247 research outputs found
Ab initio quality neural-network potential for sodium
An interatomic potential for high-pressure high-temperature (HPHT)
crystalline and liquid phases of sodium is created using a neural-network (NN)
representation of the ab initio potential energy surface. It is demonstrated
that the NN potential provides an ab initio quality description of multiple
properties of liquid sodium and bcc, fcc, cI16 crystal phases in the P-T region
up to 120 GPa and 1200 K. The unique combination of computational efficiency of
the NN potential and its ability to reproduce quantitatively experimental
properties of sodium in the wide P-T range enables molecular dynamics
simulations of physicochemical processes in HPHT sodium of unprecedented
quality.Comment: 8 pages, 11 figures, 2 table
Chinese Students in U.S. Universities: A Qualitative Study of Cross-Cultural Learning Experiences, Transition and Adaptation
Chinese students represent the largest single group among international students enrolled in the U.S, and globalization has played an important role in impacting Chinese students’ perceptions of what it means to study abroad. According to The Wall Street Journal, there are 85 percent more international students enrolled today in U.S. schools than ten years ago, adding more than 35 billion dollars to the nation’s economy in 2015 (Belkin & Purnell, 2017). This qualitative study adds to the limited research available regarding Chinese students’ cross-cultural transition and academic adaptation to American universities (Kusek, 2015; Yan & Berliner, 2009). Findings add a new perspective regarding the students’ perceptions and expectations in China compared to their true experiences in the U.S. universities. Results show that, despite significant barriers, Chinese students are eventually able to transition and adapt to the new surroundings.
Implications of findings can help in the development of effective strategies and programs to facilitate these students’ transition and adaptation in U.S. higher education institutions
Near-deterministic hybrid generation of arbitrary photonic graph states using a single quantum emitter and linear optics
Since linear-optical two-photon gates are inherently probabilistic,
measurement-based implementations are particularly well suited for photonic
platforms: a large highly-entangled photonic resource state, called a graph
state, is consumed through measurements to perform a computation. The challenge
is thus to produce these graph states. Several generation procedures, which use
either interacting quantum emitters or efficient spin-photon interface, have
been proposed to create these photonic graph states deterministically. Yet,
these solutions are still out of reach experimentally since the
state-of-the-art is the generation of a linear graph state. Here, we introduce
near-deterministic solutions for the generation of graph states using the
current quantum emitter capabilities. We propose hybridizing
quantum-emitter-based graph state generation with all-photonic fusion gates to
produce graph states of complex topology near-deterministically. Our results
should pave the way towards the practical implementation of resource-efficient
quantum information processing, including measurement-based quantum
communication and quantum computing.Comment: 19 pages, 8 figure
Modeling Variable Emission Lines in AGNs: Method and Application to NGC 5548
We present a new scheme for modeling the broad line region in active galactic
nuclei (AGNs). It involves photoionization calculations of a large number of
clouds, in several pre-determined geometries, and a comparison of the
calculated line intensities with observed emission line light curves. Fitting
several observed light curves simultaneously provides strong constraints on
model parameters such as the run of density and column density across the
nucleus, the shape of the ionizing continuum, and the radial distribution of
the emission line clouds. When applying the model to the Seyfert 1 galaxy NGC
5548, we were able to reconstruct the light curves of four ultraviolet
emission-lines, in time and in absolute flux. This has not been achieved by any
previous work. We argue that the Balmer lines light curves, and possibly also
the MgII2798 light curve, cannot be tested in this scheme because of the
limitations of present-day photoionization codes. Our fit procedure can be used
to rule out models where the particle density scales as r^{-2}, where r is the
distance from the central source. The best models are those where the density
scales as r^{-1} or r^{-1.5}. We can place a lower limit on the column density
at a distance of 1 ld, of N_{col}(r=1) >~ 10^{23} cm^{-2} and limit the
particle density to be in the range of 10^{12.5}>N(r=1)>10^{11} cm^{-3}. We
have also tested the idea that the spectral energy distribution (SED) of the
ionizing continuum is changing with continuum luminosity. None of the
variable-shape SED tried resulted in real improvement over a constant SED case
although models with harder continuum during phases of higher luminosity seem
to fit better the observed spectrum. Reddening and/or different composition
seem to play a minor role, at least to the extent tested in this work.Comment: 12 pages, including 9 embedded EPS figures, accepted for publication
in Ap
Molecular Hydrogen Formation on Amorphous Silicates Under Interstellar Conditions
Experimental results on the formation of molecular hydrogen on amorphous silicate surfaces are presented for the first time and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained earlier for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of molecular hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H_{2} formation within a temperature range which is relevant to diffuse interstellar clouds. The results also indicate that the hydrogen molecules are thermalized with the surface and desorb with low kinetic energy. Thus, they are unlikely to occupy highly excited states
The Relationship Between Luminosity and Broad-Line Region Size in Active Galactic Nuclei
We reinvestigate the relationship between the characteristic broad-line
region size (R_blr) and the Balmer emission-line, X-ray, UV, and optical
continuum luminosities. Our study makes use of the best available
determinations of R_blr for a large number of active galactic nuclei (AGNs)
from Peterson et al. Using their determinations of R_blr for a large sample of
AGNs and two different regression methods, we investigate the robustness of our
correlation results as a function of data sub-sample and regression technique.
Though small systematic differences were found depending on the method of
analysis, our results are generally consistent. Assuming a power-law relation
R_blr \propto L^\alpha, we find the mean best-fitting \alpha is about
0.67+/-0.05 for the optical continuum and the broad H\beta luminosity, about
0.56+/-0.05 for the UV continuum luminosity, and about 0.70+/-0.14 for the
X-ray luminosity. We also find an intrinsic scatter of about 40% in these
relations. The disagreement of our results with the theoretical expected slope
of 0.5 indicates that the simple assumption of all AGNs having on average same
ionization parameter, BLR density, column density, and ionizing spectral energy
distribution, is not valid and there is likely some evolution of a few of these
characteristics along the luminosity scale.Comment: 11 pages, 2 figures, emulateapj, accepted for publication in The
Astrophysical Journa
Simulating the tidal disruption of stars by stellar-mass black holes using moving-mesh hydrodynamics
In the centers of dense star clusters, close encounters between stars and
compact objects are likely to occur. We study tidal disruption events of
main-sequence (MS) stars by stellar-mass black holes (termed TDEs), which
can shed light on the processes occurring in these clusters, including being an
avenue in the mass growth of stellar-mass BHs. Using the moving-mesh
hydrodynamics code \texttt{AREPO}, we perform a suite of hydrodynamics
simulations of partial TDEs of realistic, \texttt{MESA}-generated MS stars
by varying the initial mass of the star ( and
), the age of the star (zero-age, middle-age and
terminal-age), the mass of the black hole ( and
) and the impact parameter (yielding almost no mass
loss to full disruption). We then examine the dependence of the masses, spins,
and orbital parameters of the partially disrupted remnant on the initial
encounter parameters. We find that the mass lost from a star decreases
exponentially with increasing distance of approach and that a star loses lesser mass than a . Moreover, a
more evolved star is less susceptible to mass loss. Tidal torques at the
closest approach spin up the remnant by factors of -- depending on
the impact parameter. The remnant star can be bound (eccentric) or unbound
(hyperbolic) to the black hole: hyperbolic orbits occur when the star's central
density concentration is relatively low and the black hole-star mass ratio is
high, which is the case for the disruption of a
star. Finally, we provide best-fit analytical formulae for a range of
parameters that can be incorporated into cluster codes to model star-black hole
interaction more accurately.Comment: 14 pages, 9 figures, submitted to A&
The Nature of Associated Absorption and the UV-X-ray Connection in 3C 288.1
We discuss new Hubble Space Telescope spectroscopy of the radio-loud quasar,
3C 288.1. The data cover ~590 A to ~1610 A in the quasar rest frame. They
reveal a wealth of associated absorption lines (AALs) with no accompanying
Lyman-limit absorption. The metallic AALs range in ionization from C III and N
III to Ne VIII and Mg X. We use these data and photoionization models to derive
the following properties of the AAL gas: 1) There are multiple ionization zones
within the AAL region, spanning a factor of at least ~50 in ionization
parameter. 2) The overall ionization is consistent with the ``warm'' X-ray
continuum absorbers measured in Seyfert 1 nuclei and other QSOs. However, 3)
the column densities implied by the AALs in 3C 288.1 are too low to produce
significant bound-free absorption at any UV-X-ray wavelengths. Substantial
X-ray absorption would require yet another zone, having a much higher
ionization or a much lower velocity dispersion than the main AAL region. 4) The
total hydrogen column density in the AAL gas is log N_H (cm-2)= 20.2. 5) The
metallicity is roughly half solar. 6) The AALs have deconvolved widths of ~900
km/s and their centroids are consistent with no shift from the quasar systemic
velocity (conservatively within +/-1000 km/s). 7) There are no direct
indicators of the absorber's location in our data, but the high ionization and
high metallicity both suggest a close physical relationship to the quasar/host
galaxy environment. Finally, the UV continuum shape gives no indication of a
``blue bump'' at higher energies. There is a distinct break of unknown origin
at ~1030 A, and the decline toward higher energies (with spectral index alpha =
-1.73, for f_nu ~ nu^alpha) is even steeper than a single power-law
interpolation from 1030 A to soft X-rays.Comment: 27 pages with figures and tables, in press with Ap
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