76 research outputs found

### 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

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### 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

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