8,510 research outputs found
Electron Emission from Diamondoids: A Diffusion Quantum Monte Carlo Study
We present density-functional theory (DFT) and quantum Monte Carlo (QMC)
calculations designed to resolve experimental and theoretical controversies
over the optical properties of H-terminated C nanoparticles (diamondoids). The
QMC results follow the trends of well-converged plane-wave DFT calculations for
the size dependence of the optical gap, but they predict gaps that are 1-2 eV
higher. They confirm that quantum confinement effects disappear in diamondoids
larger than 1 nm, which have gaps below that of bulk diamond. Our QMC
calculations predict a small exciton binding energy and a negative electron
affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized
nature of the lowest unoccupied molecular orbital. The NEA suggests a range of
possible applications of diamondoids as low-voltage electron emitters
Formation and Disruption of Cosmological Low Mass Objects
We investigate the evolution of cosmological low mass (low virial
temperature) objects and the formation of the first luminous objects. First,
the `cooling diagram' for low mass objects is shown. We assess the cooling rate
taking into account the contribution of H_2, which is not in chemical
equilibrium generally, with a simple argument of time scales. The reaction
rates and the cooling rate of H_2 are taken from the recent results by Galli &
Palla (1998). Using this cooling diagram, we also estimate the formation
condition of luminous objects taking into account the supernova (SN) disruption
of virialized clouds. We find that the mass of the first luminous object is
several times 10^7 solar mass, because smaller objects may be disrupted by the
SNe before they become luminous. Metal pollution of low mass (Ly-alpha) clouds
also discussed. The resultant metallicity of the clouds is about 1/1000 of the
solar metallicity.Comment: 11 pages, 2 figures, To appear in ApJ
Ab initio Molecular Dynamics in Adaptive Coordinates
We present a new formulation of ab initio molecular dynamics which exploits
the efficiency of plane waves in adaptive curvilinear coordinates, and thus
provides an accurate treatment of first-row elements. The method is used to
perform a molecular dynamics simulation of the CO_2 molecule, and allows to
reproduce detailed features of its vibrational spectrum such as the splitting
of the Raman sigma+_g mode caused by Fermi resonance. This new approach opens
the way to highly accurate ab initio simulations of organic compounds.Comment: 11 pages, 3 PostScript figure
Nonaxisymmetric Evolution of Magnetically Subcritical Clouds: Bar Growth, Core Elongation, and Binary Formation
We have begun a systematic numerical study of the nonlinear growth of
nonaxisymmetric perturbations during the ambipolar diffusion-driven evolution
of initially magnetically subcritical molecular clouds, with an eye on the
formation of binaries, multiple stellar systems and small clusters. In this
initial study, we focus on the (or bar) mode, which is shown to be
unstable during the dynamic collapse phase of cloud evolution after the central
region has become magnetically supercritical. We find that, despite the
presence of a strong magnetic field, the bar can grow fast enough that for a
modest initial perturbation (at 5% level) a large aspect ratio is obtained
during the isothermal phase of cloud collapse. The highly elongated bar is
expected to fragment into small pieces during the subsequent adiabatic phase.
Our calculations suggest that the strong magnetic fields observed in some
star-forming clouds and envisioned in the standard picture of single star
formation do not necessarily suppress bar growth and fragmentation; on the
contrary, they may actually promote these processes, by allowing the clouds to
have more than one (thermal) Jeans mass to begin with without collapsing
promptly. Nonlinear growth of the bar mode in a direction perpendicular to the
magnetic field, coupled with flattening along field lines, leads to the
formation of supercritical cores that are triaxial in general. It removes a
longstanding objection to the standard scenario of isolated star formation
involving subcritical magnetic field and ambipolar diffusion based on the
likely prolate shape inferred for dense cores. Continuted growth of the bar
mode in already elongated starless cores, such as L1544, may lead to future
binary and multiple star formation.Comment: 5 pages, 2 figures, accepted by ApJ
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
Cosmic Renaissance: The First Sources of Light
I review recent progress in understanding the formation of the first stars
and quasars. The initial conditions for their emergence are given by the now
firmly established model of cosmological structure formation. Numerical
simulations of the collapse and fragmentation of primordial gas indicate that
the first stars formed at redshifts z ~ 20 - 30, and that they were
predominantly very massive, with M_* > 100 M_sun. Important uncertainties,
however, remain. Paramount among them is the accretion process, which builds up
the final stellar mass by incorporating part of the diffuse, dust-free envelope
into the central protostellar core. The first quasars, on the other hand, are
predicted to have formed later on, at z ~ 10, in more massive dark matter
halos, with total masses, ~ 10^8 M_sun, characteristic of dwarf galaxies.Comment: 16 pages, 7 figures, invited review, to appear in PASP, Feb. 200
Order-N Density-Matrix Electronic-Structure Method for General Potentials
A new order-N method for calculating the electronic structure of general
(non-tight-binding) potentials is presented. The method uses a combination of
the ``purification''-based approaches used by Li, Nunes and Vanderbilt, and
Daw, and a representation of the density matrix based on ``travelling basis
orbitals''. The method is applied to several one-dimensional examples,
including the free electron gas, the ``Morse'' bound-state potential, a
discontinuous potential that mimics an interface, and an oscillatory potential
that mimics a semiconductor. The method is found to contain Friedel
oscillations, quantization of charge in bound states, and band gap formation.
Quantitatively accurate agreement with exact results is found in most cases.
Possible advantages with regard to treating electron-electron interactions and
arbitrary boundary conditions are discussed.Comment: 13 pages, REVTEX, 7 postscript figures (not quite perfect
Formation of the First Supermassive Black Holes
We consider the physical conditions under which supermassive black holes
could have formed inside the first galaxies. Our SPH simulations indicate that
metal-free galaxies with a virial temperature ~10^4 K and with suppressed H2
formation (due to an intergalactic UV background) tend to form a binary black
hole system which contains a substantial fraction (>10%) of the total baryonic
mass of the host galaxy. Fragmentation into stars is suppressed without
substantial H2 cooling. Our simulations follow the condensation of ~5x10^6
M_sun around the two centers of the binary down to a scale of < 0.1pc. Low-spin
galaxies form a single black hole instead. These early black holes lead to
quasar activity before the epoch of reionization. Primordial black hole
binaries lead to the emission of gravitational radiation at redshifts z>10 that
would be detectable by LISA.Comment: 11 pages, 9 figures, revised version, ApJ in press (October 10, 2003
Dust emissivity in the Submm/Mm: SCUBA and SIMBA observations of Barnard 68
We have observed the dark cloud Barnard 68 with SCUBA at 850 um and with
SIMBA at 1.2 mm. The submillimetre and millimetre dust emission correlate well
with the extinction map of Alves, Lada and Lada (2001).The A_V/850um
correlation is clearly not linear and suggests lower temperatures for the dust
in the inner core of the cloud. Assuming a model for the temperature gradient,
we derive the cloud-averaged dust emissivities (normalised to the V-Band
extinction efficiency) at 850 um and 1.2 mm. We find k_850um/k_V = 4.0 +/- 1.0
x 10^-5 and k_1.2mm/k_V = 9.0 +/- 3.0 x 10^-6. These values are compared with
other determinations in this wavelength regime and with expectations for models
of diffuse dust and grain growth in dense clouds.Comment: 8 pages, 4 figures, A&A accepted (Letter), referee forma
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