6,499 research outputs found
Modeling Convective Core Overshoot and Diffusion in Procyon Constrained by Asteroseismic Data
We compare evolved stellar models, which match Procyons mass and position in
the HR diagram, to current ground-based asteroseismic observations. Diffusion
of helium and metals along with two conventional core overshoot descriptions
and the Kuhfuss nonlocal theory of convection are considered. We establish that
one of the two published asteroseismic data reductions for Procyon, which
mainly differ in their identification of even versus odd l-values, is a
significantly more probable and self-consistent match to our models than the
other. The most probable models according to our Bayesian analysis have evolved
to just short of turnoff, still retaining a hydrogen convective core. Our most
probable models include Y and Z diffusion and have conventional core overshoot
between 0.9 and 1.5 pressure scale heights, which increases the outer radius of
the convective core by between 22% to 28%, respectively. We discuss the
significance of this comparatively higher than expected core overshoot amount
in terms of internal mixing during evolution. The parameters of our most
probable models are similar regardless of whether adiabatic or nonadiabatic
model p-mode frequencies are compared to the observations, although, the
Bayesian probabilities are greater when the nonadiabatic model frequencies are
used. All the most probable models (with or without core overshoot, adiabatic
or nonadiabatic model frequencies, diffusion or no diffusion, including priors
for the observed HRD location and mass or not) have masses that are within one
sigma of the observed mass 1.497+/-0.037 Msun
The Pulsation Properties of Procyon A
A grid of stellar evolution models for Procyon A has been calculated. These
models include the best physics available to us (including the latest opacities
and equation of state) and are based on the revised astrometric mass of Girard
et al (1996). Models were calculated with helium diffusion and with the
combined effects of helium and heavy element diffusion. Oscillation frequencies
for l=0,1,2 and 3 p-modes and the characteristic period spacing for the g-modes
were calculated for these models. We find that g-modes are sensitive to model
parameters which effect the structure of the core, such as convective core
overshoot, the heavy element abundance and the evolutionary state (main
sequence or shell hydrogen burning) of Procyon A. The p-modes are relatively
insensitive to the details of the physics used to model Procyon A, and only
depend on the evolutionary state of Procyon A. Hence, observations of p-mode
frequencies on Procyon A will serve as a robust test of stellar evolution
models.Comment: 4 pages, to appear in ApJ
A general method to determine the stability of compressible flows
Several problems were studied using two completely different approaches. The initial method was to use the standard linearized perturbation theory by finding the value of the individual small disturbance quantities based on the equations of motion. These were serially eliminated from the equations of motion to derive a single equation that governs the stability of fluid dynamic system. These equations could not be reduced unless the steady state variable depends only on one coordinate. The stability equation based on one dependent variable was found and was examined to determine the stability of a compressible swirling jet. The second method applied a Lagrangian approach to the problem. Since the equations developed were based on different assumptions, the condition of stability was compared only for the Rayleigh problem of a swirling flow, both examples reduce to the Rayleigh criterion. This technique allows including the viscous shear terms which is not possible in the first method. The same problem was then examined to see what effect shear has on stability
The nature of p-modes and granulation in HD 49933 observed by CoRoT
Context: Recent observations of HD49933 by the space-photometric mission
CoRoT provide photometric evidence of solar type oscillations in a star other
than our Sun. The first published reduction, analysis, and interpretation of
the CoRoT data yielded a spectrum of p-modes with l = 0, 1, and 2. Aims: We
present our own analysis of the CoRoT data in an attempt to compare the
detected pulsation modes with eigenfrequencies of models that are consistent
with the observed luminosity and surface temperature. Methods: We used the
Gruberbauer et al. frequency set derived based on a more conservative Bayesian
analysis with ignorance priors and fit models from a dense grid of model
spectra. We also introduce a Bayesian approach to searching and quantifying the
best model fits to the observed oscillation spectra. Results: We identify 26
frequencies as radial and dipolar modes. Our best fitting model has solar
composition and coincides within the error box with the spectroscopically
determined position of HD49933 in the H-R diagram. We also show that
lower-than-solar Z models have a lower probability of matching the observations
than the solar metallicity models. To quantify the effect of the deficiencies
in modeling the stellar surface layers in our analysis, we compare adiabatic
and nonadiabatic model fits and find that the latter reproduces the observed
frequencies better.Comment: accepted to be published in A&A, 9 pages, 5 figure
A technique for determining daytime atmospheric oxide above 50 km from backscattered ultraviolet measurements
Airglow from gamma band resonance fluorescence of nitric oxide near 255 nm is calculated at several solar zenith angles. Data from the Nimbus 4 BUV wavelengths 273.5 to 287.6 nm is used to estimate the Rayleigh and ozone scattering contributions to the BUV 255.5 nm data and the remaining signal is attributed to NO airglow. The low solar zenith angle contributions by NO is less than 0.5%, and the high latitude/high zenith angle contribution exceeds 5%. This technique allows for estimating NO content above 50 km, as well as partitioning that content between the mesosphere and thermosphere
On the detection of Lorentzian profiles in a power spectrum: A Bayesian approach using ignorance priors
Aims. Deriving accurate frequencies, amplitudes, and mode lifetimes from
stochastically driven pulsation is challenging, more so, if one demands that
realistic error estimates be given for all model fitting parameters. As has
been shown by other authors, the traditional method of fitting Lorentzian
profiles to the power spectrum of time-resolved photometric or spectroscopic
data via the Maximum Likelihood Estimation (MLE) procedure delivers good
approximations for these quantities. We, however, show that a conservative
Bayesian approach allows one to treat the detection of modes with minimal
assumptions (i.e., about the existence and identity of the modes).
Methods. We derive a conservative Bayesian treatment for the probability of
Lorentzian profiles being present in a power spectrum and describe an efficient
implementation that evaluates the probability density distribution of
parameters by using a Markov-Chain Monte Carlo (MCMC) technique.
Results. Potentially superior to "best-fit" procedure like MLE, which only
provides formal uncertainties, our method samples and approximates the actual
probability distributions for all parameters involved. Moreover, it avoids
shortcomings that make the MLE treatment susceptible to the built-in
assumptions of a model that is fitted to the data. This is especially relevant
when analyzing solar-type pulsation in stars other than the Sun where the
observations are of lower quality and can be over-interpreted. As an example,
we apply our technique to CoRoT observations of the solar-type pulsator HD
49933.Comment: 12 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
An evaluation of aft-end ignition for solid propellant rocket motors
Performance evaluation of solid propellant rocket motor ignition to determine igniter design and parameters to avoid overpressurizatio
The Absurd Pandemonium Surrounding SPACs: An Argument in Favor of Legislative and Judicial Restraint
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