1,244 research outputs found
The supernova-regulated ISM. I. The multi-phase structure
We simulate the multi-phase interstellar medium randomly heated and stirred
by supernovae, with gravity, differential rotation and other parameters of the
solar neighbourhood. Here we describe in detail both numerical and physical
aspects of the model, including injection of thermal and kinetic energy by SN
explosions, radiative cooling, photoelectric heating and various transport
processes. With 3D domain extending 1 kpc^2 horizontally and 2 kpc vertically,
the model routinely spans gas number densities 10^-5 - 10^2 cm^-3, temperatures
10-10^8 K, local velocities up to 10^3 km s^-1 (with Mach number up to 25).
The thermal structure of the modelled ISM is classified by inspection of the
joint probability density of the gas number density and temperature. We confirm
that most of the complexity can be captured in terms of just three phases,
separated by temperature borderlines at about 10^3 K and 5x10^5 K. The
probability distribution of gas density within each phase is approximately
lognormal. We clarify the connection between the fractional volume of a phase
and its various proxies, and derive an exact relation between the fractional
volume and the filling factors defined in terms of the volume and probabilistic
averages. These results are discussed in both observational and computational
contexts. The correlation scale of the random flows is calculated from the
velocity autocorrelation function; it is of order 100 pc and tends to grow with
distance from the mid-plane. We use two distinct parameterizations of radiative
cooling to show that the multi-phase structure of the gas is robust, as it does
not depend significantly on this choice.Comment: 28 pages, 22 figures and 8 table
The supernova-regulated ISM. II. The mean magnetic field
The origin and structure of the magnetic fields in the interstellar medium of
spiral galaxies is investigated with 3D, non-ideal, compressible MHD
simulations, including stratification in the galactic gravity field,
differential rotation and radiative cooling. A rectangular domain, 1x1x2
kpc^{3} in size, spans both sides of the galactic mid-plane. Supernova
explosions drive transonic turbulence. A seed magnetic field grows
exponentially to reach a statistically steady state within 1.6 Gyr. Following
Germano (1992) we use volume averaging with a Gaussian kernel to separate
magnetic field into a mean field and fluctuations. Such averaging does not
satisfy all Reynolds rules, yet allows a formulation of mean-field theory. The
mean field thus obtained varies in both space and time. Growth rates differ for
the mean-field and fluctuating field and there is clear scale separation
between the two elements, whose integral scales are about 0.7 kpc and 0.3 kpc,
respectively.Comment: 5 pages, 10 figures, submitted to Monthly Notices Letter
Number partitioning as random energy model
Number partitioning is a classical problem from combinatorial optimisation.
In physical terms it corresponds to a long range anti-ferromagnetic Ising spin
glass. It has been rigorously proven that the low lying energies of number
partitioning behave like uncorrelated random variables. We claim that
neighbouring energy levels are uncorrelated almost everywhere on the energy
axis, and that energetically adjacent configurations are uncorrelated, too.
Apparently there is no relation between geometry (configuration) and energy
that could be exploited by an optimization algorithm. This ``local random
energy'' picture of number partitioning is corroborated by numerical
simulations and heuristic arguments.Comment: 8+2 pages, 9 figures, PDF onl
Entropy-based analysis of the number partitioning problem
In this paper we apply the multicanonical method of statistical physics on
the number-partitioning problem (NPP). This problem is a basic NP-hard problem
from computer science, and can be formulated as a spin-glass problem. We
compute the spectral degeneracy, which gives us information about the number of
solutions for a given cost and cardinality . We also study an extension
of this problem for partitions. We show that a fundamental difference on
the spectral degeneracy of the generalized () NPP exists, which could
explain why it is so difficult to find good solutions for this case. The
information obtained with the multicanonical method can be very useful on the
construction of new algorithms.Comment: 6 pages, 4 figure
The supernova-regulated ISM : III. Generation of vorticity, helicity, and mean flows
Context. The forcing of interstellar turbulence, driven mainly by supernova (SN) explosions, is irrotational in nature, but the development of significant amounts of vorticity and helicity, accompanied by large-scale dynamo action, has been reported. Aims. Several earlier investigations examined vorticity production in simpler systems; here all the relevant processes can be considered simultaneously. We also investigate the mechanisms for the generation of net helicity and large-scale flow in the system. Methods. We use a three-dimensional, stratified, rotating and shearing local simulation domain of the size 1 x 1 x 2 kpc(3), forced with SN explosions occurring at a rate typical of the solar neighbourhood in the MilkyWay. In addition to the nominal simulation run with realistic Milky Way parameters, we vary the rotation and shear rates, but keep the absolute value of their ratio fixed. Reversing the sign of shear vs. rotation allows us to separate the rotation-and shear-generated contributions. Results. As in earlier studies, we find the generation of significant amounts of vorticity, the rotational flow comprising on average 65% of the total flow. The vorticity production can be related to the baroclinicity of the flow, especially in the regions of hot, dilute clustered supernova bubbles. In these regions, the vortex stretching acts as a sink of vorticity. In denser, compressed regions, the vortex stretching amplifies vorticity, but remains sub-dominant to baroclinicity. The net helicities produced by rotation and shear are of opposite signs for physically motivated rotation laws, with the solar neighbourhood parameters resulting in the near cancellation of the total net helicity. We also find the excitation of oscillatory mean flows, the strength and oscillation period of which depend on the Coriolis and shear parameters; we interpret these as signatures of the anisotropic-kinetic-alpha (AKA) effect. We use the method of moments to fit for the turbulent transport coefficients, and find alpha(AKA) values of the order 3-5 km s(-1). Conclusions. Even in a weakly rotationally and shear-influenced system, small-scale anisotropies can lead to significant effects at large scales. Here we report on two consequences of such effects, namely on the generation of net helicity and on the emergence of large-scale flows by the AKA effect, the latter detected for the first time in a direct numerical simulation of a realistic astrophysical system.Peer reviewe
Spitzer Space Telescope observations of magnetic cataclysmic variables: possibilities for the presence of dust in polars
We present Spitzer Space Telescope photometry of six short-period polars, EF
Eri, V347 Pav, VV Pup, V834 Cen, GG Leo, and MR Ser. We have combined the
Spitzer Infrared Array Camera (3.6 -8.0 microns) data with the 2MASS J, H, K_s
photometry to construct the spectral energy distributions of these systems from
the near- to mid-IR (1.235 - 8 microns). We find that five out of the six
polars have flux densities in the mid-IR that are substantially in excess of
the values expected from the stellar components alone. We have modeled the
observed SEDs with a combination of contributions from the white dwarf,
secondary star, and either cyclotron emission or a cool, circumbinary dust disk
to fill in the long-wavelength excess. We find that a circumbinary dust disk is
the most likely cause of the 8 micron excess in all cases, but we have been
unable to rule out the specific (but unlikely) case of completely optically
thin cyclotron emission as the source of the observed 8 micron flux density.
While both model components can generate enough flux at 8 microns, neither dust
nor cyclotron emission alone can match the excess above the stellar components
at all wavelengths. A model combining both cyclotron and dust contributions,
possibly with some accretion-generated flux in the near-IR, is probably
required, but our observed SEDs are not sufficiently well-sampled to constrain
such a complicated model. If the 8 micron flux density is caused by the
presence of a circumbinary dust disk, then our estimates of the masses of these
disks are many orders of magnitude below the mass required to affect CV
evolution.Comment: 58 pages, 14 figures, ApJ accepte
Ultra-strong Adhesion of Graphene Membranes
As mechanical structures enter the nanoscale regime, the influence of van der
Waals forces increases. Graphene is attractive for nanomechanical systems
because its Young's modulus and strength are both intrinsically high, but the
mechanical behavior of graphene is also strongly influenced by the van der
Waals force. For example, this force clamps graphene samples to substrates, and
also holds together the individual graphene sheets in multilayer samples. Here
we use a pressurized blister test to directly measure the adhesion energy of
graphene sheets with a silicon oxide substrate. We find an adhesion energy of
0.45 \pm 0.02 J/m2 for monolayer graphene and 0.31 \pm 0.03 J/m2 for samples
containing 2-5 graphene sheets. These values are larger than the adhesion
energies measured in typical micromechanical structures and are comparable to
solid/liquid adhesion energies. We attribute this to the extreme flexibility of
graphene, which allows it to conform to the topography of even the smoothest
substrates, thus making its interaction with the substrate more liquid-like
than solid-like.Comment: to appear in Nature Nanotechnolog
Evidence for proton acceleration up to TeV energies based on VERITAS and Fermi-LAT observations of the Cas A SNR
We present a study of -ray emission from the core-collapse supernova
remnant Cas~A in the energy range from 0.1GeV to 10TeV. We used 65 hours of
VERITAS data to cover 200 GeV - 10 TeV, and 10.8 years of \textit{Fermi}-LAT
data to cover 0.1-500 GeV. The spectral analysis of \textit{Fermi}-LAT data
shows a significant spectral curvature around GeV that is
consistent with the expected spectrum from pion decay. Above this energy, the
joint spectrum from \textit{Fermi}-LAT and VERITAS deviates significantly from
a simple power-law, and is best described by a power-law with spectral index of
with a cut-off energy of TeV. These
results, along with radio, X-ray and -ray data, are interpreted in the
context of leptonic and hadronic models. Assuming a one-zone model, we exclude
a purely leptonic scenario and conclude that proton acceleration up to at least
6 TeV is required to explain the observed -ray spectrum. From modeling
of the entire multi-wavelength spectrum, a minimum magnetic field inside the
remnant of is deduced.Comment: 33 pages, 9 Figures, 6 Table
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