1,178 research outputs found
Theoretical research program to study transition metal trimers and embedded clusters
Small transition metal clusters were studied at a high level of approximation, including all the valence electrons in the calculation and extensive electron correlation, in order to understand the electronic structure of these small metal clusters. By comparison of dimers, trimers, and possibly higher clusters, the information obtained was used to provide insights into the electronic structure of bulk transition metals. Small metal clusters are currently of considerable experimental interest and some information is becomming available both from matrix electron spin resonance studies and from gas phase spectroscopy. Collaboration between theorists and experimentalists is thus expected to be especially profitable at this time since there is some experimental information which can serve to guide the theoretical work
Crystalline silicates as a probe of disk formation history
We present a new perspective on the crystallinity of dust in protoplanetary
disks. The dominant crystallization by thermal annealing happens in the very
early phases of disk formation and evolution. Both the disk properties and the
level of crystallinity are thereby directly linked to the properties of the
molecular cloud core from which the star+disk system was formed. We show that,
under the assumption of single star formation, rapidly rotating clouds produce
disks which, after the main infall phase (i.e. in the optically revealed class
II phase), are rather massive and have a high accretion rate but low
crystallinity. Slowly rotating clouds, on the other hand, produce less massive
disks with lower accretion rate, but high levels of crystallinity. Cloud
fragmentation and the formation of multiple stars complicates the problem and
necessitates further study. The underlying physics of the model is
insufficiently understood to provide the precise relationship between
crystallinity, disk mass and accretion rate. But the fact that with `standard'
input physics the model produces disks which, in comparison to observations,
appear to have either too high levels of crystallinity or too high disk masses,
demonstrates that the comparison of these models to observations can place
strong contraints on the disk physics. The question to ask is not why some
sources are so crystalline, but why some other sources have such a low level of
crystallinity.Comment: Accepted for publication in ApJ
The SILCC (SImulating the LifeCycle of molecular Clouds) project: I. Chemical evolution of the supernova-driven ISM
The SILCC project (SImulating the Life-Cycle of molecular Clouds) aims at a
more self-consistent understanding of the interstellar medium (ISM) on small
scales and its link to galaxy evolution. We simulate the evolution of the
multi-phase ISM in a 500 pc x 500 pc x 10 kpc region of a galactic disc, with a
gas surface density of .
The Flash 4.1 simulations include an external potential, self-gravity, magnetic
fields, heating and radiative cooling, time-dependent chemistry of H and CO
considering (self-) shielding, and supernova (SN) feedback. We explore SN
explosions at different (fixed) rates in high-density regions (peak), in random
locations (random), in a combination of both (mixed), or clustered in space and
time (clustered). Only random or clustered models with self-gravity (which
evolve similarly) are in agreement with observations. Molecular hydrogen forms
in dense filaments and clumps and contributes 20% - 40% to the total mass,
whereas most of the mass (55% - 75%) is in atomic hydrogen. The ionised gas
contributes <10%. For high SN rates (0.5 dex above Kennicutt-Schmidt) as well
as for peak and mixed driving the formation of H is strongly suppressed.
Also without self-gravity the H fraction is significantly lower (
5%). Most of the volume is filled with hot gas (90% within 2 kpc).
Only for random or clustered driving, a vertically expanding warm component of
atomic hydrogen indicates a fountain flow. Magnetic fields have little impact
on the final disc structure. However, they affect dense gas () and delay H formation. We highlight that individual chemical
species, in particular atomic hydrogen, populate different ISM phases and
cannot be accurately accounted for by simple temperature-/density-based phase
cut-offs.Comment: 30 pages, 23 figures, submitted to MNRAS. Comments welcome! For
movies of the simulations and download of selected Flash data see the SILCC
website: http://www.astro.uni-koeln.de/silc
The influence of the turbulent perturbation scale on prestellar core fragmentation and disk formation
The collapse of weakly turbulent prestellar cores is a critical stage in the
process of star formation. Being highly non-linear and stochastic, the outcome
of collapse can only be explored theoretically by performing large ensembles of
numerical simulations. Standard practice is to quantify the initial turbulent
velocity field in a core in terms of the amount of turbulent energy (or some
equivalent) and the exponent in the power spectrum (n \equiv -d log Pk /d log
k). In this paper, we present a numerical study of the influence of the details
of the turbulent velocity field on the collapse of an isolated, weakly
turbulent, low-mass prestellar core. We show that, as long as n > 3 (as is
usually assumed), a more critical parameter than n is the maximum wavelength in
the turbulent velocity field, {\lambda}_MAX. This is because {\lambda}_MAX
carries most of the turbulent energy, and thereby influences both the amount
and the spatial coherence of the angular momentum in the core. We show that the
formation of dense filaments during collapse depends critically on
{\lambda}_MAX, and we explain this finding using a force balance analysis. We
also show that the core only has a high probability of fragmenting if
{\lambda}_MAX > 0.5 R_CORE (where R_CORE is the core radius); that the dominant
mode of fragmentation involves the formation and break-up of filaments; and
that, although small protostellar disks (with radius R_DISK <= 20 AU) form
routinely, more extended disks are rare. In turbulent, low-mass cores of the
type we simulate here, the formation of large, fragmenting protostellar disks
is suppressed by early fragmentation in the filaments.Comment: 11 pages, 7 figures; accepted for publication by MNRA
The SILCC project: III. Regulation of star formation and outflows by stellar winds and supernovae
We study the impact of stellar winds and supernovae on the multi-phase
interstellar medium using three-dimensional hydrodynamical simulations carried
out with FLASH. The selected galactic disc region has a size of (500 pc) x
5 kpc and a gas surface density of 10 M/pc. The simulations
include an external stellar potential and gas self-gravity, radiative cooling
and diffuse heating, sink particles representing star clusters, stellar winds
from these clusters which combine the winds from indi- vidual massive stars by
following their evolution tracks, and subsequent supernova explosions. Dust and
gas (self-)shielding is followed to compute the chemical state of the gas with
a chemical network. We find that stellar winds can regulate star (cluster)
formation. Since the winds suppress the accretion of fresh gas soon after the
cluster has formed, they lead to clusters which have lower average masses
(10 - 10 M) and form on shorter timescales (10 -
10 Myr). In particular we find an anti-correlation of cluster mass and
accretion time scale. Without winds the star clusters easily grow to larger
masses for ~5 Myr until the first supernova explodes. Overall the most massive
stars provide the most wind energy input, while objects beginning their
evolution as B-type stars contribute most of the supernova energy input. A
significant outflow from the disk (mass loading 1 at 1 kpc) can be
launched by thermal gas pressure if more than 50% of the volume near the disc
mid-plane can be heated to T > 3x10 K. Stellar winds alone cannot create a
hot volume-filling phase. The models which are in best agreement with observed
star formation rates drive either no outflows or weak outflows.Comment: 23 pages; submitted to MNRA
Diffusion and desorption of SiH3 on hydrogenated H:Si(100)-(2x1) from first principles
We have studied diffusion pathways of a silyl radical adsorbed on the
hydrogenated Si (100)-(2x1) surface by density-functional theory. The process
is of interest for the growth of crystalline silicon by plasma-enhanced
chemical vapor deposition. Preliminary searches for migration mechanisms have
been performed using metadynamics simulations. Local minima and transition
states have been further refined by using the nudged-elastic-band method.
Barriers for diffusion from plausible adsorption sites as low as 0.2 eV have
been found, but trap states have also been spotted, leading to a more stable
configuration, with escape barriers of 0.7 eV. Diffusion among weakly bound
physisorbed states is also possible with very low activation barriers (<50
meV). However, desorption mechanisms (either as SiH3 or as SiH4) from
physisorbed or more strongly bound adsorption configurations turn out to have
activation energies similar to diffusion barriers. Kinetic Monte Carlo
simulations based on ab initio activation energies show that the silyl radical
diffuses at most by a few lattice spacing before desorbing at temperatures in
the range 300-1000 K
On the resolution requirements for modelling molecular gas formation in solar neighbourhood conditions
The formation of molecular hydrogen (H) and carbon monoxide (CO) is
sensitive to the volume and column density distribution of the turbulent
interstellar medium. In this paper, we study H and CO formation in a large
set of hydrodynamical simulations of periodic boxes with driven supersonic
turbulence, as well as in colliding flows with the \textsc{Flash} code. The
simulations include a non-equilibrium chemistry network, gas self-gravity, and
diffuse radiative transfer. We investigate the spatial resolution required to
obtain a converged H and CO mass fraction and formation history. From the
numerical tests we find that H converges at a spatial resolution of
~pc, while the required resolution for CO convergence is ~pc in gas with solar metallicity which is subject to a solar
neighbourhood interstellar radiation field. We derive two critical conditions
from our numerical results: the simulation has to at least resolve the
densities at which (1) the molecule formation time in each cell in the
computational domain is equal to the dissociation time, and (2) the formation
time is equal to the the typical cell crossing time. For both H and CO, the
second criterion is more restrictive. The formulae we derive can be used to
check whether molecule formation is converged in any given simulation.Comment: 22 pages, 21 figures, submitte
Graphene-based LbL deposited films: further study of electrical and gas sensing properties
Graphene-surfactant composite materials obtained by the ultrasonic exfoliation of graphite powder in the presence of ionic surfactants (either CTAB or SDS) were utilised to construct thin films using layer-by-layer (LbL) electrostatic deposition technique. A series of graphene-based thin films were made by alternating layers of either graphene-SDS with polycations (PEI or PAH) or graphene-CTAB with polyanions (PSS). Also, graphene-phthalocyanine composite films were produced by alternating layers of graphene-CTAB with tetrasulfonated nickel phthalocyanine. Graphene-surfactant LbL films exhibited good electric conductivity (about 0.1 S/cm) of semiconductor type with a band gap of about 20 meV. Judging from UV-vis spectra measurements, graphene-phthalocyanine LbL films appeared to form joint π-electron system. Gas sensing testing of such composite films combining high conductivity of graphene with the gas sensing abilities of phthalocyanines showed substantial changes (up to 10%) in electrical conductivity upon exposure to electro-active gases such as HCl and NH3
From parallel to perpendicular -- On the orientation of magnetic fields in molecular clouds
We present synthetic dust polarization maps of simulated molecular clouds
(MCs) with the goal to systematically explore the origin of the relative
orientation of the magnetic field () with respect to the MC
sub-structures identified in density (; 3D) and column density (; 2D).
The polarization maps are generated with the radiative transfer code POLARIS,
including self-consistently calculated efficiencies for radiative torque
alignment. The MCs are formed in two sets of 3D MHD simulations: in (i)
colliding flows (CF), and (ii) the SILCC-Zoom simulations. In 3D, for the CF
simulations with an initial field strength below 5 G, is
oriented parallel or randomly with respect to the -structures. For CF runs
with stronger initial fields and all SILCC-Zoom simulations, which have an
initial field strength of 3 G, a flip from parallel to perpendicular
orientation occurs at high densities of 10 -
10 cm. We suggest that this flip happens if the MC's mass-to-flux
ratio, , is close to or below the critical value of 1. This corresponds to
a field strength around 3 - 5 G. In 2D, we use the Projected Rayleigh
Statistics (PRS) to study the orientation of . If present, the flip in
orientation occurs at 10 cm,
similar to the observed transition value from sub- to supercritical magnetic
fields in the ISM. However, projection effects can reduce the power of the PRS
method: Depending on the MC or LOS, the projected maps of the SILCC-Zoom
simulations do not always show the flip, although expected from the 3D
morphology. Such projection effects can explain the variety of recently
observed field configurations, in particular within a single MC. Finally, we do
not find a correlation between the observed orientation of and the
-PDF.Comment: 20 pages, 12 figures, accepted for publication in MNRA
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