3,818 research outputs found
A simple toy model of the advective-acoustic instability I. Perturbative approach
Some general properties of the advective-acoustic instability are described
and understood using a toy model which is simple enough to allow for analytical
estimates of the eigenfrequencies. The essential ingredients of this model, in
the unperturbed regime, are a stationary shock and a subsonic region of
deceleration. For the sake of analytical simplicity, the 2D unperturbed flow is
parallel and the deceleration is produced adiabatically by an external
potential. The instability mechanism is determined unambiguously as the
consequence of a cycle between advected and acoustic perturbations. The purely
acoustic cycle, considered alone, is proven to be stable in this flow. Its
contribution to the instability can be either constructive or destructive. A
frequency cut-off is associated to the advection time through the region of
deceleration. This cut-off frequency explains why the instability favours
eigenmodes with a low frequency and a large horizontal wavelength. The relation
between the instability occurring in this highly simplified toy model and the
properties of SASI observed in the numerical simulations of stellar
core-collapse is discussed. This simple set up is proposed as a benchmark test
to evaluate the accuracy, in the linear regime, of numerical simulations
involving this instability. We illustrate such benchmark simulations in a
companion paper.Comment: 14 pages, 10 figures, ApJ in pres
Hoyle-Lyttleton Accretion in Three Dimensions
We investigate the stability of gravitational accretion of an ideal gas onto
a compact object moving through a uniform medium at Mach 3. Previous
three-dimensional simulations have shown that such accretion is not stable, and
that strong rotational 'disk-like' flows are generated and accreted on short
time scales. We re-address this problem using overset spherical grids that
provide a factor of seven improvement in spatial resolution over previous
simulations. With our higher spatial resolution we found these 3D accretion
flows remained remarkably axisymmetric. We examined two cases of accretion with
different sized accretors. The larger accretor produced very steady flow, with
the mass accretion rate varying by less than 0.02% over 30 flow times. The
smaller accretor exhibited an axisymmetric breathing mode that modulated the
mass accretion rate by a constant 20%. Nonetheless, the flow remained highly
axisymmetric with only negligible accretion of angular momentum in both cases.Comment: 6 pages, 6 figures. Submitted to Ap
Determining the Type, Redshift, and Phase of a Supernova Spectrum
We present an algorithm to identify the types of supernova spectra, and
determine their redshift and phase. This algorithm, based on the correlation
techniques of Tonry & Davis, is implemented in the SuperNova IDentification
code (SNID). It is used by members of the ESSENCE project to determine whether
a noisy spectrum of a high-redshift supernova is indeed of type Ia, as opposed
to, e.g., type Ib/c. Furthermore, by comparing the correlation redshifts
obtained using SNID with those determined from narrow lines in the supernova
host galaxy spectrum, we show that accurate redshifts (with a typical error <
0.01) can be determined for SNe Ia for which a spectrum of the host galaxy is
unavailable. Last, the phase of an input spectrum is determined with a typical
accuracy of ~3 days.Comment: 10 pages, 7 figures. To appear in "The Multicoloured Landscape of
Compact Objects and their Explosive Progenitors: Theory vs Observations"
(Cefalu, Sicily, June 2006). Eds. L. Burderi et al. (New York: AIP
Forward Analysis for WSTS, Part III: Karp-Miller Trees
This paper is a sequel of "Forward Analysis for WSTS, Part I: Completions"
[STACS 2009, LZI Intl. Proc. in Informatics 3, 433-444] and "Forward Analysis
for WSTS, Part II: Complete WSTS" [Logical Methods in Computer Science 8(3),
2012]. In these two papers, we provided a framework to conduct forward
reachability analyses of WSTS, using finite representations of downward-closed
sets. We further develop this framework to obtain a generic Karp-Miller
algorithm for the new class of very-WSTS. This allows us to show that
coverability sets of very-WSTS can be computed as their finite ideal
decompositions. Under natural effectiveness assumptions, we also show that LTL
model checking for very-WSTS is decidable. The termination of our procedure
rests on a new notion of acceleration levels, which we study. We characterize
those domains that allow for only finitely many accelerations, based on ordinal
ranks
Neutron star masses from hydrodynamical effects in obscured sgHMXBs
A population of obscured supergiant High Mass X-ray Binaries (sgHMXBs) has
been discovered by INTEGRAL. X-ray wind tomography of IGR J17252-3616 inferred
a slow wind velocity to account for the enhanced obscuration. The main goal of
this study is to understand under which conditions high obscuration could
occur. We have used an hydrodynamical code to simulate the flow of the stellar
wind around the neutron star. A grid of simulations was used to study the
dependency of the absorbing column density and of the X-ray light-curves on the
model parameters. A comparison between the simulation results and the
observations of IGR J17252-3616 provides an estimate on these parameters. We
have constrained the wind terminal velocity to 500-600 km/s and the neutron
star mass to 1.75-2.15 solar masses. We have confirmed that the initial
hypothesis of a slow wind velocity with a moderate mass loss rate is valid. The
mass of the neutron star can be constrained by studying its impact on the
accretion flow.Comment: A&A in pres
A one-dimensional Chandrasekhar-mass delayed-detonation model for the broad-lined Type Ia supernova 2002bo
We present 1D non-local thermodynamic equilibrium (non-LTE) time-dependent
radiative-transfer simulations of a Chandrasekhar-mass delayed-detonation model
which synthesizes 0.51 Msun of 56Ni, and confront our results to the Type Ia
supernova (SN Ia) 2002bo over the first 100 days of its evolution. Assuming
only homologous expansion, this same model reproduces the bolometric and
multi-band light curves, the secondary near-infrared (NIR) maxima, and the
optical and NIR spectra. The chemical stratification of our model qualitatively
agrees with previous inferences by Stehle et al., but reveals significant
quantitative differences for both iron-group and intermediate-mass elements. We
show that +/-0.1 Msun (i.e., +/-20 per cent) variations in 56Ni mass have a
modest impact on the bolometric and colour evolution of our model. One notable
exception is the U-band, where a larger abundance of iron-group elements
results in less opaque ejecta through ionization effects, our model with more
56Ni displaying a higher near-UV flux level. In the NIR range, such variations
in 56Ni mass affect the timing of the secondary maxima but not their magnitude,
in agreement with observational results. Moreover, the variation in the I, J,
and K_s magnitudes is less than 0.1 mag within ~10 days from bolometric
maximum, confirming the potential of NIR photometry of SNe Ia for cosmology.
Overall, the delayed-detonation mechanism in single Chandrasekhar-mass white
dwarf progenitors seems well suited for SN 2002bo and similar SNe Ia displaying
a broad Si II 6355 A line. Whatever multidimensional processes are at play
during the explosion leading to these events, they must conspire to produce an
ejecta comparable to our spherically-symmetric model.Comment: Accepted for publication in MNRAS. The hydrodynamical input and
synthetic spectra are available at https://www-n.oca.eu/supernova/home.html .
Minor changes from v1: corrected several typos and updated acknowledgement
X-Ray Emission from Planetary Nebulae Calculated by 1D Spherical Numerical Simulations
We calculate the X-ray emission from both constant and time evolving shocked
fast winds blown by the central stars of planetary nebulae (PNs) and compare
with observations. Using spherically symmetric numerical simulations with
radiative cooling, we calculate the flow structure, and the X-ray temperature
and luminosity of the hot bubble formed by the shocked fast wind. We find that
a constant fast wind gives results that are very close to those obtained from
the self-similar solution. We show that in order for a fast shocked wind to
explain the observed X-ray properties of PNs, rapid evolution of the wind is
essential. More specifically, the mass loss rate of the fast wind should be
high early on when the speed is ~300-700 km/s, and then it needs to drop
drastically by the time the PN age reaches ~1000 yr. This implies that the
central star has a very short pre-PN (post-AGB) phase.Comment: accepted to MNRA
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