Stationary structures of irrotational binary systems -- models for close binary systems of compact stars

We propose a new numerical method to calculate irrotational binary systems composed of compressible gaseous stars in Newtonian gravity. Assuming irrotationality, i.e. vanishing of the vorticity vector everywhere in the star in the inertial frame, we can introduce the velocity potential for the flow field. Using this velocity potential we can derive a set of basic equations for stationary states which consist of (i) the generalized Bernoulli equation, (ii) the Poisson equation for the Newtonian gravitational potential and (iii) the equation for the velocity potential with the Neumann type boundary condition. We succeeded in developing a new code to compute numerically exact solutions to these equations for the first time. Such irrotational configurations of binary systems are appropriate models for realistic neutron star binaries composed of inviscid gases, just prior to coalescence of two stars caused by emission of gravitational waves. Accuracies of our numerical solutions are so high that we can compute reliable models for fully deformed final stationary configurations and hence determine the inner most stable circular orbit of binary neutron star systems under the approximations of weak gravity and inviscid limit.Comment: 32 pages, 25 bitmapped ps files, to appear in ApJ supplemen

The UBV Color Evolution of Classical and Symbiotic Novae

We identified a general course of classical nova outbursts in the B − V vs. U − B diagram. It has been reported that novae show spectra similar to A–F supergiants near optical light maximum. However, they do not follow the supergiant sequence in the color-color diagram, neither the blackbody nor the main-sequence sequence. Instead, we found that novae evolve along a new sequence in the pre-maximum and near-maximum phases, which we call the nova-giant sequence. This sequence is parallel to but Δ(U − B) ≈ −0.2 mag bluer than the supergiant sequence. After optical maximum, its color quickly evolves back blueward along the same nova-giant sequence and reaches the point of free-free emission (B − V = −0.03, U − B = −0.97) and stays there for a while, which is coincident with the intersection of the blackbody sequence and the nova-giant sequence. Then the color evolves leftward (blueward in B − V but almost constant in U − B) due mainly to development of strong emission lines. This is the general course of nova outbursts in the color-color diagram, which is deduced from eight well-observed novae including various speed classes. For a nova with unknown extinction, we can determine a reliable value of the color excess by matching the observed track of the target nova with this general course. This is a new and convenient method for obtaining color excesses of classical novae. Using this method, we redetermined the color excesses of nineteen well-observed novae

Theory of Nova Outbursts and Type Ia Supernovae

We briefly review the current theoretical understanding of the light curves of novae. These curves exhibit a homologous nature, dubbed the universal decline law, and when time-normalized, they almost follow a single curve independently of the white dwarf (WD) mass or chemical composition of the envelope. The optical and near-infrared light curves of novae are reproduced mainly by free-free emission from their optically thick winds. We can estimate the WD mass from multiwavelength observations because the optical, UV, and soft X-ray light curves evolve differently and we can easily resolve the degeneracy of the optical light curves. Recurrent novae and classical novae are a testbed of type Ia supernova scenarios. In the orbital period versus secondary mass diagram, recurrent novae are located in different regions from classical novae and the positions of recurrent novae are consistent with the single degenerate scenario

An approximate solver for Riemann and Riemann-like Ellipsoidal Configurations

We introduce a new technique for constructing three-dimensional (3D) models of incompressible Riemann S-type ellipsoids and compressible triaxial configurations that share the same velocity field as that of Riemann S-type ellipsoids. Our incompressible models are exact steady-state configurations; our compressible models represent approximate steady-state equilibrium configurations. Models built from this method can be used to study a variety of relevant astrophysical and geophysical problems.Comment: 25 pages, 10 figures, ApJ accepted, refereed versio

Bipolar Supernova Explosions: Nucleosynthesis & Implication on Abundances in Extremely Metal-Poor Stars

Hydrodynamics and explosive nucleosynthesis in bipolar supernova explosions are examined to account for some peculiar properties of hypernovae as well as peculiar abundance patterns of metal-poor stars. The explosion is supposed to be driven by bipolar jets which are powered by accretion onto a central remnant. We explore the features of the explosions with varying progenitors' masses and jet properties. The outcomes are different from conventional spherical models. (1) In the bipolar models, Fe-rich materials are ejected at high velocities along the jet axis, while O-rich materials occupy the central region whose density becomes very high as a consequence of continuous accretion from the side. This configuration can explain some peculiar features in the light curves and the nebular spectra of hypernovae. (2) Production of $^{56}$Ni tends to be smaller than in spherical thermal bomb models. To account for a large amount of $^{56}$Ni observed in hypernovae, the jets should be initiated when the compact remnant mass is still smaller than 2-3\msun, or the jets should be very massive and slow. (3) Ejected isotopes are distributed as follows in order of decreasing velocities: $^{64}$Zn, $^{59}$Co, $^{56}$Fe, $^{44}$Ti, and $^{4}$He at the highest velocities, $^{55}$Mn, $^{52}$Cr, $^{32}$S, and $^{28}$Si at the intermediate velocities, and $^{24}$Mg, $^{16}$O at the lowest velocities. (4) The abundance ratios (Zn, Co)/Fe are enhanced while the ratios (Mn, Cr)/Fe are suppressed. This can account for the abundance pattern of extremely metal-poor stars. These agreements between the models and observations suggest that hypernovae are driven by bipolar jets and have significantly contributed to the early Galactic chemical evolution.Comment: Accepted version, to appear in the Astrophysical Journal. Additional figures and an appendix. 58 pages including 21 figs and 9 table

Nucleosynthesis and Clump Formation in a Core Collapse Supernova

High-resolution two-dimensional simulations were performed for the first five minutes of the evolution of a core collapse supernova explosion in a 15 solar mass blue supergiant progenitor. The computations start shortly after bounce and include neutrino-matter interactions by using a light-bulb approximation for the neutrinos, and a treatment of the nucleosynthesis due to explosive silicon and oxygen burning. We find that newly formed iron-group elements are distributed throughout the inner half of the helium core by Rayleigh-Taylor instabilities at the Ni+Si/O and C+O/He interfaces, seeded by convective overturn during the early stages of the explosion. Fast moving nickel mushrooms with velocities up to about 4000 km/s are observed. This offers a natural explanation for the mixing required in light curve and spectral synthesis studies of Type Ib explosions. A continuation of the calculations to later times, however, indicates that the iron velocities observed in SN 1987 A cannot be reproduced because of a strong deceleration of the clumps in the dense shell left behind by the shock at the He/H interface.Comment: 8 pages, LaTeX, 2 postscript figures, 2 gif figures, shortened and slightly revised text and references, accepted by ApJ Letter