9,852 research outputs found
Distribution of Gaussian Process Arc Lengths
We present the first treatment of the arc length of the Gaussian Process (GP)
with more than a single output dimension. GPs are commonly used for tasks such
as trajectory modelling, where path length is a crucial quantity of interest.
Previously, only paths in one dimension have been considered, with no
theoretical consideration of higher dimensional problems. We fill the gap in
the existing literature by deriving the moments of the arc length for a
stationary GP with multiple output dimensions. A new method is used to derive
the mean of a one-dimensional GP over a finite interval, by considering the
distribution of the arc length integrand. This technique is used to derive an
approximate distribution over the arc length of a vector valued GP in
by moment matching the distribution. Numerical simulations
confirm our theoretical derivations.Comment: 10 pages, 4 figures, Accepted to The 20th International Conference on
Artificial Intelligence and Statistics (AISTATS
Bulk and surface energetics of lithium hydride crystal: benchmarks from quantum Monte Carlo and quantum chemistry
We show how accurate benchmark values of the surface formation energy of
crystalline lithium hydride can be computed by the complementary techniques of
quantum Monte Carlo (QMC) and wavefunction-based molecular quantum chemistry.
To demonstrate the high accuracy of the QMC techniques, we present a detailed
study of the energetics of the bulk LiH crystal, using both pseudopotential and
all-electron approaches. We show that the equilibrium lattice parameter agrees
with experiment to within 0.03 %, which is around the experimental uncertainty,
and the cohesive energy agrees to within around 10 meV per formula unit. QMC in
periodic slab geometry is used to compute the formation energy of the LiH (001)
surface, and we show that the value can be accurately converged with respect to
slab thickness and other technical parameters. The quantum chemistry
calculations build on the recently developed hierarchical scheme for computing
the correlation energy of a crystal to high precision. We show that the
hierarchical scheme allows the accurate calculation of the surface formation
energy, and we present results that are well converged with respect to basis
set and with respect to the level of correlation treatment. The QMC and
hierarchical results for the surface formation energy agree to within about 1
%.Comment: 16 pages, 4 figure
Double layer in ionic liquids: Overscreening vs. crowding
We develop a simple Landau-Ginzburg-type continuum theory of solvent-free
ionic liquids and use it to predict the structure of the electrical double
layer. The model captures overscreening from short-range correlations, dominant
at small voltages, and steric constraints of finite ion sizes, which prevail at
large voltages. Increasing the voltage gradually suppresses overscreening in
favor of the crowding of counterions in a condensed inner layer near the
electrode. The predicted ion profiles and capacitance-voltage relations are
consistent with recent computer simulations and experiments on room-temperature
ionic liquids, using a correlation length of order the ion size.Comment: 4 pages + supplementary informatio
The habitability of a stagnant-lid Earth
Plate tectonics is a fundamental component for the habitability of the Earth.
Yet whether it is a recurrent feature of terrestrial bodies orbiting other
stars or unique to the Earth is unknown. The stagnant lid may rather be the
most common tectonic expression on such bodies. To understand whether a
stagnant-lid planet can be habitable, i.e. host liquid water at its surface, we
model the thermal evolution of the mantle, volcanic outgassing of HO and
CO, and resulting climate of an Earth-like planet lacking plate tectonics.
We used a 1D model of parameterized convection to simulate the evolution of
melt generation and the build-up of an atmosphere of HO and CO over 4.5
Gyr. We then employed a 1D radiative-convective atmosphere model to calculate
the global mean atmospheric temperature and the boundaries of the habitable
zone (HZ). The evolution of the interior is characterized by the initial
production of a large amount of partial melt accompanied by a rapid outgassing
of HO and CO. At 1 au, the obtained temperatures generally allow for
liquid water on the surface nearly over the entire evolution. While the outer
edge of the HZ is mostly influenced by the amount of outgassed CO, the
inner edge presents a more complex behaviour that is dependent on the partial
pressures of both gases. At 1 au, the stagnant-lid planet considered would be
regarded as habitable. The width of the HZ at the end of the evolution, albeit
influenced by the amount of outgassed CO, can vary in a non-monotonic way
depending on the extent of the outgassed HO reservoir. Our results suggest
that stagnant-lid planets can be habitable over geological timescales and that
joint modelling of interior evolution, volcanic outgassing, and accompanying
climate is necessary to robustly characterize planetary habitability
Simple model of the static exchange-correlation kernel of a uniform electron gas with long-range electron-electron interaction
A simple approximate expression in real and reciprocal spaces is given for
the static exchange-correlation kernel of a uniform electron gas interacting
with the long-range part only of the Coulomb interaction. This expression
interpolates between the exact asymptotic behaviors of this kernel at small and
large wave vectors which in turn requires, among other thing, information from
the momentum distribution of the uniform electron gas with the same interaction
that have been calculated in the G0W0 approximation. This exchange-correlation
kernel as well as its complement analogue associated to the short-range part of
the Coulomb interaction are more local than the Coulombic exchange-correlation
kernel and constitute potential ingredients in approximations for recent
adiabatic connection fluctuation-dissipation and/or density functional theory
approaches of the electronic correlation problem based on a separate treatment
of long-range and short-range interaction effects.Comment: 14 pages, 14 figures, to be published in Phys. Rev.
Hubble Tarantula Treasury Project: Unraveling Tarantula's Web. II. Optical and Near Infrared Star Formation History of the Starburst Cluster NGC 2070 in 30 Doradus
We present a study of the recent star formation of 30 Doradus in the Large
Magellanic Cloud (LMC) using the panchromatic imaging survey Hubble Tarantula
Treasury Project (HTTP). In this paper we focus on the stars within 20 pc of
the center of the massive ionizing cluster of 30 Doradus, NGC 2070. We
recovered the star formation history by comparing deep optical and NIR
color-magnitude diagrams (CMDs) with state-of-the-art synthetic CMDs generated
with the latest PARSEC models, which include all stellar phases from pre-main
sequence to post- main sequence. For the first time in this region we are able
to measure the star formation using intermediate and low mass stars
simultaneously. Our results suggest that NGC2070 experienced a prolonged
activity. In particular, we find that the star formation in the region: i)
exceeded the average LMC rate ~ 20 Myr ago; ii) accelerated dramatically ~ 7
Myr ago; and iii) reached a peak value 1-3 Myr ago. We did not find significant
deviations from a Kroupa initial mass function down to 0.5 Msun. The average
internal reddening E(B-V) is found to be between 0.3 and 0.4 mag.Comment: Submitted to Ap
Carbon and nitrogen abundances of individual stars in the Sculptor dwarf spheroidal galaxy
We present [C/Fe] and [N/Fe] abundance ratios and CH({\lambda}4300) and
S({\lambda}3883) index measurements for 94 red giant branch (RGB) stars in the
Sculptor dwarf spheroidal galaxy from VLT/VIMOS MOS observations at a resolving
power R= 1150 at 4020 {\AA}. This is the first time that [N/Fe] abundances are
derived for a large number of stars in a dwarf spheroidal. We found a trend for
the [C/Fe] abundance to decrease with increasing luminosity on the RGB across
the whole metallicity range, a phenomenon observed in both field and globular
cluster giants, which can be interpreted in the framework of evolutionary
mixing of partially processed CNO material. Both our measurements of [C/Fe] and
[N/Fe] are in good agreement with the theoretical predictions for stars at
similar luminosity and metallicity. We detected a dispersion in the carbon
abundance at a given [Fe/H], which cannot be ascribed to measurement
uncertainties alone. We interpret this observational evidence as the result of
the contribution of different nucleosynthesis sources over time to a not
well-mixed interstellar medium. We report the discovery of two new
carbon-enhanced, metal-poor stars. These are likely the result of pollution
from material enriched by asymptotic giant branch stars, as indicated by our
estimates of [Ba/Fe]> +1. We also attempted a search for dissolved globular
clusters in the field of the galaxy by looking for the distinctive C-N pattern
of second population globular clusters stars in a previously detected, very
metal-poor, chemodynamical substructure. We do not detect chemical anomalies
among this group of stars. However, small number statistics and limited spatial
coverage do not allow us to exclude the hypotheses that this substructure forms
part of a tidally shredded globular cluster.Comment: 18 pages, 14 figures, 3 tables. Accepted to A&
The Star Formation History of IZw18
The star formation history in IZw18 has been inferred from HST/WFPC2 archival
data. This is done by comparing the derived V, B-V and V, V-I color-magnitude
diagrams and luminosity functions with synthetic ones, based on various sets of
stellar evolutionary tracks. At a distance of 10 Mpc, the stars resolved in the
field of IZw18 allow for a lookback time up to 1 Gyr. We find that the main
body is not experiencing its first episode of star formation. Instead, it has
been forming stars over the last 0.5-1 Gyr, at a rate of ~ 1-2 * 10**(-2) Msol
per year per kpc**2. A more intense activity of 6-16 * 10**(-2) Msol per year
per kpc**2 has taken place between 15 and 20 Myr ago. For the secondary body,
the lookback time is 0.2 Gyr at most and the uncertainty is much higher, due to
the shallower diagrams and the small number of resolved stars. The derived
range of star formation rate is 3-10 * 10**(-3) Msol per year per kpc**2. The
IMF providing the best fit to the observed stellar populations in the main body
has a slope 1.5, much flatter than in any similar galaxy analyzed with the same
method. In the secondary body, it is peaked at 2.2, closer to Salpeter's slope
(2.35).Comment: 70 pages including 18 figures, to be published in The Astronomical
Journa
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