241,067 research outputs found
Superclusters of galaxies from the 2dF redshift survey. II. Comparison with simulations
We investigate properties of superclusters of galaxies found on the basis of
the 2dF Galaxy Redshift Survey, and compare them with properties of
superclusters from the Millennium Simulation. We study the dependence of
various characteristics of superclusters on their distance from the observer,
on their total luminosity, and on their multiplicity. The multiplicity is
defined by the number of Density Field (DF) clusters in superclusters. Using
the multiplicity we divide superclusters into four richness classes: poor,
medium, rich and extremely rich. We show that superclusters are asymmetrical
and have multi-branching filamentary structure, with the degree of asymmetry
and filamentarity being higher for the more luminous and richer superclusters.
The comparison of real superclusters with Millennium superclusters shows that
most properties of simulated superclusters agree very well with real data, the
main differences being in the luminosity and multiplicity distributions.Comment: 15 pages, 13 Figures, submitted for Astronomy and Astrophysic
NEOWISE Studies of Spectrophotometrically Classified Asteroids: Preliminary Results
The NEOWISE dataset offers the opportunity to study the variations in albedo
for asteroid classification schemes based on visible and near-infrared
observations for a large sample of minor planets. We have determined the
albedos for nearly 1900 asteroids classified by the Tholen, Bus and Bus-DeMeo
taxonomic classification schemes. We find that the S-complex spans a broad
range of bright albedos, partially overlapping the low albedo C-complex at
small sizes. As expected, the X-complex covers a wide range of albedos. The
multi-wavelength infrared coverage provided by NEOWISE allows determination of
the reflectivity at 3.4 and 4.6 m relative to the visible albedo. The
direct computation of the reflectivity at 3.4 and 4.6 m enables a new
means of comparing the various taxonomic classes. Although C, B, D and T
asteroids all have similarly low visible albedos, the D and T types can be
distinguished from the C and B types by examining their relative reflectance at
3.4 and 4.6 m. All of the albedo distributions are strongly affected by
selection biases against small, low albedo objects, as all objects selected for
taxonomic classification were chosen according to their visible light
brightness. Due to these strong selection biases, we are unable to determine
whether or not there are correlations between size, albedo and space
weathering. We argue that the current set of classified asteroids makes any
such correlations difficult to verify. A sample of taxonomically classified
asteroids drawn without significant albedo bias is needed in order to perform
such an analysis.Comment: Accepted to Ap
Time-Varying Graphs and Dynamic Networks
The past few years have seen intensive research efforts carried out in some
apparently unrelated areas of dynamic systems -- delay-tolerant networks,
opportunistic-mobility networks, social networks -- obtaining closely related
insights. Indeed, the concepts discovered in these investigations can be viewed
as parts of the same conceptual universe; and the formal models proposed so far
to express some specific concepts are components of a larger formal description
of this universe. The main contribution of this paper is to integrate the vast
collection of concepts, formalisms, and results found in the literature into a
unified framework, which we call TVG (for time-varying graphs). Using this
framework, it is possible to express directly in the same formalism not only
the concepts common to all those different areas, but also those specific to
each. Based on this definitional work, employing both existing results and
original observations, we present a hierarchical classification of TVGs; each
class corresponds to a significant property examined in the distributed
computing literature. We then examine how TVGs can be used to study the
evolution of network properties, and propose different techniques, depending on
whether the indicators for these properties are a-temporal (as in the majority
of existing studies) or temporal. Finally, we briefly discuss the introduction
of randomness in TVGs.Comment: A short version appeared in ADHOC-NOW'11. This version is to be
published in Internation Journal of Parallel, Emergent and Distributed
System
A discrete model for the apparent viscosity of polydisperse suspensions including maximum packing fraction
Based on the notion of a construction process consisting of the stepwise
addition of particles to the pure fluid, a discrete model for the apparent
viscosity as well as for the maximum packing fraction of polydisperse
suspensions of spherical, non-colloidal particles is derived. The model
connects the approaches by Bruggeman and Farris and is valid for large size
ratios of consecutive particle classes during the construction process,
appearing to be the first model consistently describing polydisperse volume
fractions and maximum packing fraction within a single approach. In that
context, the consistent inclusion of the maximum packing fraction into
effective medium models is discussed. Furthermore, new generalized forms of the
well-known Quemada and Krieger equations allowing for the choice of a
second-order Taylor coefficient for the volume fraction (-coefficient),
found by asymptotic matching, are proposed. The model for the maximum packing
fraction as well as the complete viscosity model are compared to experimental
data from the literature showing good agreement. As a result, the new model is
shown to replace the empirical Sudduth model for large diameter ratios. The
extension of the model to the case of small size ratios is left for future
work.Comment: 14 pages, 4 figure
The Circum-Galactic Environment of Bright IRAS Galaxies
This paper studies systematically, for the first time, the circumgalactic
environment of bright IRAS galaxies as defined by Soifer et al. (1989). While
the role of gravitational interaction for luminous and ultraluminous IRAS
galaxies has been well established by various studies, the situation is by far
more obscure in the IR luminosity range of the bright IRAS sample, 10^{10}Lsol
< Lfir < 10^{11} Lsol. To easily identify nearby companion galaxies, the bright
IRAS sample was restricted to 87 objects with redshift range 0.008 < z < 0.018
and galactic latitude > 30^{o}. A control sample, selected from the Center for
Astrophysics redshift survey catalogue, includes 90 objects matching the Bright
IRAS sample for distribution of isophotal diameter, redshift, and morphological
type. From a search of nearby companion galaxies within 250 Kpc on the
second-generation Digitized Sky Survey (DSS-II), we found that the
circumgalactic environment of the Bright IRAS galaxies contains more large
companions than the galaxies in the optically selected control sample, and is
similar to that of Seyfert 2 galaxies. We found a weak correlation over a wide
range of far IR luminosity (10^9 Lsol < Lfir < 10^{12.5}Lsol) between projected
separation and Lfir, which confirms a very close relationship between star
formation rate of a galaxy and the strength of gravitational perturbations. We
also find that the far IR colors depend on whether a source is isolated or
interacting. Finally, we discuss the intrinsic difference and evolution
expectations for the bright IRAS galaxies and the control sample, as well as
the relationship between starbursting and active galaxies.Comment: 10 pages, 5 figs, 2 tables. Accepted for publication in Ap
Geometrical effects on energy transfer in disordered open quantum systems
We explore various design principles for efficient excitation energy
transport in complex quantum systems. We investigate energy transfer efficiency
in randomly disordered geometries consisting of up to 20 chromophores to
explore spatial and spectral properties of small natural/artificial
Light-Harvesting Complexes (LHC). We find significant statistical correlations
among highly efficient random structures with respect to ground state
properties, excitonic energy gaps, multichromophoric spatial connectivity, and
path strengths. These correlations can even exist beyond the optimal regime of
environment-assisted quantum transport. For random configurations embedded in
spatial dimensions of 30 A and 50 A, we observe that the transport efficiency
saturates to its maximum value if the systems contain 7 and 14 chromophores
respectively. Remarkably, these optimum values coincide with the number of
chlorophylls in (Fenna-Matthews-Olson) FMO protein complex and LHC II monomers,
respectively, suggesting a potential natural optimization with respect to
chromophoric density.Comment: 11 pages, 10 figures. Expanded from the former appendix to
arXiv:1104.481
Stand structure and development after selective logging with systematically aligned skid trails, directional felling and climber cutting in a dipterocarp rainforest in Sabah, Malaysia
The tropical rainforests around the world are extremely diverse and support a huge number of timber tree species and have therefore been severely logged. One area that has produced, and still produces, large amounts of timber is Southeast Asia. In this region the most commercially important timber trees belong to the family dipterocarpacae, (dipterocarps). Logging is mostly carried out relatively unplanned and causes therefore great damage to the residual stand. Better planned logging and silvicultural activities, using (for instance) planned skid trails, directional felling and climber-cutting, should reduce the damage and create healthier residual stands. This study tested a logging method involving systematically aligned skid trails combined with directional felling, called supervised logging (SL) and compared it to the conventional logging (CL) practiced in the area. Both logging methods were combined with (CC) and without (NCC) climber cutting. All trees exceeding 10 cm in diameter at breast height (dbh), and a random selection of seedlings and saplings were measured before logging (1992) and for a period of eight years after logging (1993-2001). More trees tended (0.050 < P †0.100) to be logged where CL was applied than in SL plots (the mean numbers of all stems and dipterocarps logged per ha were 13.0 and 12.4, respectively, in CL plots, compared to 9.4 and 6.7 in SL plots). The skid trail network applied in SL resulted in disturbance to around 7 % of the soil cover. The corresponding figure for the unplanned skid trail network in CL was higher (ca. 10 %), but not significantly different. Climber cutting resulted in four more dipterocarp trees being logged ha-1, compared with NCC: a statistically significant difference (P †0.050). Basal area lost or severely damaged differed between methods for some diameter classes with mainly CL leading to greater losses. The residual stand contained statistically significantly more dipterocarp seedlings after CL than after SL, while saplings and trees showed the opposite trend, i.e. more individuals were left in the residual stand after SL than after CL. There were significantly (ca. 30%) more 10-29 cm dbh dipterocarp trees in the residual stand after SL than after CL. Cutting climbers had no initial effect on the residual stand in terms of densities of seedlings, saplings trees of either all species or dipterocarps. Data collected during the subsequent period show inter alia, that the net basal area increased at similar rates following both logging methods, considering all species. There was also no significant difference between SL and CL in terms of dipterocarp net basal area, although this parameter grew by 1.0 m2 ha-1 after SL and declined by 0.1 m2 ha-1 after CL. Climber cutting significantly and positively affected net basal area growth, which amounted to 6.4 m2 ha-1 and 3.3 m2 ha-1 in the CC and NCC plots, respectively. The substantially higher growth associated with CC was mainly due to the growth and recruitment of small trees, mainly pioneer species
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