Should One Use the Ray-by-Ray Approximation in Core-Collapse Supernova Simulations?

We perform the first self-consistent, time-dependent, multi-group calculations in two dimensions (2D) to address the consequences of using the ray-by-ray+ transport simplification in core-collapse supernova simulations. Such a dimensional reduction is employed by many researchers to facilitate their resource-intensive calculations. Our new code (F{\sc{ornax}}) implements multi-D transport, and can, by zeroing out transverse flux terms, emulate the ray-by-ray+ scheme. Using the same microphysics, initial models, resolution, and code, we compare the results of simulating 12-, 15-, 20-, and 25-M$_{\odot}$ progenitor models using these two transport methods. Our findings call into question the wisdom of the pervasive use of the ray-by-ray+ approach. Employing it leads to maximum post-bounce/pre-explosion shock radii that are almost universally larger by tens of kilometers than those derived using the more accurate scheme, typically leaving the post-bounce matter less bound and artificially more "explodable." In fact, for our 25-M$_{\odot}$ progenitor, the ray-by-ray+ model explodes, while the corresponding multi-D transport model does not. Therefore, in two dimensions the combination of ray-by-ray+ with the axial sloshing hydrodynamics that is a feature of 2D supernova dynamics can result in quantitatively, and perhaps qualitatively, incorrect results.Comment: Updated and revised text; 13 pages; 13 figures; Accepted to Ap.

Theoretical Spectra and Light Curves of Close-in Extrasolar Giant Planets and Comparison with Data

We present theoretical atmosphere, spectral, and light-curve models for extrasolar giant planets (EGPs) undergoing strong irradiation for which {\it Spitzer} planet/star contrast ratios or light curves have been published (circa June 2007). These include HD 209458b, HD 189733b, TrES-1, HD 149026b, HD 179949b, and $\upsilon$ And b. By comparing models with data, we find that a number of EGP atmospheres experience thermal inversions and have stratospheres. This is particularly true for HD 209458b, HD 149026b, and $\upsilon$ And b. This finding translates into qualitative changes in the planet/star contrast ratios at secondary eclipse and in close-in EGP orbital light curves. Moreover, the presence of atmospheric water in abundance is fully consistent with all the {\it Spitzer} data for the measured planets. For planets with stratospheres, water absorption features invert into emission features and mid-infrared fluxes can be enhanced by a factor of two. In addition, the character of near-infrared planetary spectra can be radically altered. We derive a correlation between the importance of such stratospheres and the stellar flux on the planet, suggesting that close-in EGPs bifurcate into two groups: those with and without stratospheres. From the finding that TrES-1 shows no signs of a stratosphere, while HD 209458b does, we estimate the magnitude of this stellar flux breakpoint. We find that the heat redistribution parameter, P$_n$, for the family of close-in EGPs assumes values from $\sim$0.1 to $\sim$0.4. This paper provides a broad theoretical context for the future direct characterization of EGPs in tight orbits around their illuminating stars.Comment: Accepted to Ap. J., provided here in emulateapj format: 28 pages, 8 figures, many with multiple panel

Shear horizontal (SH) ultrasound wave propagation around smooth corners

Shear horizontal (SH) ultrasound guided waves are being used in an increasing number of non-destructive testing (NDT) applications. One advantage SH waves have over some wave types, is their ability to propagate around curved surfaces with little energy loss; to understand the geometries around which they could propagate, the wave reflection must be quantified. A 0.83 mm thick aluminium sheet was placed in a bending machine, and a shallow bend was introduced. Periodically-poled magnet (PPM) electromagnetic acoustic transducers (EMATs), for emission and reception of SH waves, were placed on the same side of the bend, so that reflected waves were received. Additional bending of the sheet demonstrated a clear relationship between bend angles and the reflected signal. Models suggest that the reflection is a linear superposition of the reflections from each bend segment, such that sharp turns lead to a larger peak-to-peak amplitude, in part due to increased phase coherence

Testing the models: NIR imaging and spectroscopy of the benchmark T-dwarf binary Eps Indi B

The relative roles of metallicity and surface gravity on the near-infrared spectra of late-T brown dwarfs are not yet fully understood, and evolutionary models still need to be calibrated in order to provide accurate estimates of brown dwarf physical parameters from measured spectra. The T-type brown dwarfs Eps Indi Ba and Bb forming the tightly bound binary Eps Indi B, which orbits the K4V star Eps Indi A, are nowadays the only such benchmark T dwarfs for which all important physical parameters such as metallicity, age and mass are (or soon will be) known. We present spatially resolved VLT/NACO images and low resolution spectra of Eps Indi B in the J, H and K near-infrared bands. The spectral types of Eps Indi Ba and Bb are determined by direct comparison of the flux-calibrated JHK spectra with T dwarf standard template spectra and also by NIR spectral indices. Eps Indi Bb is confirmed as a T6 while the spectral type of Eps Indi Ba is T1.5 so somewhat later than the previously reported T1. Constrained values for surface gravity and effective temperature are derived by comparison with model spectra. The evolutionary models predict masses around about 53 M_J for Eps Indi Ba and about 34 M_J for Eps Indi Bb, slightly higher than previously reported values. The suppressed J-band and enhanced K-band flux of Eps Indi Ba indicates that a noticeable cloud layer is still present in a T1.5 dwarf while no clouds are needed to model the spectrum of Eps Indi Bb.Comment: 7 pages, 5 figures, accepted by Ap

The very nearby M/T dwarf binary SCR 1845-6357

The recently discovered star SCR 1845-6357 is the first late M/T dwarf binary discovered. SCR 1845 is a particular object due to its tight orbit (currently around 4 AU) and its proximity to the Sun (3.85 pc). We present spatially resolved VLT/NACO images and low resolution spectra of SCR 1845 in the J, H and K near-infrared bands. Since the T dwarf companion, SCR 1845B, is so close to the primary SCR 1845A, orbital motion is evident even within a year. Following the orbital motion, the binary's mass can be measured accurately within a decade, making SCR 1845B a key T-dwarf mass-luminosity calibrator. The NIR spectra allow for accurate determination of spectral type and also for rough estimates of the object's physical parameters. The spectral type of SCR 1845B is determined by direct comparison of the flux calibrated JHK spectra with T dwarf standard template spectra and also by NIR spectral indices obtained from synthetic photometry. Constrained values for surface gravity, effective temperature and metallicity are derived by comparison with model spectra. Our data prove that SCR 1845B is a brown dwarf of spectral type T6 that is co-moving with and therefore gravitationally bound to the M8.5 primary. Fitting the NIR spectrum of SCR 1845B to model spectra yields an effective temperature of about 950K and a surface gravity log(g)=5.1 (cgs) assuming solar metallicity. Mass and age of SCR 1845B are in the range 40 to 50 Jupiter masses and 1.8 to 3.1 Gyr.Comment: 5 pages, 4 figures, accepted for publication in Astronomy & Astrophysic

Ab initio Translationally Invariant Nonlocal One-body Densities from No-core Shell-model Theory

[Background:] It is well known that effective nuclear interactions are in general nonlocal. Thus if nuclear densities obtained from {\it ab initio} no-core-shell-model (NCSM) calculations are to be used in reaction calculations, translationally invariant nonlocal densities must be available. [Purpose:] Though it is standard to extract translationally invariant one-body local densities from NCSM calculations to calculate local nuclear observables like radii and transition amplitudes, the corresponding nonlocal one-body densities have not been considered so far. A major reason for this is that the procedure for removing the center-of-mass component from NCSM wavefunctions up to now has only been developed for local densities. [Results:] A formulation for removing center-of-mass contributions from nonlocal one-body densities obtained from NCSM and symmetry-adapted NCSM (SA-NCSM) calculations is derived, and applied to the ground state densities of $^4$He, $^6$Li, $^{12}$C, and $^{16}$O. The nonlocality is studied as a function of angular momentum components in momentum as well as coordinate space [Conclusions:] We find that the nonlocality for the ground state densities of the nuclei under consideration increases as a function of the angular momentum. The relative magnitude of those contributions decreases with increasing angular momentum. In general, the nonlocal structure of the one-body density matrices we studied is given by the shell structure of the nucleus, and can not be described with simple functional forms.Comment: 13 pages, 11 Figure

First CRDS-measurements of water vapour continuum in the 940nm absorption band

Measurements of near-infrared water vapour continuum using continuous wave cavity ring down spectroscopy (cw- CRDS) have been performed at around 10611.6 and 10685:2 cm1. The continuum absorption coefficients for N2- broadening have been determined for two temperatures and wavenumbers. These results represent the first near-IR continuum laboratory data determined within the complex spectral environment in the 940nm water vapour band and are in reasonable agreement with simulations using the semiempirical CKD formulation