101 research outputs found
Exact results for hydrogen recombination on dust grain surfaces
The recombination of hydrogen in the interstellar medium, taking place on
surfaces of microscopic dust grains, is an essential process in the evolution
of chemical complexity in interstellar clouds. The H_2 formation process has
been studied theoretically, and in recent years also by laboratory experiments.
The experimental results were analyzed using a rate equation model. The
parameters of the surface, that are relevant to H_2 formation, were obtained
and used in order to calculate the recombination rate under interstellar
conditions. However, it turned out that due to the microscopic size of the dust
grains and the low density of H atoms, the rate equations may not always apply.
A master equation approach that provides a good description of the H_2
formation process was proposed. It takes into account both the discrete nature
of the H atoms and the fluctuations in the number of atoms on a grain. In this
paper we present a comprehensive analysis of the H_2 formation process, under
steady state conditions, using an exact solution of the master equation. This
solution provides an exact result for the hydrogen recombination rate and its
dependence on the flux, the surface temperature and the grain size. The results
are compared with those obtained from the rate equations. The relevant length
scales in the problem are identified and the parameter space is divided into
two domains. One domain, characterized by first order kinetics, exhibits high
efficiency of H_2 formation. In the other domain, characterized by second order
kinetics, the efficiency of H_2 formation is low. In each of these domains we
identify the range of parameters in which, the rate equations do not account
correctly for the recombination rate. and the master equation is needed.Comment: 23 pages + 8 figure
Brain tumour genetic network signatures of survival
Tumour heterogeneity is increasingly recognized as a major obstacle to
therapeutic success across neuro-oncology. Gliomas are characterised by
distinct combinations of genetic and epigenetic alterations, resulting in
complex interactions across multiple molecular pathways. Predicting disease
evolution and prescribing individually optimal treatment requires statistical
models complex enough to capture the intricate (epi)genetic structure
underpinning oncogenesis. Here, we formalize this task as the inference of
distinct patterns of connectivity within hierarchical latent representations of
genetic networks. Evaluating multi-institutional clinical, genetic, and outcome
data from 4023 glioma patients over 14 years, across 12 countries, we employ
Bayesian generative stochastic block modelling to reveal a hierarchical network
structure of tumour genetics spanning molecularly confirmed glioblastoma, IDH-
wildtype; oligodendroglioma, IDH-mutant and 1p/19q codeleted; and astrocytoma,
IDH- mutant. Our findings illuminate the complex dependence between features
across the genetic landscape of brain tumours, and show that generative network
models reveal distinct signatures of survival with better prognostic fidelity
than current gold standard diagnostic categories.Comment: Main article: 52 pages, 1 table, 7 figures. Supplementary material:
13 pages, 11 supplementary figure
Dust Dynamics in Compressible MHD Turbulence
We calculate the relative grain-grain motions arising from interstellar
magnetohydrodynamic (MHD) turbulence. The MHD turbulence includes both fluid
motions and magnetic fluctuations. While the fluid motions accelerate grains
through hydro-drag, the electromagnetic fluctuations accelerate grains through
resonant interactions. We consider both incompressive (Alfv\'{e}n) and
compressive (fast and slow) MHD modes and use descriptions of MHD turbulence
obtained in Cho & Lazarian (2002). Calculations of grain relative motion are
made for realistic grain charging and interstellar turbulence that is
consistent with the velocity dispersions observed in diffuse gas, including
cutoff of the turbulence from various damping processes. We show that fast
modes dominate grain acceleration, and can drive grains to supersonic
velocities. Grains are also scattered by gyroresonance interactions, but the
scattering is less important than acceleration for grains moving with
sub-Alfv\'{e}nic velocities. Since the grains are preferentially accelerated
with large pitch angles, the supersonic grains will be aligned with long axes
perpendicular to the magnetic field. We compare grain velocities arising from
MHD turbulence with those arising from photoelectric emission, radiation
pressure and H thrust. We show that for typical interstellar conditions
turbulence should prevent these mechanisms from segregating small and large
grains. Finally, gyroresonant acceleration is bound to preaccelerate grains
that are further accelerated in shocks. Grain-grain collisions in the shock may
then contribute to the overabundance of refractory elements in the composition
of galactic cosmic rays.Comment: 15 pages, 17 figure
Density of states in random lattices with translational invariance
We propose a random matrix approach to describe vibrational excitations in
disordered systems. The dynamical matrix M is taken in the form M=AA^T where A
is some real (not generally symmetric) random matrix. It guaranties that M is a
positive definite matrix which is necessary for mechanical stability of the
system. We built matrix A on a simple cubic lattice with translational
invariance and interaction between nearest neighbors. We found that for certain
type of disorder phonons cannot propagate through the lattice and the density
of states g(w) is a constant at small w. The reason is a breakdown of affine
assumptions and inapplicability of the elasticity theory. Young modulus goes to
zero in the thermodynamic limit. It strongly reminds of the properties of a
granular matter at the jamming transition point. Most of the vibrations are
delocalized and similar to diffusons introduced by Allen, Feldman et al., Phil.
Mag. B v.79, 1715 (1999).Comment: 4 pages, 5 figure
Sensitivity analyses of dense cloud chemical models
Because of new telescopes that will dramatically improve our knowledge of the
interstellar medium, chemical models will have to be used to simulate the
chemistry of many regions with diverse properties. To make these models more
robust, it is important to understand their sensitivity to a variety of
parameters. In this article, we report a study of the sensitivity of a chemical
model of a cold dense core, with homogeneous and time-independent physical
conditions, to variations in the following parameters: initial chemical
inventory, gas temperature and density, cosmic-ray ionization rate, chemical
reaction rate coefficients, and elemental abundances. From the results of the
parameter variations, we can quantify the sensitivity of the model to each
parameter as a function of time. Our results can be used in principle with
observations to constrain some parameters for different cold clouds. We also
attempted to use the Monte Carlo approach with all parameters varied
collectively. Within the parameter ranges studied, the most critical parameters
turn out to be the reaction rate coefficients at times up to 4e5 yr and
elemental abundances at later times. At typical times of best agreement with
observation, models are sensitive to both of these parameters. The models are
less sensitive to other parameters such as the gas density and temperature. The
improvement of models will require that the uncertainties in rate coefficients
of important reactions be reduced. As the chemistry becomes better understood
and more robust, it should be possible to use model sensitivities concerning
other parameters, such as the elemental abundances and the cosmic ray
ionization rate, to yield detailed information on cloud properties and history.
Nevertheless, at the current stage, we cannot determine the best values of all
the parameters simultaneously based on purely observational constraints.Comment: Accepted for publication in Astronomy & Astrophysic
A Coupled Dynamical and Chemical Model of Starless Cores of Magnetized Molecular Clouds: I. Formulation and Initial Results
We develop a detailed chemical model for the starless cores of strongly
magnetized molecular clouds, with the ambipolar diffusion-driven dynamic
evolution of the clouds coupled to the chemistry through ion abundances. We
concentrate on two representative model clouds in this initial study, one with
magnetic fields and the other without. The model predictions on the peak values
and spatial distributions of the column densities of CO, CCS, NH and
HCO are compared with those observationally inferred for the well-studied
starless core L1544, which is thought to be on the verge of star formation. We
find that the magnetic model, in which the cloud is magnetically supported for
several million years before collapsing dynamically, provides a reasonable
overall fit to the available data on L1544; the fit is significantly worse for
the non-magnetic model, in which the cloud collapses promptly. The observed
large peak column density for NH and clear central depression for CCS
favor the magnetically-retarded collapse over the free-fall collapse. A
relatively high abundance of CCS is found in the magnetic model, resulting most
likely from an interplay of depletion and late-time hydrocarbon chemistry
enhanced by CO depletion. These initial results lend some support to the
standard picture of dense core formation in strongly magnetized clouds through
ambipolar diffusion. They are at variance with those of Aikawa et al. (2001)
who considered a set of models somewhat different from ours and preferred one
in which the cloud collapses more or less freely for L1544.Comment: 25 pages, 7 figures, accepted to Ap
Observations of chemical differentiation in clumpy molecular clouds
We have extensively mapped a sample of dense molecular clouds (L1512, TMC-1C,
L1262, Per 7, L1389, L1251E) in lines of HC3N, CH3OH, SO and C^{18}O. We
demonstrate that a high degree of chemical differentiation is present in all of
the observed clouds. We analyse the molecular maps for each cloud,
demonstrating a systematic chemical differentiation across the sample, which we
relate to the evolutionary state of the cloud. We relate our observations to
the cloud physical, kinematical and evolutionary properties, and also compare
them to the predictions of simple chemical models. The implications of this
work for understanding the origin of the clumpy structures and chemical
differentiation observed in dense clouds are discussed.Comment: 20 pages, 7 figures. Higher quality figures appear in the published
journal articl
Physical Origin of the Boson Peak Deduced from a Two-Order-Parameter Model of Liquid
We propose that the boson peak originates from the (quasi-) localized
vibrational modes associated with long-lived locally favored structures, which
are intrinsic to a liquid state and are randomly distributed in a sea of
normal-liquid structures. This tells us that the number density of locally
favored structures is an important physical factor determining the intensity of
the boson peak. In our two-order-parameter model of the liquid-glass
transition, the locally favored structures act as impurities disturbing
crystallization and thus lead to vitrification. This naturally explains the
dependence of the intensity of the boson peak on temperature, pressure, and
fragility, and also the close correlation between the boson peak and the first
sharp diffraction peak (or prepeak).Comment: 5 pages, 1 figure, An error in the reference (Ref. 7) was correcte
Spitzer infrared spectrograph point source classification in the Small Magellanic Cloud
The Magellanic Clouds are uniquely placed to study the stellar contribution to dust emission. Individual stars can be resolved in these systems even in the mid-infrared, and they are close enough to allow detection of infrared excess caused by dust. We have searched the Spitzer Space Telescope data archive for all Infrared Spectrograph (IRS) staring-mode observations of the Small Magellanic Cloud (SMC) and found that 209 Infrared Array Camera (IRAC) point sources within the footprint of the Surveying the Agents of Galaxy Evolution in the Small Magellanic Cloud (SAGE-SMC) Spitzer Legacy programme were targeted, within a total of 311 staring-mode observations. We classify these point sources using a decision tree method of object classification, based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership and variability information. We find 58 asymptotic giant branch (AGB) stars, 51 young stellar objects, 4 post-AGB objects, 22 red supergiants, 27 stars (of which 23 are dusty OB stars), 24 planetary nebulae (PNe), 10 WolfâRayet stars, 3 H II regions, 3 R Coronae Borealis stars, 1 Blue Supergiant and 6 other objects, including 2 foreground AGB stars. We use these classifications to evaluate the success of photometric classification methods reported in the literature
Grain Surface Models and Data for Astrochemistry
AbstractThe cross-disciplinary field of astrochemistry exists to understand the formation, destruction, and survival of molecules in astrophysical environments. Molecules in space are synthesized via a large variety of gas-phase reactions, and reactions on dust-grain surfaces, where the surface acts as a catalyst. A broad consensus has been reached in the astrochemistry community on how to suitably treat gas-phase processes in models, and also on how to present the necessary reaction data in databases; however, no such consensus has yet been reached for grain-surface processes. A team of âŒ25 experts covering observational, laboratory and theoretical (astro)chemistry met in summer of 2014 at the Lorentz Center in Leiden with the aim to provide solutions for this problem and to review the current state-of-the-art of grain surface models, both in terms of technical implementation into models as well as the most up-to-date information available from experiments and chemical computations. This review builds on the results of this workshop and gives an outlook for future directions
- âŠ