189 research outputs found
Neutronization During Type Ia Supernova Simmering
Prior to the incineration of a white dwarf (WD) that makes a Type Ia
supernova (SN Ia), the star "simmers" for ~1000 years in a convecting, carbon
burning region. We have found that weak interactions during this time increase
the neutron excess by an amount that depends on the total quantity of carbon
burned prior to the explosion. This contribution is in addition to the
metallicity (Z) dependent neutronization through the 22Ne abundance (as studied
by Timmes, Brown, & Truran). The main consequence is that we expect a floor to
the level of neutronization that dominates over the metallicity contribution
when Z/Z_\odot<2/3, and it can be important for even larger metallicities if
substantial energy is lost to neutrinos via the convective Urca process. This
would mask any correlations between SN Ia properties and galactic environments
at low metallicities. In addition, we show that recent observations of the
dependences of SNe Ia on galactic environments make it clear that metallicity
alone cannot provide for the full observed diversity of events.Comment: Accepted for publication in The Astrophysical Journal, 5 pages, 4
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Theoretical study of Acousto-optical coherence tomography using random phase jumps on US and light
Acousto-Optical Coherence Tomography (AOCT) is variant of Acousto Optic
Imaging (called also ultrasonic modulation imaging) that makes possible to get
z resolution with acoustic and optic Continuous Wave (CW) beams. We describe
here theoretically the AOCT e ect, and we show that the Acousto Optic tagged
photons remains coherent if they are generated within a speci c z region of the
sample. We quantify the z selectivity for both the tagged photon eld, and for
the M. Lesa re et al. photorefractive signal
Detection of the tagged or untagged photons in acousto-optic imaging of thick highly scattering media by photorefractive adaptive holography
We propose an original adaptive wavefront holographic setup based on the
photorefractive effect (PR), to make real-time measurements of acousto-optic
signals in thick scattering media, with a high flux collection at high rates
for breast tumor detection. We describe here our present state of art and
understanding on the problem of breast imaging with PR detection of the
acousto-optic signal
Low-velocity shocks: signatures of turbulent dissipation in diffuse irradiated gas
Context. Large-scale motions in galaxies (supernovae explosions, galaxy collisions, galactic shear etc.) generate turbulence, which allows a fraction of the available kinetic energy to cascade down to small scales before it is dissipated.
Aims. We establish and quantify the diagnostics of turbulent dissipation in mildly irradiated diffuse gas in the specific context of shock structures.
Methods. We incorporated the basic physics of photon-dominated regions into a state-of-the-art steady-state shock code. We examined the chemical and emission properties of mildly irradiated (G_0 = 1) magnetised shocks in diffuse media (n_H = 10^2 to 10^4 cm^(-3)) at low- to moderate velocities (from 3 to 40 km s^(-1)).
Results. The formation of some molecules relies on endoergic reactions. Their abundances in J-type shocks are enhanced by several orders of magnitude for shock velocities as low as 7 km s^(-1). Otherwise most chemical properties of J-type shocks vary over less than an order of magnitude between velocities from about 7 to about 30 km s^(-1), where H_2 dissociation sets in. C-type shocks display a more gradual molecular enhancement with increasing shock velocity.
We quantified the energy flux budget (fluxes of kinetic, radiated and magnetic energies) with emphasis on the main cooling lines of the cold interstellar medium. Their sensitivity to shock velocity is such that it allows observations to constrain statistical distributions of shock velocities.
We fitted various probability distribution functions (PDFs) of shock velocities to spectroscopic observations of the galaxy-wide shock in Stephanâs Quintet and of a Galactic line of sight which samples diffuse molecular gas in Chamaeleon. In both cases, low velocities bear the greatest statistical weight and the PDF is consistent with a bimodal distribution. In the very low velocity shocks (below 5 km s^(-1)), dissipation is due to ion-neutral friction and it powers H_2 low-energy transitions and atomic lines. In moderate velocity shocks (20 km s^(-1) and above), the dissipation is due to viscous heating and accounts for most of the molecular emission. In our interpretation a significant fraction of the gas in the line of sight is shocked (from 4% to 66%). For example, C^+ emission may trace shocks in UV irradiated gas where C^+ is the dominant carbon species.
Conclusions. Low- and moderate velocity shocks are important in shaping the chemical composition and excitation state of the interstellar gas. This allows one to probe the statistical distribution of shock velocities in interstellar turbulence
The Reduction of the Electron Abundance during the Pre-explosion Simmering in White Dwarf Supernovae
Prior to the explosion of a carbon-oxygen white dwarf in a Type Ia supernova
there is a long "simmering," during which the 12C + 12C reaction gradually
heats the white dwarf on a long (~ 1000 yr) timescale. Piro & Bildsten showed
that weak reactions during this simmering set a maximum electron abundance Ye
at the time of the explosion. We investigate the nuclear reactions during this
simmering with a series of self-heating, at constant pressure, reaction network
calculations. Unlike in AGB stars, proton captures onto 22Ne and heavier trace
nuclei do not play a significant role. The 12C abundance is sufficiently high
that the neutrons preferentially capture onto 12C, rather than iron group
nuclei. As an aid to hydrodynamical simulations of the simmering phase, we
present fits to the rates of heating, electron capture, change in mean atomic
mass, and consumption of 12C in terms of the screened thermally averaged cross
section for 12C + 12C. Our evaluation of the net heating rate includes
contributions from electron captures into the 3.68 MeV excited state of 13C.
This results in a slightly larger energy release, per 12C consumed, than that
found by Piro & Bildsten, but less than that released for a burn to only 20Ne
and 23Na. We compare our one-zone results to more accurate integrations over
the white dwarf structure to estimate the amount of 12C that must be consumed
to raise the white dwarf temperature, and hence to determine the net reduction
of Ye during simmering.Comment: Accepted for publication in The Astrophysical Journal, 9 pages, 6
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Model and Stability Analysis of a Flexible Bladed Rotor
This paper presents a fully bladed flexible rotor and outlines the
associated stability analysis. From an energetic approach based on
the complete energies and potentials for Euler-Bernoulli beams, a
system of equations is derived, in the rotational frame, for the
rotor. This later one is made of a hollow shaft modelled by an
Euler-Bernoulli beam supported by a set of bearings. It is
connected to a rigid disk having a rotational inertia. A full set
of flexible blades is also modelled by Euler-Bernoulli beams
clamped in the disk. The flexural vibrations of the blades as well
as those of the shaft are considered. The evolution of the
eigenvalues of this rotor, in the corotational frame,
is studied. A stability detection method, bringing coalescence and
loci separation phenomena to the fore, in case of an asymmetric
rotor, is undertaken in order to determine a parametric domain
where turbomachinery cannot encounter damage. Finally, extensive
parametric studies including the length and the stagger angle of
the blades as well as their flexibility are presented in order to
obtain robust criteria for stable and unstable areas prediction
Stability Analysis of Beams Rotating on an Elastic Ring Application to Turbo machinery Rotor-Stator Contacts
Summary This paper presents a model of flexible beams rotating on the inner surface of an elastic stationary ring. The beams possesses two degrees of freedom, traction/compression and flexure. The in-plane deformations of the ring are considered and a single mode approximation is used. The model has been developed within the rotating frame by use of an energetic method. To better understand the phenomena occurring, the degrees of freedom of the beams can first be treated separately then together. Stability analysis show that even without rubbing, the radial degree of freedom of a beam rotating on an elastic ring can create divergence instabilities as well as mode couplings of the circular structure. When rubbing is considered, the system is unstable as soon as the rotational speed is non null. Moreover rubbing can couple the beams and the ring giving rise to mode coupling instabilities and locus veering phenomena. Finally, a comparison to a more complicated model of a flexible bladed-rotor in contact with an elastic casing shows a very good accordance with the phenomena occurring
3-D image-based numerical computations of snow permeability: links to specific surface area, density, and microstructural anisotropy
We used three-dimensional (3-D) images of snow microstructure to carry out numerical estimations of the full tensor of the intrinsic permeability of snow (<b>K</b>). This study was performed on 35 snow samples, spanning a wide range of seasonal snow types. For several snow samples, a significant anisotropy of permeability was detected and is consistent with that observed for the effective thermal conductivity obtained from the same samples. The anisotropy coefficient, defined as the ratio of the vertical over the horizontal components of <b>K</b>, ranges from 0.74 for a sample of decomposing precipitation particles collected in the field to 1.66 for a depth hoar specimen. Because the permeability is related to a characteristic length, we introduced a dimensionless tensor <b>K</b>*=<b>K</b>/<i>r</i><sub>es</sub><sup>2</sup>, where the equivalent sphere radius of ice grains (<i>r</i><sub>es</sub>) is computed from the specific surface area of snow (SSA) and the ice density (&rho;<sub>i</sub>) as follows: <i>r</i><sub>es</sub>=3/(SSA&times;&rho;<sub>i</sub>. We define <i>K</i> and <i>K</i>* as the average of the diagonal components of <b>K</b> and <b>K</b>*, respectively. The 35 values of <i>K</i>* were fitted to snow density (&rho;<sub>s</sub>) and provide the following regression: <i>K</i> = (3.0 &pm; 0.3) <i>r</i><sub>es</sub><sup>2</sup> exp((&minus;0.0130 &pm; 0.0003)&rho;<sub>s</sub>). We noted that the anisotropy of permeability does not affect significantly the proposed equation. This regression curve was applied to several independent datasets from the literature and compared to other existing regression curves or analytical models. The results show that it is probably the best currently available simple relationship linking the average value of permeability, <i>K</i>, to snow density and specific surface area
Delayed Decision-making in Real-time Beatbox Percussion Classification
This is an electronic version of an article published in Journal of New Music Research, 39(3), 203-213, 2010. doi:10.1080/09298215.2010.512979. Journal of New Music Research is available online at: www.tandfonline.com/openurl?genre=article&issn=1744-5027&volume=39&issue=3&spage=20
Stability analysis of rotating beams rubbing on an elastic circular structure
This paper presents the stability analysis of a system composed of rotating
beams on a flexible, circular fixed ring, using the Routh-Hurwitz criterion.
The model displayed has been fully developed within the rotating frame by use
of an energy approach. The beams considered possess two degrees of freedom
(dofs), a flexural motion as well as a traction/compression motion. In-plane
deformations of the ring will be considered. Divergences and mode couplings
have thus been underscored within the rotating frame and in order to simplify
understanding of all these phenomena, the dofs of the beams will first be
treated separately and then together. The dynamics of radial rotating loads on
an elastic ring can create divergence instabilities as well as post-critical
mode couplings. Moreover, the flexural motion of beam rubbing on the ring can
also lead to mode couplings and to the locus-veering phenomenon. The presence
of rubbing seems to make the system unstable as soon as the rotational speed of
the beams is greater than zero. Lastly, the influence of an angle between the
beams and the normal to the ring's inner surface will be studied with respect
to system stability, thus highlighting a shift frequency phenomenon
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