157 research outputs found
Fast and Accurate OOV Decoder on High-Level Features
This work proposes a novel approach to out-of-vocabulary (OOV) keyword search
(KWS) task. The proposed approach is based on using high-level features from an
automatic speech recognition (ASR) system, so called phoneme posterior based
(PPB) features, for decoding. These features are obtained by calculating
time-dependent phoneme posterior probabilities from word lattices, followed by
their smoothing. For the PPB features we developed a special novel very fast,
simple and efficient OOV decoder. Experimental results are presented on the
Georgian language from the IARPA Babel Program, which was the test language in
the OpenKWS 2016 evaluation campaign. The results show that in terms of maximum
term weighted value (MTWV) metric and computational speed, for single ASR
systems, the proposed approach significantly outperforms the state-of-the-art
approach based on using in-vocabulary proxies for OOV keywords in the indexed
database. The comparison of the two OOV KWS approaches on the fusion results of
the nine different ASR systems demonstrates that the proposed OOV decoder
outperforms the proxy-based approach in terms of MTWV metric given the
comparable processing speed. Other important advantages of the OOV decoder
include extremely low memory consumption and simplicity of its implementation
and parameter optimization.Comment: Interspeech 2017, August 2017, Stockholm, Sweden. 201
Critical ingredients of supernova Ia radiative-transfer modeling
We explore the physics of SN Ia light curves and spectra using the 1-D
non-LTE time-dependent radiative-transfer code CMFGEN. Rather than adjusting
ejecta properties to match observations, we select as input one "standard" 1-D
Chandrasekhar-mass delayed-detonation hydrodynamical model, and then explore
the sensitivity of radiation and gas properties on radiative-transfer modeling
assumptions. The correct computation of SN Ia radiation is not exclusively a
solution to an "opacity problem", characterized by the treatment of a large
number of lines. It is also key to treat important atomic processes
consistently. Besides handling line blanketing in non-LTE, we show that
including forbidden line transitions of metals is increasingly important for
the temperature and ionization of the gas beyond maximum light. Non-thermal
ionization and excitation are also critical since they affect the color
evolution and the Delta-M15 of our model. While impacting little the bolometric
luminosity, a more complete treatment of decay routes leads to enhanced line
blanketing, e.g., associated with 48Ti in the U and B bands. Overall, we find
that SN Ia radiation properties are influenced in a complicated way by the
atomic data we employ, so that obtaining converged results is a challenge. We
nonetheless obtain a good match to the golden standard type Ia SN 2005cf in the
optical and near-IR, from 5 to 60d after explosion, suggesting that assuming
spherical symmetry is not detrimental to SN Ia radiative-transfer modeling at
these times. Multi-D effects no doubt matter, but they are perhaps less
important than accurately treating non-LTE processes [abridged].Comment: Accepted to MNRA
Constraints on the explosion mechanism and progenitors of type Ia supernovae
Observations of SN 2011fe at early times reveal an evolution analogous to a
fireball model of constant color. In contrast, our unmixed delayed detonations
of Chandrasekhar-mass white dwarfs (DDC series) exhibit a faster brightening
concomitant with a shift in color to the blue. In this paper, we study the
origin of these discrepancies. We find that strong chemical mixing largely
resolves the photometric mismatch at early times, but it leads to an enhanced
line broadening that contrasts, for example, with the markedly narrow SiII6355A
line of SN 2011fe. We also explore an alternative configuration with
pulsational-delayed detonations (PDDEL model series). Because of the pulsation,
PDDEL models retain more unburnt carbon, have little mass at high velocity, and
have a much hotter outer ejecta after the explosion. The pulsation does not
influence the inner ejecta, so PDDEL and DDC models exhibit similar radiative
properties beyond maximum. However, at early times, PDDEL models show bluer
optical colors and a higher luminosity, even for weak mixing. Their early-time
radiation is derived primarily from the initial shock-deposited energy in the
outer ejecta rather than radioactive decay heating. Furthermore, PDDEL models
show short-lived CII lines, reminiscent of SN 2013dy. They typically exhibit
lines that are weaker, narrower, and of near-constant width, reminiscent of SN
2011fe. In addition to multi-dimensional effects, varying configurations for
such ``pulsations" offer a source of spectral diversity amongst SNe Ia. PDDEL
and DDC models also provide one explanation for low- and high-velocity gradient
SNe Ia.Comment: Accepted to MNRA
[CoIII] versus NaID in type Ia supernova spectra
The high metal content and fast expansion of supernova (SN) Ia ejecta lead to
considerable line overlap in their optical spectra. Uncertainties in
composition and ionization further complicate the process of line
identification. In this paper, we focus on the 5900A emission feature seen in
SN Ia spectra after bolometric maximum, a line which in the last two decades
has been associated with [CoIII]5888A or NaID. Using non-LTE time-dependent
radiative-transfer calculations based on Chandrasekhar-mass delayed-detonation
models, we find that NaID line emission is extremely weak at all post-maximum
epochs. Instead, we predict the presence of [CoIII]5888A after maximum in all
our SN Ia models, which cover a range from 0.12 to 0.87Msun of 56Ni. We also
find that the [CoIII]5888A forbidden line is present within days of bolometric
maximum, and strengthens steadily for weeks thereafter. Both predictions are
confirmed by observations. Rather than trivial taxonomy, these findings confirm
that it is necessary to include forbidden-line transitions in
radiative-transfer simulations of SNe Ia, both to obtain the correct ejecta
cooling rate and to match observed optical spectra.Comment: Accepted to MNRA
Evidence for sub-Chandrasekhar-mass progenitors of Type Ia supernovae at the faint end of the width-luminosity relation
The faster light-curve evolution of low-luminosity Type Ia supernovae (SNe
Ia) suggests that they could result from the explosion of white dwarf (WD)
progenitors below the Chandrasekhar mass (). Here we present 1D
non-LTE time-dependent radiative transfer simulations of pure central
detonations of carbon-oxygen WDs with a mass (M_\rm{tot}) between 0.88
M and 1.15 M, and a yield between 0.08
M and 0.84 M. Their lower ejecta density compared to
models results in a more rapid increase of the luminosity at early
times and an enhanced -ray escape fraction past maximum light.
Consequently, their bolometric light curves display shorter rise times and
larger post-maximum decline rates. Moreover, the higher
M(^{56}\rm{Ni})/M_\rm{tot} ratio at a given mass enhances the
temperature and ionization level in the spectrum-formation region for the less
luminous models, giving rise to bluer colours at maximum light and a faster
post-maximum evolution of the colour. For sub- models fainter
than mag at peak, the greater bolometric decline and faster
colour evolution lead to a larger -band post-maximum decline rate, . In particular, all of our previously-published models
(standard and pulsational delayed detonations) are confined to mag, while the sub- models with
M_\rm{tot}\lesssim 1 M extend beyond this limit to mag for a peak mag, in better agreement
with the observed width-luminosity relation (WLR). Regardless of the precise
ignition mechanism, these simulations suggest that fast-declining SNe Ia at the
faint end of the WLR could result from the explosion of WDs whose mass is
significantly below the Chandrasekhar limit.Comment: 10 pages, 6 figures. Accepted for publication in MNRA
On the Evolution of Thermonuclear Flames on Large Scales
The thermonuclear explosion of a massive white dwarf in a Type Ia supernova
explosion is characterized by vastly disparate spatial and temporal scales. The
extreme dynamic range inherent to the problem prevents the use of direct
numerical simulation and forces modelers to resort to subgrid models to
describe physical processes taking place on unresolved scales.
We consider the evolution of a model thermonuclear flame in a constant
gravitational field on a periodic domain. The gravitational acceleration is
aligned with the overall direction of the flame propagation, making the flame
surface subject to the Rayleigh-Taylor instability. The flame evolution is
followed through an extended initial transient phase well into the steady-state
regime. The properties of the evolution of flame surface are examined. We
confirm the form of the governing equation of the evolution suggested by
Khokhlov (1995). The mechanism of vorticity production and the interaction
between vortices and the flame surface are discussed. The results of our
investigation provide the bases for revising and extending previous
subgrid-scale model.Comment: 15 pages, 22 postscript figures. Accepted for publication by the
Astrophysical Journal. High resolution figures can be found at
http://flash.uchicago.edu/~zhang/research_paper.htm
Laser Pulse Heating of Spherical Metal Particles
We consider a general problem of laser pulse heating of spherical metal
particles with the sizes ranging from nanometers to millimeters. We employ the
exact Mie solutions of the diffraction problem and solve heat-transfer
equations to determine the maximum temperature at the particle surface as a
function of optical and thermometric parameters of the problem. The main
attention is paid to the case when the thermometric conductivity of the
particle is much larger than that of the environment, as it is in the case of
metal particles in fluids. We show that in this case at any given finite
duration of the laser pulse the maximum temperature rise as a function of the
particle size reaches an absolute maximum at a certain finite size of the
particle, and we suggest simple approximate analytical expressions for this
dependence which covers the entire range of variations of the problem
parameters and agree well with direct numerical simulations.Comment: 7 pages, 6 figure
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