86 research outputs found
The Thermonuclear Explosion Of Chandrasekhar Mass White Dwarfs
The flame born in the deep interior of a white dwarf that becomes a Type Ia
supernova is subject to several instabilities. We briefly review these
instabilities and the corresponding flame acceleration. We discuss the
conditions necessary for each of the currently proposed explosion mechanisms
and the attendant uncertainties. A grid of critical masses for detonation in
the range - g cm is calculated and its
sensitivity to composition explored. Prompt detonations are physically
improbable and appear unlikely on observational grounds. Simple deflagrations
require some means of boosting the flame speed beyond what currently exists in
the literature. ``Active turbulent combustion'' and multi-point ignition are
presented as two plausible ways of doing this. A deflagration that moves at the
``Sharp-Wheeler'' speed, , is calculated in one dimension
and shows that a healthy explosion is possible in a simple deflagration if the
front moves with the speed of the fastest floating bubbles. The relevance of
the transition to the ``distributed burning regime'' is discussed for delayed
detonations. No model emerges without difficulties, but detonation in the
distributed regime is plausible, will produce intermediate mass elements, and
warrants further study.Comment: 28 pages, 4 figures included, uses aaspp4.sty. Submitted to Ap
Constraints On The Delayed Transition to Detonation in Type Ia Supernovae
We investigate the possibility of a delayed detonation in a type Ia supernova
under the assumption that the transition to detonation is triggered by
turbulence only. Our discussion is based on the Zeldovich mechanism and
suggests that typical turbulent velocities present during the explosion are not
strong enough to allow this transition to occur. Although we are able to show
that in carbon-rich matter (e.g., C) the possibility of a
deflagration to detonation transition (DDT) is enhanced, even in this case the
turbulent velocities needed are larger than the expected value of on a length-scale of cm. Thus we
conclude that a DDT may not be a common event during a thermonuclear explosion
of a Chandrasekhar-mass white dwarf.Comment: 18 pages, 5 figures, accepted for publication in the Ap
Deflagration to Detonation Transition in Thermonuclear Supernovae
We derive the criteria for deflagration to detonation transition (DDT) in a
Type Ia supernova. The theory is based on the two major assumptions: (i)
detonation is triggered via the Zeldovich gradient mechanism inside a region of
mixed fuel and products, (ii) the mixed region is produced by a turbulent
mixing of fuel and products either inside an active deflagration front or
during the global expansion and subsequent contraction of an exploding white
dwarf. We determine the critical size of the mixed region required to initiate
a detonation in a degenerate carbon-oxygen mixture. This critical length is
much larger than the width of the reaction front of a Chapman-Jouguet
detonation. However, at densities greater than simeq 5 x 10^6 g cm^-3, it is
much smaller than the size of a white dwarf. We derive the critical turbulent
intensity required to create the mixed region inside an active deflagration
front in which a detonation can form. We conclude that the density rho_tr at
which a detonation can form in a carbon-oxygem white dwarf is low, less than 2
- 5 x 10^7 g cm^-3, but greater than 5 x 10^6 g cm^-3.Comment: 28 pages, 19 figs, Latex (epsf), submitted to The Astrophysical
Journa
Can Deflagration-Detonation-Transitions occur in Type Ia Supernovae?
The mechanism for deflagration-detonation-transition (DDT) by turbulent
preconditioning, suggested to explain the possible occurrence of delayed
detonations in Type Ia supernova explosions, is argued to be conceptually
inconsistent. It relies crucially on diffusive heat losses of the burned
material on macroscopic scales. Regardless of the amplitude of turbulent
velocity fluctuations, the typical gradient scale for temperature fluctuations
is shown to be the laminar flame width or smaller, rather than the factor of
thousand more required for a DDT. Furthermore, thermonuclear flames cannot be
fully quenched in regions much larger than the laminar flame width as a
consequence of their simple ``chemistry''. Possible alternative explosion
scenarios are briefly discussed.Comment: 8 pages, uses aastex; added references. Accepted by ApJ Letter
Spontaneous Transition of Turbulent Flames to Detonations in Unconfined Media
Deflagration-to-detonation transition (DDT) can occur in environments ranging
from experimental and industrial systems to astrophysical thermonuclear (type
Ia) supernovae explosions. Substantial progress has been made in explaining the
nature of DDT in confined systems with walls, internal obstacles, or
pre-existing shocks. It remains unclear, however, whether DDT can occur in
unconfined media. Here we use direct numerical simulations (DNS) to show that
for high enough turbulent intensities unconfined, subsonic, premixed, turbulent
flames are inherently unstable to DDT. The associated mechanism, based on the
nonsteady evolution of flames faster than the Chapman-Jouguet deflagrations, is
qualitatively different from the traditionally suggested spontaneous reaction
wave model, and thus does not require the formation of distributed flames.
Critical turbulent flame speeds, predicted by this mechanism for the onset of
DDT, are in agreement with DNS results.Comment: 4 pages, 3 figures; accepted to Physical Review Letter
In-situ nanoindentation of titania microspheres with different crystallinity
This work was supported by the Lomonosov MSU Program of Development
Prevention of recurrent bleeding from varicose veins of the esophagus and stomach among patients with decompensated liver cirrhosis
Aim. To evaluate the efficiency of laparoscopic devascularization of the esophagus and stomach with endoscopic ligation of varicose esophageal veins in the prevention of esophageal-gastric bleeding among patients with decompensated liver cirrhosis.
Methods. The results of treatment of 73 patients with decompensated liver cirrhosis and high risk of bleeding were analyzed. To prevent recurrent bleeding from esophageal and gastric veins, all patients underwent endoscopic ligation at the first step of treatment. In case of inefficiency of ligation and recurrence of varicose veins of esophagus, laparoscopic devascularization of esophagus and stomach was performed. The efficiency of laparoscopic devascularization with intraoperative endoscopic ligation of varicose esophageal veins and ligation as an independent method of treatment for the prevention of upper gastrointestinal bleeding was estimated by comparison of the frequency of recurrence of esophageal and gastric bleeding and recurrence of esophageal varices according to upper endoscopy in comparison groups.
Results. In 6 months, 1 and 2 years after laparoscopic devascularization of the esophagus and stomach in combination with endoscopic ligation, the risk of bleeding is less compared to endoscopic ligation as an isolated treatment method (p=0.05; p=0.052; p=0.06). Laparoscopic devascularization with ligation reduces the risk of recurrence of esophageal varices during the first year after surgery by 20% (χ2=2.61; p=0.106), in 2 years by 23% (χ2=1.75; p=0.091) compared to endoscopic ligation only.
Conclusion. Patients with liver cirrhosis with decompensated hepatic failure satisfactorily postpone endovideosurgical interventions; laparoscopic gastric devascularization with the intersection of the main inflows to the esophageal varicose veins is an effective method to prevent esophageal-gastric hemorrhage among patients with decompensated liver cirrhosis after ineffective endoscopic ligation
The Response of Model and Astrophysical Thermonuclear Flames to Curvature and Stretch
Critically understanding the `standard candle'-like behavior of Type Ia
supernovae requires understanding their explosion mechanism. One family of
models for Type Ia Supernovae begins with a deflagration in a Carbon-Oxygen
white dwarf which greatly accelerates through wrinkling and flame
instabilities. While the planar speed and behavior of astrophysically-relevant
flames is increasingly well understood, more complex behavior, such as the
flame's response to stretch and curvature, has not been extensively explored in
the astrophysical literature; this behavior can greatly enhance or suppress
instabilities and local flame-wrinkling, which in turn can increase or decrease
the bulk burning rate. In this paper, we explore the effects of curvature on
both nuclear flames and simpler model flames to understand the effect of
curvature on the flame structure and speed.Comment: 25 pages; accepted to ApJ; fixed author field
Thermonuclear Burning Regimes and the Use of SNe Ia in Cosmology
The calculations of the light curves of thermonuclear supernovae are carried
out by a method of multi-group radiation hydrodynamics. The effects of spectral
lines and expansion opacity are taken into account. The predictions for UBVI
fluxes are given. The values of rise time for B and V bands found in our
calculations are in good agreement with the observed values. We explain why our
results for the rise time have more solid physical justification than those
obtained by other authors. It is shown that small variations in the chemical
composition of the ejecta, produced in the explosions with different regimes of
nuclear burning, can influence drastically the light curve decline in the B
band and, to a lesser extent, in the V band. We argue that recent results on
positive cosmological constant Lambda, found from the high redshift supernova
observations, could be wrong in the case of possible variations of the
preferred mode of nuclear burning in the earlier Universe.Comment: 20 pages, 5 figures, presented at the conference "Astronomy at the
Eve of the New Century", Puschino, May 17-22, 1999. A few references and a
table added, typos correcte
Observation of a J^PC = 1-+ exotic resonance in diffractive dissociation of 190 GeV/c pi- into pi- pi- pi+
The COMPASS experiment at the CERN SPS has studied the diffractive
dissociation of negative pions into the pi- pi- pi+ final state using a 190
GeV/c pion beam hitting a lead target. A partial wave analysis has been
performed on a sample of 420000 events taken at values of the squared
4-momentum transfer t' between 0.1 and 1 GeV^2/c^2. The well-known resonances
a1(1260), a2(1320), and pi2(1670) are clearly observed. In addition, the data
show a significant natural parity exchange production of a resonance with
spin-exotic quantum numbers J^PC = 1-+ at 1.66 GeV/c^2 decaying to rho pi. The
resonant nature of this wave is evident from the mass-dependent phase
differences to the J^PC = 2-+ and 1++ waves. From a mass-dependent fit a
resonance mass of 1660 +- 10+0-64 MeV/c^2 and a width of 269+-21+42-64 MeV/c^2
is deduced.Comment: 7 page, 3 figures; version 2 gives some more details, data unchanged;
version 3 updated authors, text shortened, data unchange
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