4,170 research outputs found
Do cyber-birds flock together? Comparing deviance among social network members of cyber-dependent offenders and traditional offenders
The distinct setting in which cyber-dependent crime takes place may reduce the similarity in the deviance of social network members. We test this assumption by analysing the deviance of the most important social contacts of cyber-dependent offenders and traditional offenders in the Netherlands (N = 344 offenders; N = 1131 social contacts). As expected, similarity in deviance is weaker for cyber-dependent crime. Because this is a strong predictor of traditional offending, this has important implications for criminological research and practice. Additionally, for both crime types the offending behaviour of a person is more strongly linked to the deviance of social ties if those ties are of the same gender and age, and if the offender has daily contact with them. Implications and future criminological research suggestions are discussed
Bistability and regular spatial patterns in arid ecosystems.
A variety of patterns observed in ecosystems can be explained by resource–concentration mechanisms. A resource–concentration mechanism occurs when organisms increase the lateral flow of a resource toward them, leading to a local concentration of this resource and to its depletion from areas farther away. In resource–concentration systems, it has been proposed that certain spatial patterns could indicate proximity to discontinuous transitions where an ecosystem abruptly shifts from one stable state to another. Here, we test this hypothesis using a model of vegetation dynamics in arid ecosystems. In this model, a resource– concentration mechanism drives a positive feedback between vegetation and soil water availability. We derived the conditions leading to bistability and pattern formation. Our analysis revealed that bistability and regular pattern formation are linked in our model. This means that, when regular vegetation patterns occur, they indicate that the system is along a discontinuous transition to desertification. Yet, in real systems, only observing regular vegetation patterns without identifying the pattern-driving mechanism might not be enough to conclude that an ecosystem is along a discontinuous transition because similar patterns can emerge from different ecological mechanisms
Nutrients and Hydrology Indicate the Driving Mechanisms of Peatland Surface Patterning
Peatland surface patterning motivates studies that identify underlying structuring mechanisms. Theoretical studies so far suggest that different mechanisms may drive similar types of patterning. The long time span associated with peatland surface pattern formation, however, limits possibilities for empirically testing model predictions by field manipulations. Here, we present a model that describes spatial interactions between vegetation, nutrients, hydrology, and peat. We used this model to study pattern formation as driven by three different mechanisms: peat accumulation, water ponding, and nutrient accumulation. By on-and-off switching of each mechanism, we created a full-factorial design to see how these mechanisms affected surface patterning (pattern of vegetation and peat height) and underlying patterns in nutrients and hydrology. Results revealed that different combinations of structuring mechanisms lead to similar types of peatland surface patterning but contrasting underlying patterns in nutrients and hydrology. These contrasting underlying patterns suggest that the presence or absence of the structuring mechanisms can be identified by relatively simple short-term field measurements of nutrients and hydrology, meaning that longer-term field manipulations can be circumvented. Therefore, this study provides promising avenues for future empirical studies on peatland patternin
Towards an understanding of Type Ia supernovae from a synthesis of theory and observations
Motivated by the fact that calibrated light curves of Type Ia supernovae (SNe
Ia) have become a major tool to determine the expansion history of the
Universe, considerable attention has been given to, both, observations and
models of these events over the past 15 years. Here, we summarize new
observational constraints, address recent progress in modeling Type Ia
supernovae by means of three-dimensional hydrodynamic simulations, and discuss
several of the still open questions. It will be be shown that the new models
have considerable predictive power which allows us to study observable
properties such as light curves and spectra without adjustable non-physical
parameters. This is a necessary requisite to improve our understanding of the
explosion mechanism and to settle the question of the applicability of SNe Ia
as distance indicators for cosmology. We explore the capabilities of the models
by comparing them with observations and we show how such models can be applied
to study the origin of the diversity of SNe Ia.Comment: 26 pages, 13 figures, Frontiers of Physics, in prin
Type Ia Supernovae and Accretion Induced Collapse
Using the population synthesis binary evolution code StarTrack, we present
theoretical rates and delay times of Type Ia supernovae arising from various
formation channels. These channels include binaries in which the exploding
white dwarf reaches the Chandrasekhar mass limit (DDS, SDS, and helium-rich
donor scenario) as well as the sub-Chandrasekhar mass scenario, in which a
white dwarf accretes from a helium-rich companion and explodes as a SN Ia
before reaching the Chandrasekhar mass limit. We find that using a common
envelope parameterization employing energy balance with alpha=1 and lambda=1,
the supernova rates per unit mass (born in stars) of sub-Chandrasekhar mass SNe
Ia exceed those of all other progenitor channels at epochs t=0.7 - 4 Gyr for a
burst of star formation at t=0. Additionally, the delay time distribution of
the sub-Chandrasekhar model can be divided in to two distinct evolutionary
channels: the `prompt' helium-star channel with delay times < 500 Myr, and the
`delayed' double white dwarf channel with delay times > 800 Myr spanning up to
a Hubble time. These findings are in agreement with recent
observationally-derived delay time distributions which predict that a large
number of SNe Ia have delay times < 1 Gyr, with a significant fraction having
delay times < 500 Myr. We find that the DDS channel is also able to account for
the observed rates of SNe Ia. However, detailed simulations of white dwarf
mergers have shown that most of these mergers will not lead to SNe Ia but
rather to the formation of a neutron star via accretion-induced collapse. If
this is true, our standard population synthesis model predicts that the only
progenitor channel which can account for the rates of SNe Ia is the
sub-Chandrasekhar mass scenario, and none of the other progenitors considered
can fully account for the observed rates.Comment: 6 pages, 1 figure, 1 table, to appear in proceedings for "Binary Star
Evolution: Mass Loss, Accretion and Mergers
The LISA Gravitational Wave Foreground: A Study of Double White Dwarfs
Double white dwarfs are expected to be a source of confusion-limited noise
for the future gravitational wave observatory LISA. In a specific frequency
range, this 'foreground noise' is predicted to rise above the instrumental
noise and hinder the detection of other types of signals, e.g., gravitational
waves arising from stellar mass objects inspiraling into massive black holes.
In many previous studies only detached populations of compact object binaries
have been considered in estimating the LISA gravitational wave foreground
signal. Here, we investigate the influence of compact object detached and
Roche-Lobe Overflow Galactic binaries on the shape and strength of the LISA
signal. Since >99% of remnant binaries which have orbital periods within the
LISA sensitivity range are white dwarf binaries, we consider only these
binaries when calculating the LISA signal. We find that the contribution of
RLOF binaries to the foreground noise is negligible at low frequencies, but
becomes significant at higher frequencies, pushing the frequency at which the
foreground noise drops below the instrumental noise to >6 mHz. We find that it
is important to consider the population of mass transferring binaries in order
to obtain an accurate assessment of the foreground noise on the LISA data
stream. However, we estimate that there still exists a sizeable number (~11300)
of Galactic double white dwarf binaries which will have a signal-to-noise ratio
>5, and thus will be potentially resolvable with LISA. We present the LISA
gravitational wave signal from the Galactic population of white dwarf binaries,
show the most important formation channels contributing to the LISA disc and
bulge populations and discuss the implications of these new findings.Comment: ApJ accepted. 28 pages, 11 figures (low resolution), 5 tables, some
new references and changed content since last astro-ph versio
SN1991bg-like supernovae are a compelling source of most Galactic antimatter
The Milky Way Galaxy glows with the soft gamma ray emission resulting from
the annihilation of electron-positron pairs every
second. The origin of this vast quantity of antimatter and the peculiar
morphology of the 511keV gamma ray line resulting from this annihilation have
been the subject of debate for almost half a century. Most obvious positron
sources are associated with star forming regions and cannot explain the rate of
positron annihilation in the Galactic bulge, which last saw star formation some
ago, or else violate stringent constraints on the positron
injection energy. Radioactive decay of elements formed in core collapse
supernovae (CCSNe) and normal Type Ia supernovae (SNe Ia) could supply
positrons matching the injection energy constraints but the distribution of
such potential sources does not replicate the required morphology. We show that
a single class of peculiar thermonuclear supernova - SN1991bg-like supernovae
(SNe 91bg) - can supply the number and distribution of positrons we see
annihilating in the Galaxy through the decay of Ti synthesised in these
events. Such Ti production simultaneously addresses the observed
abundance of Ca, the Ti decay product, in solar system material.Comment: Accepted for publication in Proceedings of IAU Symposium 322: The
Multimessenger Astrophysics of the Galactic Center 4 page
Resource contrast in patterned peatlands increases along a climatic gradient
Copyright by the Ecological Society of America 2010, for personal or educational use only. Article is available at <http://dx.doi.org/10.1890/09-1313.1
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