2,186 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
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
Progenitors of Supernovae Type Ia
Despite the significance of Type Ia supernovae (SNeIa) in many fields in
astrophysics, SNeIa lack a theoretical explanation. The standard scenarios
involve thermonuclear explosions of carbon/oxygen white dwarfs approaching the
Chandrasekhar mass; either by accretion from a companion or by a merger of two
white dwarfs. We investigate the contribution from both channels to the SNIa
rate with the binary population synthesis (BPS) code SeBa in order to constrain
binary processes such as the mass retention efficiency of WD accretion and
common envelope evolution. We determine the theoretical rates and delay time
distribution of SNIa progenitors and in particular study how assumptions affect
the predicted rates.Comment: 6 pages, 6 figures, appeared in proceedings for "The 18th European
White Dwarf Workshop
Temperature dependent transport characteristics of graphene/n-Si diodes
Realizing an optimal Schottky interface of graphene on Si is challenging, as
the electrical transport strongly depends on the graphene quality and the
fabrication processes. Such interfaces are of increasing research interest for
integration in diverse electronic devices as they are thermally and chemically
stable in all environments, unlike standard metal/semiconductor interfaces. We
fabricate such interfaces with n-type Si at ambient conditions and find their
electrical characteristics to be highly rectifying, with minimal reverse
leakage current (10 A) and rectification of more than . We
extract Schottky barrier height of 0.69 eV for the exfoliated graphene and 0.83
eV for the CVD graphene devices at room temperature. The temperature dependent
electrical characteristics suggest the influence of inhomogeneities at the
graphene/n-Si interface. A quantitative analysis of the inhomogeneity in
Schottky barrier heights is presented using the potential fluctuation model
proposed by Werner and G\"{u}ttler.Comment: 5 pages, 5 figure
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
Options for Improved Biomass Production in Feeding Systems for Dairying in High Rainfall Environments in New Zealand
New Zealand dairy production has expanded into marginal climates and soil types on the premise of excellent profitability and efficient utilisation of forage. Annual pasture production in the cool West Coast of the South Island (rainfall 2042-2933 mm) is15,000 kg. Increased farm production and feeding of high quality biomass, from imported feed or supplementary feed crops grown on- farm, are needed to improve milk solid output. Small plot trials with spring and early summer-sown brassicas, cereals and maize were the focus for development of systems to maximise and manage the seasonal feed supply. The effect of sowing time, fertiliser timing and rate of N and K fertiliser application were studied to quantify the risks of crop failure in the high rainfall and low radiation environment. The aim was to increase forage supply/ha in a predominantly grass-based system and reduce associated risks to environmental sustainability
Deflagrations in hybrid CONe white dwarfs: a route to explain the faint Type Iax supernova 2008ha
Stellar evolution models predict the existence of hybrid white dwarfs (WDs)
with a carbon-oxygen core surrounded by an oxygen-neon mantle. Being born with
masses ~1.1 Msun, hybrid WDs in a binary system may easily approach the
Chandrasekhar mass (MCh) by accretion and give rise to a thermonuclear
explosion. Here, we investigate an off-centre deflagration in a near-MCh hybrid
WD under the assumption that nuclear burning only occurs in carbon-rich
material. Performing hydrodynamics simulations of the explosion and detailed
nucleosynthesis post-processing calculations, we find that only 0.014 Msun of
material is ejected while the remainder of the mass stays bound. The ejecta
consist predominantly of iron-group elements, O, C, Si and S. We also calculate
synthetic observables for our model and find reasonable agreement with the
faint Type Iax SN 2008ha. This shows for the first time that deflagrations in
near-MCh WDs can in principle explain the observed diversity of Type Iax
supernovae. Leaving behind a near-MCh bound remnant opens the possibility for
recurrent explosions or a subsequent accretion-induced collapse in faint Type
Iax SNe, if further accretion episodes occur. From binary population synthesis
calculations, we find the rate of hybrid WDs approaching MCh to be on the order
of 1 percent of the Galactic SN Ia rate.Comment: 9 pages, 7 figures, 2 tables, accepted for publication in MNRA
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