4,304 research outputs found
Excitation of stellar p-modes by turbulent convection: 1. Theoretical formulation
Stochatic excitation of stellar oscillations by turbulent convection is
investigated and an expression for the power injected into the oscillations by
the turbulent convection of the outer layers is derived which takes into
account excitation through turbulent Reynolds stresses and turbulent entropy
fluctuations. This formulation generalizes results from previous works and is
built so as to enable investigations of various possible spatial and temporal
spectra of stellar turbulent convection. For the Reynolds stress contribution
and assuming the Kolmogorov spectrum we obtain a similar formulation than those
derived by previous authors. The entropy contribution to excitation is found to
originate from the advection of the Eulerian entropy fluctuations by the
turbulent velocity field. Numerical computations in the solar case in a
companion paper indicate that the entropy source term is dominant over Reynold
stress contribution to mode excitation, except at high frequencies.Comment: 14 pages, accepted for publication in A&
Porto Oscillation Code (POSC)
The Porto Oscillation Code (POSC) has been developed in 1995 and improved
over the years, with the main goal of calculating linear adiabatic oscillations
for models of solar-type stars. It has also been used to estimate the
frequencies and eigenfunctions of stars from the pre-main sequence up to the
sub-giant phase, having a mass between 0.8 and 4 solar masses.
The code solves the linearised perturbation equations of adiabatic pulsations
for an equilibrium model using a second order numerical integration method. The
possibility of using Richardson extrapolation is implemented. Several options
for the surface boundary condition can be used. In this work we briefly review
the key ingredients of the calculations, namely the equations, the numerical
scheme and the output.Comment: Accepted for publication in Astrophysics and Space Science
On the transfer of resonant-line radiation in mesh simulations
The last decade has seen applications of Adaptive Mesh Refinement (AMR)
methods for a wide range of problems from space physics to cosmology. With the
advent of these methods, in which space is discretized into a mesh of many
individual cubic elements, the contemporary analog of the extensively studied
line radiative transfer (RT) in a semi-infinite slab is that of RT in a cube.
In this study we provide an approximate solution of the RT equation, as well as
analytic expressions for the probability distribution functions (pdfs) of the
properties of photons emerging from a cube, and compare them with the
corresponding slab problem. These pdfs can be used to perform fast
resonant-line RT in optically thick AMR cells where, otherwise, it could take
unrealistically long times to transfer even a handful of photons.Comment: 5 two-column pages, 2 figures; matches accepted version, to appear on
Ap
Improved Fokker-Planck Equation for Resonance Line Scattering
A new Fokker-Planck equation is developed for treating resonance line
scattering, especially relevant to the treatment of Lyman alpha in the early
universe. It is a "corrected" form of the equation of Rybicki & Dell'Antonio
that now obeys detailed balance, so that the approach to thermal equilibrium is
properly described. The new equation takes into account the energy changes due
to scattering off moving particles, the recoil term of Basko, and stimulated
scattering. One result is a surprising unification of the equation for
resonance line scattering and the Kompaneets equation. An improved energy
exchange formula due to resonance line scattering is derived. This formula is
compared to previous formulas of Madau, Meikson, & Rees (1997) and Chen &
Miralda-Escud\'e (2004).Comment: 10 pages, no figures, submitted to Ap
The chaotic behavior of the black hole system GRS 1915+105
A modified non-linear time series analysis technique, which computes the
correlation dimension , is used to analyze the X-ray light curves of the
black hole system GRS 1915+105 in all twelve temporal classes. For four of
these temporal classes saturates to which indicates that
the underlying dynamical mechanism is a low dimensional chaotic system. Of the
other eight classes, three show stochastic behavior while five show deviation
from randomness. The light curves for four classes which depict chaotic
behavior have the smallest ratio of the expected Poisson noise to the
variability () while those for the three classes which depict
stochastic behavior is the highest (). This suggests that the temporal
behavior of the black hole system is governed by a low dimensional chaotic
system, whose nature is detectable only when the Poisson fluctuations are much
smaller than the variability.Comment: Accepted for publication in Astrophysical Journa
A GPU-Computing Approach to Solar Stokes Profile Inversion
We present a new computational approach to the inversion of solar
photospheric Stokes polarization profiles, under the Milne-Eddington model, for
vector magnetography. Our code, named GENESIS (GENEtic Stokes Inversion
Strategy), employs multi-threaded parallel-processing techniques to harness the
computing power of graphics processing units GPUs, along with algorithms
designed to exploit the inherent parallelism of the Stokes inversion problem.
Using a genetic algorithm (GA) engineered specifically for use with a GPU, we
produce full-disc maps of the photospheric vector magnetic field from polarized
spectral line observations recorded by the Synoptic Optical Long-term
Investigations of the Sun (SOLIS) Vector Spectromagnetograph (VSM) instrument.
We show the advantages of pairing a population-parallel genetic algorithm with
data-parallel GPU-computing techniques, and present an overview of the Stokes
inversion problem, including a description of our adaptation to the
GPU-computing paradigm. Full-disc vector magnetograms derived by this method
are shown, using SOLIS/VSM data observed on 2008 March 28 at 15:45 UT
On the possible existence of short-period g-mode instabilities powered by nuclear burning shells in post-AGB H-deficient (PG1159-type) stars
We present a pulsational stability analysis of hot post-AGB H-deficient
pre-white dwarf stars with active He-burning shells. The stellar models
employed are state-of-the-art equilibrium structures representative of PG1159
stars derived from the complete evolution of the progenitor stars. On the basis
of fully nonadiabatic pulsation computations, we confirmed theoretical evidence
for the existence of a separate PG1159 instability strip in the diagram characterized by short-period -modes excited by the
-mechanism. This instability strip partially overlaps the already
known GW Vir instability strip of intermediate/long period -modes
destabilized by the classical -mechanism acting on the partial
ionization of C and/or O in the envelope of PG1159 stars. We found that PG1159
stars characterized by thick He-rich envelopes and located inside this
overlapping region could exhibit both short and intermediate/long periods
simultaneously. we study the particular case of VV 47, a pulsating planetary
nebula nucleus that has been reported to exhibit a series of unusually short
pulsation periods. We found that the long periods exhibited by VV 47 can be
readily explained by the classical -mechanism, while the observed
short-period branch below s could correspond to modes triggered
by the He-burning shell through the -mechanism, although more
observational work is needed to confirm the reality of these short-period
modes. Were the existence of short-period -modes in this star convincingly
confirmed by future observations, VV 47 could be the first known pulsating star
in which both the -mechanism and the -mechanism of mode
driving are simultaneously operating.Comment: 9 pages, 5 figures, 2 tables. To be published in The Astrophysical
Journa
Radiative Transfer Effects in He I Emission Lines
We consider the effect of optical depth of the 2 ^{3}S level on the nebular
recombination spectrum of He I for a spherically symmetric nebula with no
systematic velocity gradients. These calculations, using many improvements in
atomic data, can be used in place of the earlier calculations of Robbins. We
give representative Case B line fluxes for UV, optical, and IR emission lines
over a range of physical conditions: T=5000-20000 K, n_{e}=1-10^{8} cm^{-3},
and tau_{3889}=0-100. A FORTRAN program for calculating emissivities for all
lines arising from quantum levels with n < 11 is also available from the
authors.
We present a special set of fitting formulae for the physical conditions
relevant to low metallicity extragalactic H II regions: T=12,000-20,000 K,
n_{e}=1-300 cm^{-3}, and tau_{3889} < 2.0. For this range of physical
conditions, the Case B line fluxes of the bright optical lines 4471 A, 5876 A,
and 6678 A, are changed less than 1%, in agreement with previous studies.
However, the 7065 A corrections are much smaller than those calculated by
Izotov & Thuan based on the earlier calculations by Robbins. This means that
the 7065 A line is a better density diagnostic than previously thought. Two
corrections to the fitting functions calculated in our previous work are also
given.Comment: To be published in 10 April 2002 ApJ; relevant code available at
ftp://wisp.physics.wisc.edu/pub/benjamin/Heliu
A Five-year Spectroscopic and Photometric Campaign on the Prototypical alpha Cygni Variable and A-type Supergiant Star Deneb
Deneb is often considered the prototypical A-type supergiant, and is one of
the visually most luminous stars in the Galaxy. A-type supergiants are
potential extragalactic distance indicators, but the variability of these stars
needs to be better characterized before this technique can be considered
reliable. We analyzed 339 high resolution echelle spectra of Deneb obtained
over the five-year span of 1997 through 2001 as well as 370 Stromgren
photometric measurements obtained during the same time frame. Our spectroscopic
analysis included dynamical spectra of the H-alpha profile, H-alpha equivalent
widths, and radial velocities measured from Si II 6347, 6371. Time-series
analysis reveals no obvious cyclic behavior that proceeds through multiple
observing seasons, although we found a suspected 40 day period in two,
non-consecutive observing seasons. Some correlations are found between
photometric and radial velocity data sets, and suggest radial pulsations at two
epochs. No correlation is found between the variability of the H-alpha profiles
and that of the radial velocities or the photometry. Lucy (1976) found evidence
that Deneb was a long period single-lined spectroscopic binary star, but our
data set shows no evidence for radial velocity variations caused by a binary
companion.Comment: 49 pages, 9 figures, 5 tables, accepted for publication in the
Astronomical Journa
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