16,529 research outputs found
Regimes of Helium Burning
No full textThe burning regimes encountered by laminar deflagrations and Zeldovich von Neumann Döring (ZND) detonations propagating through helium-rich compositions in the presence of buoyancy-driven turbulence are analyzed. Particular attention is given to models of X-ray bursts that start with a thermonuclear runaway on the surface of a neutron star and to the thin-shell helium instability of intermediate-mass stars. In the X-ray burst case, turbulent deflagrations propagating in the lateral or radial direction encounter a transition from the distributed regime to the flamelet regime at a density of ~108 g cm-3. In the radial direction, the purely laminar deflagration width is larger than the pressure scale height for densities smaller than ~106 g cm-3. Self-sustained laminar deflagrations traveling in the radial direction cannot exist below this density. Similarly, the planar ZND detonation width becomes larger than the pressure scale height at ~107 g cm-3, suggesting that steady state, self-sustained detonations cannot come into existence in the radial direction. In the thin helium shell case, turbulent deflagrations traveling in the lateral or radial direction encounter the distributed regime at densities below ~107 g cm-3 and the flamelet regime at larger densities. In the radial direction, the purely laminar deflagration width is larger than the pressure scale height for densities smaller than ~104 g cm-3, indicating that steady state laminar deflagrations cannot form below this density. The planar ZND detonation width becomes larger than the pressure scale height at ~5 × 104 g cm-3, suggesting that steady state, self-sustained detonations cannot come into existence in the radial direction
News (νs) on the Galactic evolution of lithium.
No full textGalactic chemical evolution models are presented that take into account several recent developments: (1) models of classical novae nucleosynthesis question the total amount of ^7^Li produced by these systems; (2) observations and models for lithium production in asymptotic giant branch stars and the mass of the planetary nebula that they eject; (3) ^7^Li production from ν-process nucleosynthesis in Type II supernova explosions. The best fitting models for the upper envelope of the observed N(Li) as a function of [Fe/H] suggest that Type II supernovae and intermediate-low mass stars each produce about 1/2 of the solar lithium abundance, although fractions between 1/4 to 3/4 for each source are not conclusively ruled out. The variance of these fractions primarily reflect the uncertainties in the input nucleosynthesis
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
Relations between x-ray timing features and spectral parameters of galactic black hole x-ray binaries
No full textWe present a study of correlations between spectral and timing parameters for a sample of black hole X-ray binary candidates. Data are taken from GX
339-4, H 1743-322, and XTE J1650-500, as the Rossi X-ray Timing Explorer
(RXTE) observed complete outbursts of these sources. In our study we investigate outbursts that happened before the end of 2009 to make use of the high-energy coverage of the HEXTE detector and select observations that
show a certain type of quasi-periodic oscillations (type-C QPOs). The spectral parameters are derived using the empirical convolution model simpl to model the Comptonized component of the emission together with a disc blackbody for the emission of the accretion disc. Additional spectral features, namely a reflection component, a high-energy cut-off, and excess emission at 6.4 keV, are taken into account. Our investigations confirm the known positive
correlation between photon index and centroid frequency of the QPOs and reveal an anti-correlation between the fraction of up-scattered photons and the QPO frequency. We show that both correlations behave as expected in the “sombrero”
geometry. Furthermore, we find that during outburst decay the correlation between photon index and QPO frequency follow a general track, independent of individual outbursts
Statistical methods for thermonuclear reaction rates and nucleosynthesis simulations
No full textabstract: Rigorous statistical methods for estimating thermonuclear reaction rates and nucleosynthesis are becoming increasingly established in nuclear astrophysics. The main challenge being faced is that experimental reaction rates are highly complex quantities derived from a multitude of different measured nuclear parameters (e.g., astrophysical S-factors, resonance energies and strengths, particle and γ-ray partial widths). We discuss the application of the Monte Carlo method to two distinct, but related, questions. First, given a set of measured nuclear parameters, how can one best estimate the resulting thermonuclear reaction rates and associated uncertainties? Second, given a set of appropriate reaction rates, how can one best estimate the abundances from nucleosynthesis (i.e., reaction network) calculations? The techniques described here provide probability density functions that can be used to derive statistically meaningful reaction rates and final abundances for any desired coverage probability. Examples are given for applications to s-process neutron sources, core-collapse supernovae, classical novae, and Big Bang nucleosynthesis.Copyright IOP Publishing. This is the authors' final, peer-reviewed manuscript. Iliadis, Christian, Longland, Richard, Coc, Alain, Timmes, F. X., & Champagne, Art E. (2015). Statistical methods for thermonuclear reaction rates and nucleosynthesis simulations. JOURNAL OF PHYSICS G-NUCLEAR AND PARTICLE PHYSICS, 42, 034007. The final version as published can be viewed online at http://dx.doi.org/10.1088/0954-3899/42/3/03400
Logarithmic variance profiles and the corresponding f-1 spectra of temperature fluctuations in turbulent Rayleigh-Bénard convection
No full textWe report experimental results for the temperature variance 2(z) and the corresponding frequency spectra P(f) in turbulent Rayleigh-Bénard convection (RBC) in a cylindrical sample of aspect ratioT= D/L = 1:00 (D = 1:12 m is the diameter and L = 1:12 m the height). The measurements were conducted in the Rayleigh-number range 1011 < Ra < 1:35 1014 and Pr ' 0:8. For Ra = 1:35x1014, 2(z) could be described well by a logarithmic dependence on the vertical position z in a range of z 1 < z < z 2 with z 1 ' 70 and z 2 = 0:1L. Here L=(2Nu) is the thickness of a thin thermal sublayer adjacent to the horizontal plate where the heat flux (denoted by the Nusselt number Nu) is carried mostly by thermal diffusion. In the log layer, we found that the temperature spectra had a significant frequency range over which P(f) f with close to 1. As Ra decreased, increased so that the log layer became thinner. At Ra = 2:05 1011, z 2 < z 1 and therefore there was no range for a log layer. Correspondingly, the temperature spectrum near the horizontal plate did not have the f1 scaling form either
Simulations of X-ray bursts and superbursts
No full textObservations of neutron star in binary systems provide powerful constrains on the physics at the surface of neutron stars. During the accretion of matter from the companion star, periodic nuclear explosion are triggered in the outer layers of the neutron star, increasing the luminosity during a time range of a few minutes. Rarely, one can also detect day-long explosions in accreting binary systems. The nature of those two kind of bursts is still not well understood. In fact, simplified simulations of the outer layers of an accreting neutron star in a binary are not yet able to reproduce all observable features.
The work presented in this thesis is devoted to the one-dimensional simulations of X-ray bursts and superbursts. The numerical code used in this work has initially been programmed by J. Fisker in 2006. By updating and optimizing the code, we are able to simulate X-ray bursts as well as superbursts in a feasible time range. Using a large nulear network, we study the features of X-ray bursts and compare them with observations. To understand the link between various properties entering our simulations as parameters or boundary conditions, we present several models which reproduces hunderds of X-ray burst. In this current work, we focus mainly on changes in crustal heating, accretion rate and accretion composition. Analyzing the influence on the light curve as well as on the ashes of X-ray bursts, we are able to compare our results with observations. To shed some light on the self-consistent ignition of a superburst, we model a setup which may lead to the ignition of a superburst.
Our results suggest that additional helium, heavier isotopes and the lack of hydrogen in the accretion composition help to generate carbon-rich X-ray burst ashes. Strong heating below the superburst ignition layer prevents the destruction of carbon after an X-ray bursts and might be the key ingredience in the self-consistent ignition of a superburst within the time range of the observed recurrence tim
On the periodic orbits of the Third-order differential equation x ' ' '- x ' ' x'- x= F(x,x',x ' ')
No full textAgraïments: The second author is partially supported by CAPES/MECD-DGU 222/2010 Brazil and SpainIn this paper we study the periodic orbits of the third-order differential equation x''' − µx'' + x' − µx = εF(x, x', x''), where ε is a small parameter and the function F is of class C2
The generalized Liénard polynomial differential systems x'=y,y'= -g(x) - f (x)y with deg g = deg f 1 are not Liouvillian integrable
No full textAgraïments: The second author was supported by Portuguese National Funds through FCT - Fundação para a Ciência e a Tecnologia within the project PTDC/MAT/117106/2010 and by CAMGSD (PEst-OE/EEI/LA0009/2013)We prove the nonexistence of Liouvillian first integrals for the generalized Li\'enard polynomial differential systems of the form x' = y, y'=-g(x)-f(x)y, where g(x) and f(x) are arbitrary polynomials such that g = f 1
The generalized Liénard polynomial differential systems x'=y,y'= -g(x) - f (x)y with deg g = deg f 1 are not Liouvillian integrable
No full textAgraïments: The second author was supported by Portuguese National Funds through FCT - Fundação para a Ciência e a Tecnologia within the project PTDC/MAT/117106/2010 and by CAMGSD (PEst-OE/EEI/LA0009/2013)We prove the nonexistence of Liouvillian first integrals for the generalized Li\'enard polynomial differential systems of the form x' = y, y'=-g(x)-f(x)y, where g(x) and f(x) are arbitrary polynomials such that g = f 1
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