Flash Code Simulations of Rayleigh-Taylor and Richtmyer-Meshkov Instabilities in Laser-Driven Experiments

We present two- and three-dimensional simulations involving Richtmyer–Meshkov and Rayleigh-Taylor instabilities run with the adaptive mesh refinement code, flash. Variations in the rate of mixing layer growth due to dimensionality, perturbation modes, and simulation resolution are explored. These simulations are designed for detailed comparisons with experiments run on the Omega laser to gain understanding of the mixing processes and to prepare for validation of the Flash code.     

Studying Hydrodynamic Instability Using Shock-Tube Experiments

The hydrodynamic instability, which develops on the contact surface between two fluids, has great importance in astrophysical phenomena such as the inhomogeneous density distribution following a supernova event. In this event acceleration waves pass across a material interface and initiate and enhance unstable conditions in which small perturbations grow dramatically. In the present study, an experimental technique aimed at investigating the above-mentioned hydrodynamic instability is presented. The experimental investigation is based on a shock-tube apparatus by which a shock wave is generated and initiates the instability that develops on the contact surface between two gases. The flexibility of the system enables one to vary the initial shape of the contact surface, the shock-wave Mach number, and the density ratio across the contact surface. Three selected sets of shock-tube experiments are presented in order to demonstrate the system capabilities: (1) large-initial amplitudes with low-Mach-number incident shock waves; (2) small-initial amplitudes with moderate-Mach-number incident shock waves; and (3) shock bubble interaction. In the large-amplitude experiments a reduction of the initial velocity with respect to the linear growth prediction was measured. The results were compared to those predicted by a vorticity-deposition model and to previous experiments with moderate- and high-Mach number incident shock waves that were conducted by others. In this case, a reduction of the initial velocity was noted. However, at late times the growth rate had a 1/t behavior as in the small-amplitude low-Mach number case. In the small-amplitude moderate-Mach number shock experiments a reduction from the impulsive theory was noted at the late stages. The passage of a shock wave through a spherical bubble results in the formation of a vortex ring. Simple dimensional analysis shows that the circulation depends linearly on the speed of sound of the surrounding material and on the initial bubble radius.     

Non-Linear Dynamics of the Richtmyer–Meshkov Instability in Supernovae

We report analytical and numerical solutions describing the evolution of the coherent structure of bubbles and spikes in the Richtmyer–Meshkov instability in supernovae. It is shown that the dynamics of the flow is essentially non-local, and the nonlinear Richtmyer–Meshkov bubble flattens and decelerates.     

Evolution of the optical spectrum of SN 1987a in the large magellanic cloud

The evolution of the spectrum of SN1987a is traced from 1987 February 26 to March 31. Based on the low-resolution spectroscopic data we identify the lines of H, He I, Na I, Fe II, Sc II, Ca II which are known to be present in Type II Supernovae, and also present evidence for the existence of lines of Mg I, Ca_I, O I, and N I. We discuss the evolution of the Hα profile, and draw attention to its complex structure around March 30. Close to the rest wavelength of Ha a double-peaked structure appeared in the profile with a peak-to-peak separation of ∼ 1400 km s⁻¹, suggestive of an expanding shell or disc of gas. Using the available broadband photometric information, we also trace the evolution of the photosphere of SN1987a assuming that it radiates like a supergiant.     

非平衡动力学中恒星结构稳定性研究

采用非平衡动力学方法研究由CNO循环决定的恒星结构稳定性,本文得出:CNO循环自身是稳定的;扩散效应不会改变处于稳定的恒星结构稳定性;对流效应对其稳定性有重要影响,波数小的扰动更容易激发不稳定性;巨大的温度梯度也能激发不稳定性。本文给出了稳定性条件和恒星结构保持稳定所允许的最小扰动波数。     

An atlas of the optical spectrum of supernova 1987A in the large magellanic cloud

Photographic spectra of SN1987A in the LMC have been obtained from 1987 February 25 to 1988 June 30. Microdensitometer tracings of these have been reduced to intensity and corrections for instrumental response have been applied to the spectra. This paper presents these data in an atlas format, discusses the reduction procedures in detail, and presents radial velocity measurements of selected lines in the spectra     

超新星光谱研究的进展：观测

介绍了超新星完整样品的分类概况，依次转述了各类超新星光谱观测的新进展。两颗特殊Ｉａ型超新星－ＳＮ１９９１Ｔ和ＳＮ１９９１ｂｇ一的发现对原先认为Ｉａ型超新星是均质的观点提出了挑战，对Ｉｂ／Ｉｃ型超新星应加以特别关注，ＩＩ型超新星ＳＮ１９９３Ｊ和ＳＮ１９８７Ｋ由光极大时的ＩＩ型演化星云类似于Ｉｂ／Ｉｃ的光谱，对传统的超新星Ｉ型和ＩＩ型的区分提出了质疑，对某些特殊ＩＩ型超新星也许列为“ＩＩｎ”型，其Ｈα     

Richtmyer-Meshkov Experiments on the Omega Laser

Observations of the interstellar medium reveal a dynamic realm permeated by shocks. These shocks are generated on a large range of scales by galactic rotation, supernovae, stellar winds, and other processes. Whenever a shock encounters a density interface, Richtmyer-Meshkov instabilities may develop. Perturbations along the interface grow, leading to structure formation and material mixing. An understanding of the evolution of Richtmyer-Meshkov instabilities is essential for understanding galactic structure, molecular cloud morphology, and the early stages of star formation. An ongoing experimental campaign studies Richtmyer-Meshkov mixing in a convergent, compressible, miscible plasma at the Omega laser facility. Cylindrical targets, consisting of a low density foam core and an aluminum shell covered by an epoxy ablator, are directly driven by fifty laser beams. The aluminum shell is machined to produce different perturbation spectra. Surface types include unperturbed (smooth), single-mode sinusoids, multi-mode (rough), and multi-mode with particular modes accentuated (specified-rough). Experimental results are compared to theory and numerical simulations.     

Final Stages of Stellar Evolution and Nucleosynthesis: What do We Know and What Would We Like to Know?

Recent developments in theoretical model-calculations for the synthesis of the chemical elements during late stages of stellar evolution are reviewed. Special emphasis is put on a discussion of various astrophysical sites, including core-collapse and thermonuclear supernovae, and the physics of turbulent reactive fluids. Results of numerical simulations are presented and discussed, together with new results concerning solar-system abundances as well as abundances observed in very metal-poor stars, in the context of searches for constraints on the still rather uncertain nuclear physics data and astrophysical models. This revised version was published online in September 2006 with corrections to the Cover Date.     

Validating the Flash Code: Vortex-Dominated Flows

As a component of the Flash Center’s validation program, we compare FLASH simulation results with experimental results from Los Alamos National Laboratory. The flow of interest involves the lateral interaction between a planar M _a = 1.2 shock wave with a cylinder of gaseous sulfur hexafluoride (SF₆) in air, and in particular the development of primary and secondary instabilities after the passage of the shock. While the overall evolution of the flow is comparable in the simulations and experiments, small-scale features are difficult to match. We focus on the sensitivity of numerical results to simulation parameters.     

The Impact of Historical Chinese Astronomical Records

The impact of Chinese historical astronomical records is important in the study of astronomy today. In particular, the impact of the Chinese records related to historical supernovae have made important contributions to modern astronomy, contributing to the rapid progress of space sciences and high-energy astrophysics made in the recent two decades. These historical records could also be of assistance in the future. In this connection, the main topics discussed in this paper are the great new star which occurred in the 14th century Before Christ (BC), the historical supernovae Anno Domini (AD) 185 and AD 393, and the new concept of the “Po star” and its application.     

Limits for the Firehose Instability in Space Plasmas

Electromagnetic instabilities in high-β plasmas, where β is the ratio of the kinetic plasma energy to the magnetic energy, have a broad range of astrophysical applications. The presence of temperature anisotropies T _∥ /T _⊥ >1 (where ∥ and ⊥ denote directions relative to the background magnetic field) in solar flares and the solar wind is sustained by the observations and robust acceleration mechanisms that heat plasma particles in the parallel direction. The surplus of parallel kinetic energy can excite either the Weibel-like instability (WI) of the ordinary mode perpendicular to the magnetic field or the firehose instability (FHI) of the circularly polarized waves at parallel propagation. The interplay of these two instabilities is examined. The growth rates and the thresholds provided by the kinetic Vlasov – Maxwell theory are compared. The WI is the fastest growing one with a growth rate that is several orders of magnitude larger than that of the FHI. These instabilities are however inhibited by the ambient magnetic field by introducing a temperature anisotropy threshold. The WI admits a larger anisotropy threshold, so that, under this threshold, the FHI remains the principal mechanism of relaxation. The criteria provided here by describing the interplay of the WI and FHI are relevant for the existence of these two instabilities in any space plasma system characterized by an excess of parallel kinetic energy.     

Optimal extraction of cosmological information from supernova data in the presence of calibration uncertainties

We present a new technique to extract the cosmological information from high-redshift supernova data in the presence of calibration errors and extinction due to dust. While in the traditional technique the distance modulus of each supernova is determined separately, in our approach we determine all distance moduli at once, in a process that achieves a significant degree of self-calibration. The result is a much reduced sensitivity of the cosmological parameters to the calibration uncertainties. As an example, for a strawman mission similar to that outlined in the SNAP satellite proposal, the increased precision obtained with the new approach is roughly equivalent to a factor of five decrease in the calibration uncertainty.     

Spectroscopic observations of SN1987a in the LMC

This paper describes the results of optical spectroscopy of SN1987a carried out at Mt John University Observatory on a regular basis since 1987 February 25. Typical spectra are presented in an evolutionary sequence, and velocities of Balmer and selected metallic lines are measured in either emission or absorption. The velocities are interpreted together with the results of photoelectric photometry. The Barnes-Evans relationship has been applied to the photometry to give an angular diameter of the photosphere. The expansion velocity of the photosphere is initially about 3700 km s^-1 which is similar to the asymptotic value from weaker absorption lines such as the MgIb line. After 40 days the photosphere appears to expand more slowly and it reaches a maximum size of about 145 A.U. after about 100 days before receding inwards. The photometry and spectroscopy together result in a distance modulus of 18.3 ± 0.2.     

Diagnostics of Titan's stratospheric dynamics using Cassini/CIRS data and the 2-dimensional IPSL circulation model

The dynamics of Titan's stratosphere is discussed in this study, based on a comparison between observations by the CIRS instrument on board the Cassini spacecraft, and results of the 2-dimensional circulation model developed at the Institute Pierre-Simon Laplace, available at http://www.lmd.jussieu.fr/titanDbase [Rannou, P., Lebonnois, S., Hourdin, F., Luz, D., 2005. Adv. Space Res. 36, 2194-2198]. The comparison aims at both evaluating the model's capabilities and interpreting the observations concerning: (1) dynamical and thermal structure using temperature retrievals from Cassini/CIRS and the vertical profile of zonal wind at the Huygens landing site obtained by Huygens/DWE; and (2) vertical and latitudinal profiles of stratospheric gases deduced from Cassini/CIRS data. The modeled thermal structure is similar to that inferred from observations (Cassini/CIRS and Earth-based observations). However, the upper stratosphere (above 0.05 mbar) is systematically too hot in the 2D-CM, and therefore the stratopause region is not well represented. This bias may be related to the haze structure and to misrepresented radiative effects in this region, such as the cooling effect of hydrogen cyanide (HCN). The 2D-CM produces a strong atmospheric superrotation, with zonal winds reaching 200 m s⁻¹ at high winter latitudes between 200 and 300 km altitude (0.1-1 mbar). The modeled zonal winds are in good agreement with retrieved wind fields from occultation observations, Cassini/CIRS and Huygens/DWE. Changes to the thermal structure are coupled to changes in the meridional circulation and polar vortex extension, and therefore affect chemical distributions, especially in winter polar regions. When a higher altitude haze production source is used, the resulting modeled meridional circulation is weaker and the vertical and horizontal mixing due to the polar vortex is less extended in latitude. There is an overall good agreement between modeled chemical distributions and observations in equatorial regions. The difference in observed vertical gradients of C₂H₂ and HCN may be an indicator of the relative strength of circulation and chemical loss of HCN. The negative vertical gradient of ethylene in the low stratosphere at 15° S, cannot be modeled with simple 1-dimensional models, where a strong photochemical sink in the middle stratosphere would be necessary. It is explained here by dynamical advection from the winter pole towards the equator in the low stratosphere and by the fact that ethylene does not condense. Near the winter pole (80° N), some compounds (C₄H₂, C₃H₄) exhibit an (interior) minimum in the observed abundance vertical profiles, whereas 2D-CM profiles are well mixed all along the atmospheric column. This minimum can be a diagnostic of the strength of the meridional circulation, and of the spatial extension of the winter polar vortex where strong descending motions are present. In the summer hemisphere, observed stratospheric abundances are uniform in latitude, whereas the model maintains a residual enrichment over the summer pole from the spring cell due to a secondary meridional overturning between 1 and 50 mbar, at latitudes south of 40-50° S. The strength, as well as spatial and temporal extensions of this structure are a difficulty, that may be linked to possible misrepresentation of horizontally mixing processes, due to the restricted 2-dimensional nature of the model. This restriction should also be kept in mind as a possible source of other discrepancies.     

Detecting extra-galactic supernova neutrinos in the Antarctic ice

Building on the technological success of the IceCube neutrino telescope, we outline a prospective low-energy extension that utilizes the clear ice of the South Pole. Aiming at a 10 Mton effective volume and a 10 MeV threshold, the detector would provide sufficient sensitivity to detect neutrino bursts from core-collapse supernovae (SNe) in nearby galaxies. The detector geometry and required density of instrumentation are discussed along with the requirements to control the various sources of background, such as solar neutrinos. In particular, the suppression of spallation events induced by atmospheric muons poses a challenge that will need to be addressed. Assuming this background can be controlled, we find that the resulting detector will be able to detect SNe from beyond 10 Mpc, delivering between 10 and 41 regular core-collapse SN detections per decade. It would further allow to study more speculative phenomena, such as optically dark (failed) SNe, where the collapse proceeds directly to a black hole, at a detection rate similar to that of regular SNe. We find that the biggest technological challenge lies in the required number of large area photo-sensors, with simultaneous strict limits on the allowed noise rates. If both can be realized, the detector concept we present will reach the required sensitivity with a comparatively small construction effort and hence offers a route to future routine observations of SNe with neutrinos.     

Self-consistent theory of white dwarf burning in supernova ia events

A self-consistent model of white dwarf burning in the Supernovae Ia events is proposed which includes the consequent stages of the flame, the spontaneous explosion and the detonation. The flame is ignited locally at several points near the center of the star, but soon it is pushed out of the center due to the Rayleigh-Taylor instability. Most of the hot fuel of the central part of the star remains unburnt by the flame. The unburnt fuel explodes spontaneously after the time delay determined by the average temperature near the star center. Until the time of the explosion the flame consumes more than 0.1 of the white dwarf mass out of the star center, which causes pre-expansion of the unburnt fuel in the outer layers of the star. The spontaneous explosion triggers the detonation which incinerates the rest of the star and produces the intermediate mass elements in the pre-expanded layers. The detonation overcomes gravitational binding and causes mass ejection. The proposed theory provides the physical basis for the explanation of the observed spectrum of Supernovae Ia.     

The galactic evolution of the supernova rates

Supernova rates (hypernova, type II, type Ib/c and type Ia) in a particular galaxy depend on the metallicity (i.e. on the galaxy age), on the physics of star formation and on the binary population. In order to study the time evolution of the galactic supernova rates, we use our chemical evolutionary model that accounts in detail for the evolution of single stars and binaries. In particular, supernovae of type Ia are considered to arise from exploding white dwarfs in interacting binaries and we adopt the two most plausible physical models: the single degenerate model and the double degenerate model. Comparison between theoretical prediction and observations of supernova rates in different types of galaxies allows to put constraints on the population of intermediate mass and massive close binaries.

The temporal evolution of the absolute galactic rates of different types of supernovae (including the type Ia rate) is presented in such a way that the results can be directly implemented into a galactic chemical evolutionary model. Particularly for type Ia’s the inclusion of binary evolution leads to results considerably different from those in earlier population synthesis approaches, in which binary evolution was not included in detail.     

Binary progenitors of Supernovae

Supernovae of both Type I (hydrogen-poor) and Type II (hydrogen-rich) can be expected to occur among binary stars. Among massive stars (>10 M•), the companion makes it more difficult for the primary to develop an unstable core of >1.4.M• while still retaining the extended, hydrogen-rich envelope needed to make a typical Type II light curve. Among 1–10 M• stars, on the other hand, a companion plays a vital role in currently popular models for Type I events, by transferring material to the primary after it has become a stable white dwarf, and so driving it to conditions where either core collapse or explosive nuclear burning will occur. Several difficulties (involving nucleosynthesis, numbers and lifetimes of progenitors, the mass-transfer mechanism,etc.) still exist in these models. Some of them are overcome by a recent, promising scenario in which the secondary also evolves to a degenerate configuration, and the two white dwarfs spiral together to produce a hydrogen-free explosion, long after single stars of the same initial masses have ceased to be capable of fireworks.     

Photometry of the Supernova SN 2002ap in M 74 during 2002

UBVR _c I _c observations of SN 2002ap during February, October, and November 2002 at the Crimean Astrophysical Observatory are reported. An examination of our photometric data, along with published data, shows that over a period of about a year from the day the SN 2002ap supernova burst, the light curve passed through three developmental stages: a sharp rise, followed by a stage of rapid exponential decrease, and then a slower fading. Based on the shape of the light curve, this supernova is of type SN I, but according to the variation in its color indices, it more likely belongs to the SN Ic supernovae. In the premaximum stage, the energy distribution from 3000 to 6000 resembles the emission from a star of spectral class F5V. In the second stage of the light curve evolution, when the brightness falls off rapidly, the changes in the color indices are associated with a change in the radiation temperature indicative of rapid cooling of the ejected material. Taking the effective radiation temperature in the premaximum stage to be T _eff 6500 K, we estimate the expansion velocity of the quasiphotosphere to be about 9700 km/s.