4,973 research outputs found
Detecting the signatures of helium in type Iax supernovae
Recent studies have argued that the progenitor system of type Iax supernovae
must consist of a carbon-oxygen white dwarf accreting from a helium star
companion. Based on existing explosion models invoking the pure deflagration of
carbon-oxygen white dwarfs, we investigate the likelihood of producing spectral
features due to helium in type Iax supernovae. From this scenario, we select
those explosion models producing ejecta and Ni masses that are broadly
consistent with those estimated for type Iax supernovae (0.014 -
0.478~ and - 0.183~, respectively). To this
end, we present a series of models of varying luminosities (~mag) with helium abundances accounting for up to
36\% of the ejecta mass, and covering a range of epochs beginning a few
days before Bband maximum to approximately two weeks after maximum. We find
that the best opportunity for detecting \ion{He}{i} features is at
near-infrared wavelengths, and in the post-maximum spectra of the fainter
members of this class. We show that the optical spectrum of SN~2007J is
potentially consistent with a large helium content (a few 10),
but argue that current models of accretion and material stripping from a
companion struggle to produce compatible scenarios. We also investigate the
presence of helium in all objects with near-infrared spectra. We show that
SNe~2005hk, 2012Z, and 2015H contain either no helium or their helium
abundances are constrained to much lower values
(10). Our results demonstrate the differences in
helium content among type Iax supernovae, perhaps pointing to different
progenitor channels. Either SN~2007J is an outlier in terms of its progenitor
system, or it is not a true member of the type Iax supernova class.Comment: 15 pages, 12 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysic
Fluent temporal logic for discrete-time event-based models
Fluent model checking is an automated technique for verifying that an event-based operational model satisfies some state-based declarative properties. The link between the event-based and state-based formalisms is defined through fluents which are state predicates whose value are determined by the occurrences of initiating and terminating events that make the fluents values become true or false, respectively. The existing fluent temporal logic is convenient for reasoning about untimed event-based models but difficult to use for timed models. The paper extends fluent temporal logic with temporal operators for modelling timed properties of discrete-time event-based models. It presents two approaches that differ on whether the properties model the system state after the occurrence of each event or at a fixed time rate. Model checking of timed properties is made possible by translating them into the existing untimed framework. Copyright 2005 ACM
Helical structures from an isotropic homopolymer model
We present Monte Carlo simulation results for square-well homopolymers at a
series of bond lengths. Although the model contains only isotropic pairwise
interactions, under appropriate conditions this system shows spontaneous chiral
symmetry breaking, where the chain exists in either a left- or a right-handed
helical structure. We investigate how this behavior depends upon the ratio
between bond length and monomer radius.Comment: 10 pages, 3 figures, accepted for publication by Physical Review
Letter
Two photon absorption laser induced fluorescence measurements of neutral density in a helicon plasma
Modelling the early time behaviour of type Ia supernovae: effects of the 56Ni distribution
Recent studies have demonstrated the diversity in type Ia supernovae (SNe Ia)
at early times and highlighted a need for a better understanding of the
explosion physics as manifested by observations soon after explosion. To this
end, we present a Monte Carlo code designed to model the light curves of
radioactively driven, hydrogen-free transients from explosion to approximately
maximum light. In this initial study, we have used a parametrised description
of the ejecta in SNe Ia, and performed a parameter study of the effects of the
Ni distribution on the observed colours and light curves for a fixed
Ni mass of 0.6 . For a given density profile, we find that
models with Ni extending throughout the entirety of the ejecta are
typically brighter and bluer shortly after explosion. Additionally, the shape
of the density profile itself also plays an important role in determining the
shape, rise time, and colours of observed light curves. We find that the
multi-band light curves of at least one SNe Ia (SN 2009ig) are inconsistent
with less extended Ni distributions, but show good agreement with models
that incorporate Ni throughout the entire ejecta. We further demonstrate
that comparisons with full colour light curves are powerful tools in
discriminating various Ni distributions, and hence explosion models.Comment: 14 pages, 8 figures, 2 tables. Minor changes in notation to match
published version in Astronomy & Astrophysic
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