22,989 research outputs found
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
Molecular Hydrogen and Global Star Formation Relations in Galaxies
(ABRIDGED) We use hydrodynamical simulations of disk galaxies to study
relations between star formation and properties of the molecular interstellar
medium (ISM). We implement a model for the ISM that includes low-temperature
(T<10^4K) cooling, directly ties the star formation rate to the molecular gas
density, and accounts for the destruction of H2 by an interstellar radiation
field from young stars. We demonstrate that the ISM and star formation model
simultaneously produces a spatially-resolved molecular-gas surface density
Schmidt-Kennicutt relation of the form Sigma_SFR \propto Sigma_Hmol^n_mol with
n_mol~1.4 independent of galaxy mass, and a total gas surface density -- star
formation rate relation Sigma_SFR \propto Sigma_gas^n_tot with a power-law
index that steepens from n_tot~2 for large galaxies to n_tot>~4 for small dwarf
galaxies. We show that deviations from the disk-averaged Sigma_SFR \propto
Sigma_gas^1.4 correlation determined by Kennicutt (1998) owe primarily to
spatial trends in the molecular fraction f_H2 and may explain observed
deviations from the global Schmidt-Kennicutt relation.Comment: Version accepted by ApJ, high-res version available at
http://kicp.uchicago.edu/~brant/astro-ph/molecular_ism/rk2007.pd
Crystal Nucleation in Liquids: Open Questions and Future Challenges in Molecular Dynamics Simulations
The nucleation of crystals in liquids is one of nature's most ubiquitous
phenomena, playing an important role in areas such as climate change and the
production of drugs. As the early stages of nucleation involve exceedingly
small time and length scales, atomistic computer simulations can provide unique
insight into the microscopic aspects of crystallization. In this review, we
take stock of the numerous molecular dynamics simulations that in the last few
decades have unraveled crucial aspects of crystal nucleation in liquids. We put
into context the theoretical framework of classical nucleation theory and the
state of the art computational methods, by reviewing simulations of e.g. ice
nucleation or crystallization of molecules in solutions. We shall see that
molecular dynamics simulations have provided key insight into diverse
nucleation scenarios, ranging from colloidal particles to natural gas hydrates,
and that in doing so the general applicability of classical nucleation theory
has been repeatedly called into question. We have attempted to identify the
most pressing open questions in the field. We believe that by improving (i.)
existing interatomic potentials; and (ii.) currently available enhanced
sampling methods, the community can move towards accurate investigations of
realistic systems of practical interest, thus bringing simulations a step
closer to experiments
Triggering the Formation of Halo Globular Clusters with Galaxy Outflows
We investigate the interactions of high-redshift galaxy outflows with
low-mass virialized (Tvir < 10,000K) clouds of primordial composition. While
atomic cooling allows star formation in larger primordial objects, such
"minihalos" are generally unable to form stars by themselves. However, the
large population of high-redshift starburst galaxies may have induced
widespread star formation in these objects, via shocks that caused intense
cooling both through nonequilibrium H2 formation and metal-line emission. Using
a simple analytic model, we show that the resulting star clusters naturally
reproduce three key features of the observed population of halo globular
clusters (GCs). First, the 10,000 K maximum virial temperature corresponds to
the ~ 10^6 solar mass upper limit on the stellar mass of GCs. Secondly, the
momentum imparted in such interactions is sufficient to strip the gas from its
associated dark matter halo, explaining why GCs do not reside in dark matter
potential wells. Finally, the mixing of ejected metals into the primordial gas
is able to explain the ~ 0.1 dex homogeneity of stellar metallicities within a
given GC, while at the same time allowing for a large spread in metallicity
between different clusters. To study this possibility in detail, we use a
simple 1D numerical model of turbulence transport to simulate mixing in
cloud-outflow interactions. We find that as the shock shears across the side of
the cloud, Kelvin-Helmholtz instabilities arise, which cause mixing of enriched
material into > 20% of the cloud. Such estimates ignore the likely presence of
large-scale vortices, however, which would further enhance turbulence
generation. Thus quantitative mixing predictions must await more detailed
numerical studies.Comment: 21 pages, 11 figures, Apj in pres
Molecular Biology at the Quantum Level: Can Modern Density Functional Theory Forge the Path?
Recent years have seen vast improvements in the ability of rigorous
quantum-mechanical methods to treat systems of interest to molecular biology.
In this review article, we survey common computational methods used to study
such large, weakly bound systems, starting from classical simulations and
reaching to quantum chemistry and density functional theory. We sketch their
underlying frameworks and investigate their strengths and weaknesses when
applied to potentially large biomolecules. In particular, density functional
theory---a framework that can treat thousands of atoms on firm theoretical
ground---can now accurately describe systems dominated by weak van der Waals
interactions. This newfound ability has rekindled interest in using this
tried-and-true approach to investigate biological systems of real importance.
In this review, we focus on some new methods within density functional theory
that allow for accurate inclusion of the weak interactions that dominate
binding in biological macromolecules. Recent work utilizing these methods to
study biologically-relevant systems will be highlighted, and a vision for the
future of density functional theory within molecular biology will be discussed
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