gas should reside in
filaments filling the large-scale structure (LSS) in the form of a Warm-Hot
Intergalactic Medium (WHIM), at temperatures of 10^5 - 10^7 K, thus emitting in
the soft X-ray energies via free-free interaction and line emission from heavy
elements. In this work we characterize the properties of the X-ray emission of
the WHIM, and the LSS in general, focusing on the influence of different
physical mechanisms, namely galactic winds (GWs), black-hole feedback and
star-formation, and providing estimates of possible observational constraints.
To this purpose we use a set of cosmological hydrodynamical simulations that
include a self-consistent treatment of star-formation and chemical enrichment
of the intergalactic medium, that allows us to follow the evolution of
different metal species. We construct a set of simulated light-cones to make
predictions of the emission in the 0.3-10 keV energy range. We obtain that GWs
increase by a factor of 2 the emission of both galaxy clusters and WHIM. The
amount of oxygen at average temperature and, consequently, the amount of
expected bright Ovii and Oviii lines is increased by a factor of 3 due to GWs
and by 20 per cent when assuming a top-heavy IMF. We compare our results with
current observational constraints and find that the emission from faint groups
and WHIM should account from half to all of the unresolved X-ray background in
the 1-2 keV band.Comment: 15 pages, 8 figures, 4 tables. Accepted for publication in the MNRAS.
Minor changes after referee repor
- âŠ