Radial and Nonradial Oscillation Modes in Rapidly Rotating Stars

Radial and nonradial oscillations offer the opportunity to investigate the interior properties of stars. We use 2D stellar models and a 2D finite difference integration of the linearized pulsation equations to calculate non-radial oscillations. This approach allows us to directly calculate the pulsation modes for a distorted rotating star without treating the rotation as a perturbation. We are also able to express the finite difference solution in the horizontal direction as a sum of multiple spherical harmonics for any given mode. Using these methods, we have investigated the effects of increasing rotation and the number of spherical harmonics on the calculated eigenfrequencies and eigenfunctions and compared the results to perturbation theory. In slowly rotating stars, current methods work well, and we show that the eigenfunction can be accurately modelled using 2nd order perturbation theory and a single spherical harmonic. We use 10 Msun models with velocities ranging from 0 to 420 km/s (0.89 Omega_c) and examine low order p modes. We find that one spherical harmonic remains reasonable up to a rotation rate around 300km s^{-1} (0.69 Omega_c) for the radial fundamental mode, but can fail at rotation rates as low as 90 km/s (0.23 Omega_c) for the 2H mode or l = 2 p_2 mode, based on the eigenfrequencies alone. Depending on the mode in question, a single spherical harmonic may fail at lower rotation rates if the shape of the eigenfunction is taken into consideration. Perturbation theory, in contrast, remains valid up to relatively high rotation rates for most modes. We find the lowest failure surface equatorial velocity is 120 km/s (0.30 Omega_c) for the l = 2 p_2 mode, but failure velocities between 240 and 300 km/s (0.58-0.69 Omega_c)are more typical.Comment: accepted for publication in Ap

The Structure of Close Binaries in Two Dimensions

The structure and evolution of close binary stars has been studied using the two-dimensional (2D) stellar structure algorithm developed by Deupree (1995). We have calculated a series of solar composition stellar evolution sequences of binary models, where the mass of the 2D model is 8Msun with a point-mass 5Msun companion. We have also studied the structure of the companion in 2D, by considering the zero-age main-sequence (ZAMS) structure of a 5Msun model with an 8Msun point-mass companion. In all cases the binary orbit was assumed to be circular and co-rotating with the rotation rate of the stars. We considered binary models with three different initial separations, a = 10, 14 and 20Rsun. These models were evolved through central hydrogen burning or until the more massive star expanded to fill its critical potential surface or Roche lobe. The calculations show that evolution of the deep interior quantities is only slightly modified from those of single star evolution. Describing the model surface as a Roche equipotential is also satisfactory until very close to the time of Roche lobe overflow, when the self gravity of the model about to lose mass develops a noticeable aspherical component and the surface time scale becomes sufficiently short that it is conceivable that the actual surface is not an equipotential.Comment: 22 pages, 10 figures, accepted by Ap

LACHESIS -- An instrumentation system for obtaining containment and environmental data

The instrumentation system developed to obtain containment and environmental data in the LYNER complex is presented. The primary purpose of this report is to familiarize potential operators of the system with the details of its use. The instrumentation system has three major hardware modules: (1) the sensor power source, amplifier, and signal conditioner module, (2) the digitizers, and (3) the computer controller. Each of these is described with emphasis on the steps required to make that component perform effectively. In addition the roles of activities of other people besides the Los Alamos shot engineer who are required to ensure the success of the system are outlined

CCD Photometry of Galactic Globular Clusters. IV. The NGC 1851 RR Lyraes

The variable star population of the galactic globular cluster NGC 1851 (C0512-400) has been studied by CCD photometry, from observations made in the B, V, and I bands during 1993-4. Light curves are presented for 29 variables, seven of which are new discoveries. The behavior of the RR lyraes in the period-temperature diagram appears normal when compared to clusters which bracket the NGC 1851 metallicity. Reddening and metallicity are re-evaluated, with no compelling evidence to change from accepted values. Photometry for stars within an annulus with radii 80 and 260 arcsec agrees to better than 0.02 mag in all colors with extensive earlier photometry, to at least V = 18.5. Instability strip boundary positions for several clusters shows a trend for the red boundary to move to redder colors as the metallicity increases.Comment: 29 pages, 9 figures, accepted by A.

Calibrating the Mixing Length Parameter for a Red Giant Envelope

Two-dimensional hydrodynamical simulations were made to calibrate the mixing length parameter for modeling red giant's convective envelope. As was briefly reported in Asida & Tuchman (97), a comparison of simulations starting with models integrated with different values of the mixing length parameter, has been made. In this paper more results are presented, including tests of the spatial resolution and Large Eddy Simulation terms used by the numerical code. The consistent value of the mixing length parameter was found to be 1.4, for a red giant of mass 1.2 solar-mass, core mass of 0.96 solar-mass, luminosity of 200 solar-luminosities, and metallicity Z=0.001.Comment: 18 pages, 1 table, 13 figures. Accepted for publication in Ap.

Effects of Uniform and Differential Rotation on Stellar Pulsations

We have investigated the effects of uniform rotation and a specific model for differential rotation on the pulsation frequencies of 10 \Msun\ stellar models. Uniform rotation decreases the frequencies for all modes. Differential rotation does not appear to have a significant effect on the frequencies, except for the most extreme differentially rotating models. In all cases, the large and small separations show the effects of rotation at lower velocities than do the individual frequencies. Unfortunately, to a certain extent, differential rotation mimics the effects o f more rapid rotation, and only the presence of some specific observed frequencies with well identified modes will be able to uniquely constrain the internal rotation of pulsating stars.Comment: 33 pages, 16 figures. Accepted for publication in Ap

Surface temperature and synthetic spectral energy distributions for rotationally deformed stars

The spectral energy distribution (SED) of a non-spherical star could differ significantly from the SED of a spherical star with the same average temperature and luminosity. Calculation of the SED of a deformed star is often approximated as a composite of several spectra, each produced by a plane parallel model of given effective temperature and gravity. The weighting of these spectra over the stellar surface, and hence the inferred effective temperature and luminosity, will be dependent on the inclination of the rotation axis of the star with respect to the observer, as well as the temperature and gravity distribution on the stellar surface. Here we calculate the surface conditions of rapidly rotating stars with a 2D stellar structure and evolution code and compare the effective temperature distribution to that predicted by von Zeipel's law. We calculate the composite spectrum for a deformed star by interpolating within a grid of intensity spectra of plane parallel model atmospheres and integrating over the surface of the star. Using this method, we find that the deduced variation of effective temperature with inclination can be as much as 3000 K for an early B star, depending on the details of the underlying model.Comment: 38 pages, 9 figures (AAStex preprint format). Accepted for publication in the Ap

Vertical abundance stratification in the blue horizontal branch star HD135485

It is commonly believed that the observed overabundances of many chemical species relative to the expected cluster metallicity in blue horizontal branch (BHB) stars appear as a result of atomic diffusion in the photosphere. The slow rotation of BHB stars (with T_eff > 11,500K), typically v sin{i} < 10 km/s, is consistent with this idea. In this work we search for observational evidence of vertical chemical stratification in the atmosphere of HD135485. If this evidence exists, it will demonstrate the importance of atomic diffusion processes in the atmospheres of BHB stars. We undertake an extensive abundance stratification analysis of the atmosphere of HD135485, based on recently acquired high resolution and S/N CFHT ESPaDOnS spectra and a McDonald-CE spectrum. Our numerical simulations show that nitrogen and sulfur reveal signatures of vertical abundance stratification in the stellar atmosphere. It appears that the abundances of these elements increase toward the upper atmosphere. This fact cannot be explained by the influence of microturbulent velocity, because oxygen, carbon, neon, argon, titanium and chromium do not show similar behavior and their abundances remain constant throughout the atmosphere. It seems that the iron abundance may increase marginally toward the lower atmosphere. This is the first demonstration of vertical abundance stratification of metals in a BHB star.Comment: 8 pages, 5 figures, accepted to A&

Presupernova Structure of Massive Stars

Issues concerning the structure and evolution of core collapse progenitor stars are discussed with an emphasis on interior evolution. We describe a program designed to investigate the transport and mixing processes associated with stellar turbulence, arguably the greatest source of uncertainty in progenitor structure, besides mass loss, at the time of core collapse. An effort to use precision observations of stellar parameters to constrain theoretical modeling is also described.Comment: Proceedings for invited talk at High Energy Density Laboratory Astrophysics conference, Caltech, March 2010. Special issue of Astrophysics and Space Science, submitted for peer review: 7 pages, 3 figure

Interaction Between Convection and Pulsation

This article reviews our current understanding of modelling convection dynamics in stars. Several semi-analytical time-dependent convection models have been proposed for pulsating one-dimensional stellar structures with different formulations for how the convective turbulent velocity field couples with the global stellar oscillations. In this review we put emphasis on two, widely used, time-dependent convection formulations for estimating pulsation properties in one-dimensional stellar models. Applications to pulsating stars are presented with results for oscillation properties, such as the effects of convection dynamics on the oscillation frequencies, or the stability of pulsation modes, in classical pulsators and in stars supporting solar-type oscillations.Comment: Invited review article for Living Reviews in Solar Physics. 88 pages, 14 figure