1,689 research outputs found
Angular momentum transport and disk morphology in SPH simulations of galaxy formation
We perform controlled N-Body/SPH simulations of disk galaxy formation by
cooling a rotating gaseous mass distribution inside equilibrium cuspy spherical
and triaxial dark matter halos. We systematically study the angular momentum
transport and the disk morphology as we increase the number of dark matter and
gas particles from 10^4 to 10^6, and decrease the gravitational softening from
2 kpc to 50 parsecs. The angular momentum transport, disk morphology and radial
profiles depend sensitively on force and mass resolution. At low resolution,
similar to that used in most current cosmological simulations, the cold gas
component has lost half of its initial angular momentum via different
mechanisms. The angular momentum is transferred primarily to the hot halo
component, by resolution-dependent hydrodynamical and gravitational torques,
the latter arising from asymmetries in the mass distribution. In addition,
disk-particles can lose angular momentum while they are still in the hot phase
by artificial viscosity. In the central disk, particles can transfer away over
99% of their initial angular momentum due to spiral structure and/or the
presence of a central bar. The strength of this transport also depends on force
and mass resolution - large softening will suppress the bar instability, low
mass resolution enhances the spiral structure. This complex interplay between
resolution and angular momentum transfer highlights the complexity of
simulations of galaxy formation even in isolated haloes. With 10^6 gas and dark
matter particles, disk particles lose only 10-20% of their original angular
momentum, yet we are unable to produce pure exponential profiles.Comment: 17 pages, 16 figures, MNRAS accepted. Minor changes in response to
referee comments. High resolution version of the paper can be found at
http://krone.physik.unizh.ch/~tkaufman/papers.htm
Electronic Structure Shift of Deep Nanoscale Silicon by SiO- vs. SiN-Embedding as Alternative to Impurity Doping
Conventional impurity doping of deep nanoscale silicon (dns-Si) used in ultra
large scale integration (ULSI) faces serious challenges below the 14 nm
technology node. We report on a new fundamental effect in theory and
experiment, namely the electronic structure of dns-Si experiencing energy
offsets of ca. 1 eV as a function of SiO- vs. SiN-embedding with a
few monolayers (MLs). An interface charge transfer (ICT) from dns-Si specific
to the anion type of the dielectric is at the core of this effect and arguably
nested in quantum-chemical properties of oxygen (O) and nitrogen (N) vs. Si. We
investigate the size up to which this energy offset defines the electronic
structure of dns-Si by density functional theory (DFT), considering interface
orientation, embedding layer thickness, and approximants featuring two Si
nanocrystals (NCs); one embedded in SiO and the other in SiN.
Working with synchrotron ultraviolet photoelectron spectroscopy (UPS), we use
SiO- vs. SiN-embedded Si nanowells (NWells) to obtain their energy
of the top valence band states. These results confirm our theoretical findings
and gauge an analytic model for projecting maximum dns-Si sizes for NCs,
nanowires (NWires) and NWells where the energy offset reaches full scale,
yielding to a clear preference for electrons or holes as majority carriers in
dns-Si. Our findings can replace impurity doping for n/p-type dns-Si as used in
ultra-low power electronics and ULSI, eliminating dopant-related issues such as
inelastic carrier scattering, thermal ionization, clustering, out-diffusion and
defect generation. As far as majority carrier preference is concerned, the
elimination of those issues effectively shifts the lower size limit of Si-based
ULSI devices to the crystalization limit of Si of ca. 1.5 nm and enables them
to work also under cryogenic conditions.Comment: 14 pages, 17 Figures with a total 44 graph
Massive Black Hole Recoil in High Resolution Hosts
The final inspiral and coalescence of a black hole binary can produce highly
beamed gravitational wave radiation. To conserve linear momentum, the black
hole remnant can recoil with "kick" velocity as high as 4000 km/s. We present
two sets of full N-body simulations of recoiling massive black holes (MBH) in
high-resolution, non-axisymmetric potentials. The host to the first set of
simulations is the main halo of the Via Lactea I simulation (Diemand et al.
2007). The nature of the resulting orbits is investigated through a numerical
model where orbits are integrated assuming an evolving, triaxial NFW potential,
and dynamical friction is calculated directly from the velocity dispersion
along the major axes of the main halo of Via Lactea I. By comparing the
triaxial case to a spherical model, we find that the wandering time spent by
the MBH is significantly increased due to the asphericity of the halo. For
kicks larger than 200 km/s, the remnant MBH does not return to the inner 200 pc
within 1 Gyr, a timescale an order of magnitude larger than the upper limit of
the estimated QSO lifetime. The second set of simulations is run using the
outcome of a high-resolution gas-rich merger (Mayer et al. 2007) as host
potential. In this case, a recoil velocity of 500 km/s cannot remove the MBH
from the nuclear region.Comment: 4 pages, 4 figures. Proceedings of the conference Galactic & Stellar
Dynamics In the Era of High Resolution Survey
Gravitational instability in binary protoplanetary discs: new constraints on giant planet formation
We use high-resolution three-dimensional smoothed particle hydrodynamic (SPH) simulations to study the evolution of self-gravitating binary protoplanetary discs. Heating by shocks and cooling is included. We consider different orbital separations and masses of the discs. Massive discs (M∼ 0.1 M⊙) that fragment in isolation as a result of gravitational instability develop only transient overdensities in binary systems with a separation of about 60 au. This is true even when the cooling time is significantly shorter than the orbital time because efficient heating owing to strong tidally induced spiral shocks dominates. The resulting temperatures, above 200 K, would vaporize water ice in the outer disc, posing a problem even for the other model of giant planet formation, core accretion. Light discs (M∼ 0.01 M⊙) do not fragment but remain cold because their low self-gravity inhibits strong shocks. Core accretion would not be hampered in them. At separations of about 120 au, tidally induced spiral shocks weaken significantly and fragmentation occurs similarly to isolated systems. If disc instability is the main formation mechanism for giant planets, ongoing surveys targeting binary systems should find considerably fewer planets in systems with separations below 100 a
Primordial Earth mantle heterogeneity caused by the Moon-forming giant impact
The giant impact hypothesis for Moon formation successfully explains the
dynamic properties of the Earth-Moon system but remains challenged by the
similarity of isotopic fingerprints of the terrestrial and lunar mantles.
Moreover, recent geochemical evidence suggests that the Earth's mantle
preserves ancient (or "primordial") heterogeneity that predates the
Moon-forming giant impact. Using a new hydrodynamical method, we here show that
Moon-forming giant impacts lead to a stratified starting condition for the
evolution of the terrestrial mantle. The upper layer of the Earth is
compositionally similar to the disk, out of which the Moon evolves, whereas the
lower layer preserves proto-Earth characteristics. As long as this predicted
compositional stratification can at least partially be preserved over the
subsequent billions of years of Earth mantle convection, the compositional
similarity between the Moon and the accessible Earth's mantle is a natural
outcome of realistic and high-probability Moon-forming impact scenarios. The
preservation of primordial heterogeneity in the modern Earth not only
reconciles geochemical constraints but is also consistent with recent
geophysical observations. Furthermore, for significant preservation of a
proto-Earth reservoir, the bulk composition of the Earth-Moon system may be
systematically shifted towards chondritic values.Comment: Comments are welcom
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