Models of X-ray stars.
Scorpius X-1 X ray star model based on relativistic electron distribution due to magnetic field fluctuations, considering role of Alfven waves
Manley, O. P.↗
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Models for neutron-core stars based on realistic nuclear-matter calculations
Neutron core star models based on realistic nuclear matter calculations and hyperons equation of state
Rose, W. K.↗
New limb-darkening coefficients for modeling binary star light curves
We present monochromatic, passband-specific, and bolometric limb-darkening coefficients for a linear as well as nonlinear logarithmic and square root limb-darkening laws. These coefficients, including the bolometric ones, are needed when modeling binary star light curves with the latest version of the Wilson-Devinney light curve progam. We base our calculations on the most recent ATLAS stellar atmosphere models for solar chemical composition stars with a wide range of effective temperatures and surface gravitites. We examine how well various limb-darkening approximations represent the variation of the emerging specific intensity across a stellar surface as computed according to the model. For binary star light curve modeling purposes, we propose the use of a logarithmic or a square root law. We design our tables in such a manner that the relative quality of either law with respect to another can be easily compared. Since the computation of bolometric limb-darkening coefficients first requires monochromatic coefficients, we also offer tables of these coefficients (at 1221 wavelength values between 9.09 nm and 160 micrometer) and tables of passband-specific coefficients for commonly used photometric filters.
Van Hamme, W.↗
The structure and evolution of galacto-detonation waves - Some analytic results in sequential star formation models of spiral galaxies
Waves of star formation in a uniform, differentially rotating disk galaxy are treated analytically as a propagating detonation wave front. It is shown, that if single solitary waves could be excited, they would evolve asymptotically to one of two stable spiral forms, each of which rotates with a fixed pattern speed. Simple numerical solutions confirm these results. However, the pattern of waves that develop naturally from an initially localized disturbance is more complex and dies out within a few rotation periods. These results suggest a conclusive observational test for deciding whether sequential star formation is an important determinant of spiral structure in some class of galaxies.
Cowie, L. L.↗
Anisotropic stars for spherically symmetric spacetimes satisfying the Karmarkar condition
A new class of solution describing an anisotropic stellar configuration satisfying Karmarkar’s condition i.e. spherically symmetric metric of embedding class 1, is reported. It has been shown that the compact star model is physically well-behaved and meet all the physical requirements for a stable configuration in hydrostatic equilibrium. Our model describes compact stars like Vela X-1 and 4U1608-52 to a very good approximation. - Highlights: • We consider the Einstein field equations for spherically symmetric spacetimes of embedding class one. • We explore the Karmarkar condition for spherically symmetric, bounded matter configurations. • The Karmarkar condition reduces the solution-generating method to the Einstein field equations to a single metric function. • By specifying this metric function we generate physically plausible stellar configurations with anisotropic pressure.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Convective shells in the interior of Cepheid variable stars: Overshooting models based on hydrodynamic simulations
Context. Because Cepheid variable stars have long been used as a cosmic benchmark for scaling distances in our Galaxy and beyond, the accuracy of stellar evolution models for Cepheids have wide-reaching effects. However, our understanding of the dynamics in the interiors of these physically complex stars is limited. Aims. Our goal is to provide a detailed multi-dimensional picture of hydrodynamic convection and convective boundary mixing in the interior of Cepheids. Methods. Using the Modules for Experiments in Stellar Astrophysics (MESA), we studied the structure of intermediate-mass stars that cross the instability strip. Then, we performed two-dimensional hydrodynamic simulations of six stars with the fully compressible Multidimensional Stellar Implicit Code (MUSIC). Our simulations did not model the radial pulsations but focused on the interior structure of this family of stars. We developed and applied a new statistical analysis to examine convection and convective boundary mixing in the interior of these stellar simulations. Results. Based on a grid of MESA models, we demonstrated that a common structure for intermediate mass Cepheids includes an interior convective shell as well as a thin outer convective envelope. Using the extreme value theory approach to analyze our MUSIC simulation data, we found that overshooting above the convective shell fills the space between these convectively unstable layers. We developed a new statistical analysis that provides a clearer picture of how overshooting fills this layer; it also allowed us to formulate a detailed comparison between overshooting above and below the convective shell. Our analysis effectively decomposes the overshooting layer into two layers: a weak overshooting layer and a strong overshooting layer. Statistically, this is accomplished by decomposing the strongly non-Gaussian probability density function into a mixture of gamma distributions. Using our mixture model, we showed that the ratio of overshooting lengths above and below the convective shell depends directly on the radial extent of the convective shell as well as its depth in the star. We proposed a new form for the diffusion coefficient that addresses the need for overlapping overshooting layers between convective shells. We introduced the idea of a “super-mixing layer” where overshooting from both the convective shell and the convective envelope results in efficient mixing and could be viewed as merging the two adjacent convective zones.
79 ASTRONOMY AND ASTROPHYSICS↗
Direct Measurement of Internal Flow Velocities in a Star-Slot Model
This paper presents the results of a cold flow experiment to make direct measurements of the velocity distribution in a model of a solid rocket motor star grain propellant slot. The experimental procedure utilizes a multi-component laser Doppler velocimeter (LDV) and an apparatus for seeding the flow with aluminum particles to determine the velocity components at various discrete locations within the star slot. The test article used in this investigation was a one-tenth scale, cold flow model based on the geometry of the Space Shuttle solid rocket motor head-end section. The results obtained for the direct measurements of velocity are compared to velocities calculated from measured pressure distributions to data obtained from oil smear experiments and flow visualization videos, and to heat transfer calorimeter data.
Foster, Winfred A., Jr.↗
Numerical models of star formation in X-ray cluster cooling flows
A grid of numerical models is used here to study how accretion flows are affected by star formation. The observables associated with these models are investigated, presenting their X-ray surface brightness profiles. An X-ray diagnostic is searched for that, in the absence of spatially resolved X-ray spectral data, can be an unambiguous indicator for ongoing star formation in cooling flows. The spatial distributions of newly formed stars are presented and compared to profiles of the accreting galaxy in each model. Whether the accretion population can form the luminous parts of the accreting galaxy or whether it forms a dark component is considered.
White, Raymond E., III↗
BL Herculis stars - Theoretical models for field variables
Type II Cepheids with periods between 1 and 3 days, commonly designated as Bl Herculis stars, have been modeled here with the aim of interpreting the wide variety of light curves observed among the field variables. Previously modeled globular cluster members are used as standard calibration objects. The major finding is that only a small range of luminosities is capable of generating a large variety of light curve types at a given period. For a mass of approximately 0.60 solar mass, the models are able to reproduce the observed mean luminosities, dispersion of mean luminosities, periods, light amplitudes, light asymmetries, and phases of secondary features in the light curves of known BL Her stars. It is possible that the metal-rich variables (which are found only in the field) have luminosities lower than those of most metal-poor variables. The present revised mass for BL Her, a metal-rich object, is not significantly different from the mean mass of the metal-poor variables.
Carson, R.↗
Surface Temperature Mapping Models for STAR and CTF
This milestone presents a demonstration of a surface mapping model to map the surface temperature for single-phase STAR-CCM+ to Cobra-TF using average temperature data. This model can be used to generate high-resolution surface temperature data. This can be accomplished with linear equations or with an alternative non-linear model. Improvements and a path forward for the surface mapping model to be applied to two-phase temperature mappings is also laid out in this milestone report.
42 ENGINEERING↗
Second-overtone models of RR Lyrae stars
Linear models of RR Lyrae stars pulsating in the second overtone indicate that this mode is self-excited only for rather low luminosities if the masses are normal. An unstable second-overtone model has been selected from the linear survey and has been followed up to limiting amplitude by using a nonlinear hydrodynamical computer program. Deep splitting of the main light peak in the theoretical light curve occurs but does not resemble any observed feature. Moreover, the distribution of periods of RR Lyrae stars in globular clusters can be accounted for adequately by fundamental-mode and first-overtone pulsation. It is concluded that second-overtone pulsators probably do not exist among RR Lyrae stars.
Stothers, Richard B.↗
An analytical model of eclipsing binary star systems
Model of eclipsing binary star system based on digital computers
Wood, D. B.↗
Helium-burning flashes on an accreting neutron star - A model for X-ray burst sources
Detailed numerical models of X-ray bursts resulting from thermonuclear flashes near the surface of an accreting neutron star have been constructed. The models assume a nonrotating nonmagnetized spherically accreting neutron star of 1.4 solar masses, radius 6.6 km, core temperature in the range from 250 million to 570 million K, and accretion rate in the range (0.3-3) x 10 to the 17th power g/s. Under many conditions the helium-burning shell undergoes thermonuclear flashes that result in the emission of X-ray bursts, the gross properties of which are remarkably similar to those of most observed X-ray burst sources. Neutron stars with moderately low core temperatures, low accretion rates, and weak magnetic fields are most likely to produce X-ray bursts.
Joss, P. C.↗
Intermediate-mass Asymptotic Giant Branch Stars and Sources of {sup 26}Al, {sup 60}Fe, {sup 107}Pd, and {sup 182}Hf in the Solar System
We explore the possibility that the short-lived radionuclides {sup 26}Al, {sup 60}Fe, {sup 107}Pd, and {sup 182}Hf inferred to be present in the proto-solar cloud originated from 3–8 M{sub ⊙} asymptotic giant branch (AGB) stars. Models of AGB stars with initial mass above 5 M{sub ⊙} are prolific producers of {sup 26}Al owing to hot bottom burning (HBB). In contrast, {sup 60}Fe, {sup 107}Pd, and {sup 182}Hf are produced by neutron captures: {sup 107}Pd and {sup 182}Hf in models ≲5 M{sub ⊙}, and {sup 60}Fe in models with higher mass. We mix stellar yields from solar-metallicity AGB models into a cloud of solar mass and composition to investigate whether it is possible to explain the abundances of the four radioactive nuclides at the Sun’s birth using one single value of the mixing ratio between the AGB yields and the initial cloud material. We find that AGB stars that experience efficient HBB (⩾6 M{sub ⊙}) cannot provide a solution because they produce too little {sup 182}Hf and {sup 107}Pd relative to {sup 26}Al and {sup 60}Fe. Lower-mass AGB stars cannot provide a solution because they produce too little {sup 26}Al relative to {sup 107}Pd and {sup 182}Hf. A self-consistent solution may be found for AGB stars with masses in between (4–5.5 M{sub ⊙}), provided that HBB is stronger than in our models and the {sup 13}C(α, n){sup 16}O neutron source is mildly activated. If stars of M<5.5 M{sub ⊙} are the source of the radioactive nuclides, then some basis for their existence in proto-solar clouds needs to be explored, given that the stellar lifetimes are longer than the molecular cloud lifetimes.
79 ASTRONOMY AND ASTROPHYSICS↗
NuGrid stellar data set – II. Stellar yields from H to Bi for stellar models with MZAMS = 1–25 M⊙ and Z = 0.0001–0.02
We provide here a significant extension of the NuGrid Set 1 models in mass coverage and towards lower metallicity, adopting the same physics assumptions. The combined data set now includes the initial masses M ZAMS /M ⊙ = 1, 1.65, 2, 3, 4, 5, 6, 7, 12, 15, 20, 25 for Z = 0.02, 0.01, 0.006, 0.001, 0.0001 with α-enhanced composition for the lowest three metallicities. These models are computed with the MESA stellar evolution code and are evolved up to the AGB, the white dwarf stage, or until core collapse. The nucleosynthesis was calculated for all isotopes in post-processing with the NuGrid MPPNP code. Explosive nucleosynthesis is based on semi-analytic 1D shock models. Metallicity-dependent mass-loss, convective boundary mixing in low- and intermediate-mass models and H and He core burning massive star models are included. Convective O-C shell mergers in some stellar models lead to the strong production of odd-Z elements P, Cl, K, and Sc. In AGB models with hot dredge-up, the convective boundary mixing efficiency is reduced to accommodate for its energetic feedback. In both low-mass and massive star models at the lowest metallicity, H-ingestion events are observed and lead to i-process nucleosynthesis and substantial 15 N production. Finally, complete yield data tables, derived data products and online analytic data access are provided.
79 ASTRONOMY AND ASTROPHYSICS↗
A computer model of disks of stars.
Computer model for evolution investigation of disks of stars, discussing methods to obtain gravitational potential
Hockney, R. W.↗
Modeling Neutron Star Matter in the Age of Multimessenger Astrophysics
Abstract The interpretation of available and forthcoming data obtained from multimessenger astrophysical observations—potentially providing unprecedented access to neutron star properties—will require the development of novel, accurate theoretical models of dense matter. Of great importance, in this context, will be the capability for devising a description of thermal effects applicable to the study of quantities other than the equation of state, such as the transport coefficients and the neutrino mean free path in the nuclear medium. A formalism based on correlated basis states and the cluster expansion technique has been previously employed to derive a well-behaved effective interaction—suitable for use in standard perturbation theory—from a state-of-the-art nuclear Hamiltonian, including phenomenological two- and three-nucleon potentials. Here, we provide a comprehensive and self-contained account of the extension of this approach to the treatment of finite-temperature effects, and report the results of numerical calculations of a number of properties of nuclear matter with arbitrary neutron excess and temperature up to 50 MeV.
79 ASTRONOMY AND ASTROPHYSICS↗
Stochastic stellar wind models - Evolved stars
Observational, semiempirical, and theoretical results are presented for individual cool giants and supergiants which show that stochastic stellar wind flows are an extremely important feature in these stars. It is shown that the status of the existing theoretical models is very uncertain, due to the fact that appropriate models must simultaneously take into account a broad variety of strongly coupled physical processes. The potentially important physical processes include global oscillation modes; convection; magnetic fields; the generation, propagation, and interaction of stochastic waves; three-dimensional radiative transfer effects; time-dependent hydrogen ionization; thermal bifurcation and instabilities; and nonequilibrium chemistry, particularly the formation and destruction of molecules and dust.
Cuntz, Manfred↗