1,499 research outputs found
All-Electron Path Integral Monte Carlo Simulations of Warm Dense Matter: Application to Water and Carbon Plasmas
We develop an all-electron path integral Monte Carlo (PIMC) method with
free-particle nodes for warm dense matter and apply it to water and carbon
plasmas. We thereby extend PIMC studies beyond hydrogen and helium to elements
with core electrons. PIMC pressures, internal energies, and pair-correlation
functions compare well with density functional theory molecular dynamics
(DFT-MD) at temperatures of (2.5-7.5) K and both methods together
form a coherent equation of state (EOS) over a density-temperature range of
3--12 g/cm and 10--10 K
Thermodynamics of hot dense H-plasmas: Path integral Monte Carlo simulations and analytical approximations
This work is devoted to the thermodynamics of high-temperature dense hydrogen
plasmas in the pressure region between and Mbar. In particular
we present for this region results of extensive calculations based on a
recently developed path integral Monte Carlo scheme (direct PIMC). This method
allows for a correct treatment of the thermodynamic properties of hot dense
Coulomb systems. Calculations were performed in a broad region of the
nonideality parameter and degeneracy parameter . We give a comparison with a few available results from
other path integral calculations (restricted PIMC) and with analytical
calculations based on Pade approximations for strongly ionized plasmas. Good
agreement between the results obtained from the three independent methods is
found.Comment: RevTex file, 21 pages, 5 ps-figures include
SPEAR Far Ultraviolet Spectral Images of the Cygnus Loop
We present far-ultraviolet (FUV) spectral images, measured at C IV 1550, He
II 1640, Si IV+O IV] 1400, and O III] 1664, of the entire Cygnus Loop, observed
with the Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR)
instrument, also known as FIMS. The spatial distribution of FUV emission
generally corresponds with a limb-brightened shell, and is similar to optical,
radio and X-ray images. The features found in the present work include a
``carrot'', diffuse interior, and breakout features, which have not been seen
in previous FUV studies. Shock velocities of 140-160 km/s is found from a line
ratio of O IV] to O III], which is insensitive not only to resonance scattering
but also to elemental abundance. The estimated velocity indicates that the fast
shocks are widespread across the remnant. By comparing various line ratios with
steady-state shock models, it is also shown that the resonance scattering is
widespread.Comment: 13 pages, 3 figures, 1 table, accepted for publication in ApJ
Discovery of an overdensity of faint red galaxies in the vicinity of the z=1.786 radio galaxy 3C 294
We report the discovery of an overdensity of faint red galaxies in the
vicinity of the z=1.786 radio galaxy 3C 294. The overdensity, discovered in a
84 min Ks-band ISAAC/VLT image is significant at the 2.4 sigma level (compared
to the local field density), and overlaps with the extended X-ray emission
around 3C 294 detected with the Chandra X-ray Observatory. The near-infrared
colours of the galaxies making up the overdensity show a large scatter and the
galaxies do not follow a red sequence in the colour magnitude diagram. If the
galaxies are in a cluster at z=1.786 they must be dominated by young stellar
populations with different star-formation histories.Comment: Accepted for publication in MNRA
Measurements of Sunyaev-Zel'dovich Effect Scaling Relations for Clusters of Galaxies
We present new measurements of the Sunyaev-Zel'dovich (SZ) effect from
clusters of galaxies using the Sunyaev-Zel'dovich Infrared Experiment (SuZIE
II). We combine these new measurements with previous cluster observations with
the SuZIE instrument to form a sample of 15 clusters of galaxies. For this
sample we calculate the central Comptonization, y, and the integrated SZ flux
decrement, S, for each of our clusters. We find that the integrated SZ flux is
a more robust observable derived from our measurements than the central
Comptonization due to inadequacies in the spatial modelling of the
intra-cluster gas with a standard Beta model. This is highlighted by comparing
our central Comptonization results with values calculated from measurements
using the BIMA and OVRO interferometers. On average, the SuZIE calculated
central Comptonizations are approximately 60% higher in the cooling flow
clusters than the interferometric values, compared to only approximately 12%
higher in the non-cooling flow clusters. We believe this discrepancy to be in
large part due to the spatial modelling of the intra-cluster gas. From our
cluster sample we construct y-T and S-T scaling relations. The y-T scaling
relation is inconsistent with what we would expect for self-similar clusters;
however this result is questionable because of the large systematic uncertainty
in the central Comptonization. The S-T scaling relation has a slope and
redshift evolution consistent with what we expect for self-similar clusters
with a characteristic density that scales with the mean density of the
universe. We rule out zero redshift evolution of the S-T relation at 90%
confidence.Comment: Accepted to Astrophysical Journal. 52 pages, 14 tables, 7 figures
;replaced to match ApJ accepted versio
Reconstruction of a first-order phase transition from computer simulations of individual phases and subphases
We present a new method for investigating first-order phase transitions using
Monte Carlo simulations. It relies on the multiple-histogram method and uses
solely histograms of individual phases. In addition, we extend the method to
include histograms of subphases. The free energy difference between phases,
necessary for attributing the correct statistical weights to the histograms, is
determined by a detour in control parameter space via auxiliary systems with
short relaxation times. We apply this method to a recently introduced model for
structure formation in polypeptides for which other methods fail.Comment: 13 pages in preprint mode, REVTeX, 2 Figures available from the
authors ([email protected], [email protected]
Statistical Mechanics of Canonical-Dissipative Systems and Applications to Swarm Dynamics
We develop the theory of canonical-dissipative systems, based on the
assumption that both the conservative and the dissipative elements of the
dynamics are determined by invariants of motion. In this case, known solutions
for conservative systems can be used for an extension of the dynamics, which
also includes elements such as the take-up/dissipation of energy. This way, a
rather complex dynamics can be mapped to an analytically tractable model, while
still covering important features of non-equilibrium systems. In our paper,
this approach is used to derive a rather general swarm model that considers (a)
the energetic conditions of swarming, i.e. for active motion, (b) interactions
between the particles based on global couplings. We derive analytical
expressions for the non-equilibrium velocity distribution and the mean squared
displacement of the swarm. Further, we investigate the influence of different
global couplings on the overall behavior of the swarm by means of
particle-based computer simulations and compare them with the analytical
estimations.Comment: 14 pages incl. 13 figures. v2: misprints in Eq. (40) corrected, ref.
updated. For related work see also:
http://summa.physik.hu-berlin.de/~frank/active.htm
Can coarse-graining introduce long-range correlations in a symbolic sequence?
We present an exactly solvable mean-field-like theory of correlated ternary
sequences which are actually systems with two independent parameters. Depending
on the values of these parameters, the variance on the average number of any
given symbol shows a linear or a superlinear dependence on the length of the
sequence. We have shown that the available phase space of the system is made up
a diffusive region surrounded by a superdiffusive region. Motivated by the fact
that the diffusive portion of the phase space is larger than that for the
binary, we have studied the mapping between these two. We have identified the
region of the ternary phase space, particularly the diffusive part, that gets
mapped into the superdiffusive regime of the binary. This exact mapping implies
that long-range correlation found in a lower dimensional representative
sequence may not, in general, correspond to the correlation properties of the
original system.Comment: 10 pages including 1 figur
Equation of state of a strongly magnetized hydrogen plasma
The influence of a constant uniform magnetic field on the thermodynamic
properties of a partially ionized hydrogen plasma is studied. Using the method
of Green' s function various interaction contributions to the thermodynamic
functions are calculated. The equation of state of a quantum magnetized plasma
is presented within the framework of a low density expansion up to the order
e^4 n^2 and, additionally, including ladder type contributions via the bound
states in the case of strong magnetic fields (2.35*10^{5} T << B << 2.35*10^{9}
T). We show that for high densities (n=10^{27-30} m^{-3}) and temperatures
T=10^5 - 10^6 K typical for the surface of neutron stars nonideality effects
as, e.g., Debye screening must be taken into account.Comment: 12 pages, 2 Postscript figures. uses revtex, to appear in Phys. Rev.
Separating the BL Lac and Cluster X-ray Emissions in Abell 689 with Chandra
We present the results of a Chandra observation of the galaxy cluster Abell
689 (z=0.279). Abell 689 is one of the most luminous clusters detected in the
ROSAT All Sky Survey (RASS), but was flagged as possibly including significant
point source contamination. The small PSF of the Chandra telescope allows us to
confirm this and separate the point source from the extended cluster X-ray
emission. For the cluster we determine a bolometric luminosity of
L_{bol}=(3.3+/-0.3)x10^{44} erg s-1 and a temperature of kT=5.1^{+2.2}_{-1.3}
keV when including a physically motivated background model. We compare our
measured luminosity for A689 to that quoted in the Rosat All Sky Survey (RASS)
and find L_{0.1-2.4,keV}=2.8x10^{44} erg s-1, a value \sim10 times lower than
the ROSAT measurement. Our analysis of the point source shows evidence for
significant pileup, with a pile-up fraction of ~60%. SDSS spectra and HST
images lead us to the conclusion that the point source within Abell 689 is a BL
Lac object. Using radio and optical observations from the VLA and HST archives,
we determine {\alpha}_{ro}=0.50, {\alpha}_{ox}=0.77 and {\alpha}_{rx}=0.58 for
the BL Lac, which would classify it as being of 'High-energy peak BL Lac' (HBL)
type. Spectra extracted of A689 show a hard X-ray excess at energies above 6
keV that we interpret as inverse Compton emission from aged electrons that may
have been transported into the cluster from the BL Lac.Comment: 11 pages, 15 figures, MNRAS in pres
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