A higher-order split-step Fourier parabolic-equation sound propagation solution scheme

Author Posting. © Acoustical Society of America, 2012. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 132 (2012): EL61-EL67, doi:10.1121/1.4730328.A three-dimensional Cartesian parabolic-equation model with a higher-order approximation to the square-root Helmholtz operator is presented for simulating underwater sound propagation in ocean waveguides. The higher-order approximation includes cross terms with the free-space square-root Helmholtz operator and the medium phase speed anomaly. It can be implemented with a split-step Fourier algorithm to solve for sound pressure in the model. Two idealized ocean waveguide examples are presented to demonstrate the performance of this numerical technique.This work was sponsored by the Office of Naval Research under Grants No. N00014-10- 1-0040 and No.N00014-11-1-0701

A nonstationary form of the range refraction parabolic equation and its application as an artificial boundary condition for the wave equation in a waveguide

The time-dependent form of Tappert's range refraction parabolic equation is derived using Daletskiy-Krein formula form noncommutative analysis and proposed as an artificial boundary condition for the wave equation in a waveguide. The numerical comparison with Higdon's absorbing boundary conditions shows sufficiently good quality of the new boundary condition at low computational cost.Comment: 12 pages, 9 figure

The cataclysmic variable QZ Lib : a period bouncer

While highly evolved cataclysmic variables (CVs) with brown dwarf donors, often called “period bouncers”, are predicted to make up ≃ 40 − 70% of the Galactic CV population, only a handful of such systems are currently known. The identification and characterization of additional period bouncers is therefore important to probe this poorly understood phase of CV evolution. We investigate the evolution of the CV QZ Lib following its 2004 super–outburst using multi–epoch spectroscopy. From time– resolved spectroscopic observations we measure the orbital period of the system, Porb = 0.06436(20) d, which, combined with the superhump period PSH = 0.064602(24) d, yields the system mass ratio, q = 0.040(9). From the analysis of the spectral energy distribution we determine the structure of the accretion disc and the white dwarf effective temperature, Teff = 10 500 ± 1500 K. We also derive an upper limit on the effective temperature of the secondary, Teff < 1700 K, corresponding to a brown dwarf of T spectral type. The low temperature of the white dwarf, the small mass ratio and the fact that the donor is not dominating the near–infrared emission are all clues of a post bounce system. Although it is possible that QZ Lib could have formed as a white dwarf plus a brown dwarf binary, binary population synthesis studies clearly suggest this scenario to be less likely than a period bouncer detection and we conclude that QZ Lib is a CV that has already evolved through the period minimum

K-band spectroscopy of pre-cataclysmic variables

Aims. There exists now substantial evidence for abundance anomalies in a number of cataclysmic variables (CVs), indicating that the photosphere of the secondary star incorporates thermonuclear processed material. However, the spectral energy distribution in CVs is usually dominated by the radiation produced by the accretion process, severely hindering an investigation of the stellar components. On the other hand, depending on how the secondary star has acquired such material, the above mentioned abundance anomalies could also be present in pre-CVs, i.e. detached white/red dwarf binaries that will eventually evolve into CVs, but have not yet started mass transfer, and therefore allow for an unobstructed view on the secondary star at infrared wavelengths. Methods. We have taken K-band spectroscopy of a sample of 13 pre-CVs in order to examine them for anomalous chemical abundances. In particular, we study the strength of the 12CO and 13CO absorption bands that have been found diminished and enhanced, respectively, in similar studies of CVs. Results. All our systems show CO abundances that are within the range observed for single stars. The weakest 12CO bands with respect to the spectral type are found in the pre-CV BPM 71214, although on a much smaller scale than observed in CVs. Furthermore there is no evidence for enhanced 13CO. Taking into account that our sample is subject to the present observational bias that favours the discovery of young pre-CVs with secondary stars of late spectral types, we can conclude the following: 1) our study provides observational proof that the CO anomalies discovered in certain CVs are not due to any material acquired during the common envelope phase, and 2) if the CO anomalies in certain CVs are not due to accretion of processed material during nova outburst, then the progenitors of these CVs are of a significantly different type than the currently known sample of pre-CVs

An iterative three-dimensional parabolic equation solver for propagation above irregular boundaries

This paper describes the development of an iterative three-dimensional parabolic equation solver that takes into account the effects of irregular boundaries and refraction from a layered atmosphere. A terrain-following coordinate transformation, based on the well-known Beilis-Tappert mapping, is applied to the narrow-angle parabolic equation in an inhomogeneous media. The main advantage of this approach, which has been used in two dimensions in the past, is the simplification of the impedance boundary condition at the earth surface. The transformed initial-boundary value problem is discretized using the Crank-Nicholson marching scheme in the propagating direction and second-order finite-differences in the transversal plane. The proposed method relies on an efficient iterative fixed-point solver which involves the inversion of tridiagonal matrices only. The accuracy of the method is evaluated through a comparison with boundary element simulations in a homogeneous atmosphere above a Gaussian hill. Results show that transversal scattering occur in the shadow zone of the obstacle where the 2D parabolic equation underestimates the pressure amplitude. The model is particularly suited for the simulation of infrasound in a three-dimensional environment with realistic topographie

The first pre-supersoft X-ray binary

We report the discovery of an extremely close white dwarf plus F dwarf main-sequence star in a 12 h binary identified by combining data from the Radial Velocity Experiment survey and the Galaxy Evolution Explorer survey. A combination of spectral energy distribution fitting and optical and Hubble Space Telescope ultraviolet spectroscopy allowed us to place fairly precise constraints on the physical parameters of the binary. The system, TYC 6760-497-1, consists of a hot Teff ∼ 20 000 K, MWD∼0.6M⊙MWD∼0.6M⊙ white dwarf and an F8 star (MMS∼1.23M⊙MMS∼1.23M⊙, RMS∼1.3R⊙RMS∼1.3R⊙) seen at a low inclination (i ∼ 37°). The system is likely the descendant of a binary that contained the F star and an ∼2 M⊙ A-type star that filled its Roche lobe on the thermally pulsating asymptotic giant branch, initiating a common envelope phase. The F star is extremely close to Roche lobe filling and there is likely to be a short phase of thermal time-scale mass transfer on to the white dwarf during which stable hydrogen burning occurs. During this phase, it will grow in mass by up to 20 per cent, until the mass ratio reaches close to unity, at which point it will appear as a standard cataclysmic variable star. Therefore, TYC 6760-497-1 is the first known progenitor of a supersoft source system, but will not undergo a Type Ia supernova explosion. Once an accurate distance to the system is determined by Gaia, we will be able to place very tight constraints on the stellar and binary parameters

Ray-based description of normal mode amplitudes in a range-dependent waveguide

An analogue of the geometrical optics for description of the modal structure of a wave field in a range-dependent waveguide is considered. In the scope of this approach the mode amplitude is expressed through solutions of the ray equations. This analytical description accounts for mode coupling and remains valid in a nonadiabatic environment. It has been used to investigate the applicability condition of the adiabatic approximation. An applicability criterion is formulated as a restriction on variations of the action variable of the ray.Comment: 11 pages, 5 figure