327 research outputs found
Simulations of core convection in rotating A-type stars: Differential rotation and overshooting
We present the results of 3--D simulations of core convection within A-type
stars of 2 solar masses, at a range of rotation rates. We consider the inner
30% by radius of such stars, thereby encompassing the convective core and some
of the surrounding radiative envelope. We utilize our anelastic spherical
harmonic (ASH) code, which solves the compressible Navier-Stokes equations in
the anelastic approximation, to examine highly nonlinear flows that can span
multiple scale heights. The cores of these stars are found to rotate
differentially, with central cylindrical regions of strikingly slow rotation
achieved in our simulations of stars whose convective Rossby number (R_{oc}) is
less than unity. Such differential rotation results from the redistribution of
angular momentum by the nonlinear convection that strongly senses the overall
rotation of the star. Penetrative convective motions extend into the overlying
radiative zone, yielding a prolate shape (aligned with the rotation axis) to
the central region in which nearly adiabatic stratification is achieved. This
is further surrounded by a region of overshooting motions, the extent of which
is greater at the equator than at the poles, yielding an overall spherical
shape to the domain experiencing at least some convective mixing. We assess the
overshooting achieved as the stability of the radiative exterior is varied, and
the weak circulations that result in that exterior. The convective plumes serve
to excite gravity waves in the radiative envelope, ranging from localized
ripples of many scales to some remarkable global resonances.Comment: 48 pages, 16 figures, some color. Accepted to Astrophys. J. Color
figures compressed with appreciable loss of quality; a PDF of the paper with
better figures is available at
http://lcd-www.colorado.edu/~brownim/core_convectsep24.pd
Line asymmetry of solar p-modes: Reversal of asymmetry in intensity power spectra
The sense of line asymmetry of solar p-modes in the intensity power spectra
is observed to be opposite of that seen in the velocity power spectra.
Theoretical calculations provide a good understanding and fit to the observed
velocity power spectra whereas the reverse sense of asymmetry in the intensity
power spectrum has been poorly understood. We show that when turbulent eddies
arrive at the top of the convection zone they give rise to an observable
intensity fluctuation which is correlated with the oscillation they generate,
thereby affecting the shape of the line in the p-mode power spectra and
reversing the sense of asymmetry (this point was recognized by Nigam et al. and
Roxburgh & Vorontsov). The addition of the correlated noise displaces the
frequencies of peaks in the power spectrum. Depending on the amplitude of the
noise source the shift in the position of the peak can be substantially larger
than the frequency shift in the velocity power spectra. In neither case are the
peak frequencies precisely equal to the eigenfrequencies of p-modes. We suggest
two observations which can provide a test of the model discussed here.Comment: Revised version. To appear in Ap
Response of a spaceborne gravitational wave antenna to solar oscillations
We investigate the possibility of observing very small amplitude low
frequency solar oscillations with the proposed laser interferometer space
antenna (LISA). For frequencies below the
dominant contribution is from the near zone time dependent gravitational
quadrupole moments associated with the normal modes of oscillation. For
frequencies above the dominant contribution
is from gravitational radiation generated by the quadrupole oscillations which
is larger than the Newtonian signal by a factor of the order , where is the distance to the Sun, and is the velocity of light.
The low order solar quadrupole pressure and gravity oscillation modes have
not yet been detected above the solar background by helioseismic velocity and
intensity measurements. We show that for frequencies , the signal due to solar oscillations will have a higher
signal to noise ratio in a LISA type space interferometer than in
helioseismology measurements. Our estimates of the amplitudes needed to give a
detectable signal on a LISA type space laser interferometer imply surface
velocity amplitudes on the sun of the order of 1-10 mm/sec in the frequency
range . If such modes exist with
frequencies and amplitudes in this range they could be detected with a LISA
type laser interferometer.Comment: 16 pages, 6 figures, 1 table. A reworked and considerably improved
version of ArXiv:astro-ph/0103472, Published in PR
Open issues in probing interiors of solar-like oscillating main sequence stars: 2. Diversity in the HR diagram
We review some major open issues in the current modelling of low and
intermediate mass, main sequence stars based on seismological studies. The
solar case was discussed in a companion paper, here several issues specific to
other stars than the Sun are illustrated with a few stars observed with CoRoT
and expectations from Kepler data.Comment: GONG 2010 - SoHO 24, A new era of seismology of the Sun and
solar-like stars, To be published in the Journal of Physics: Conference
Series (JPCS
Peaks and Troughs in Helioseismology: The Power Spectrum of Solar Oscillations
I present a matched-wave asymptotic analysis of the driving of solar
oscillations by a general localised source. The analysis provides a simple
mathematical description of the asymmetric peaks in the power spectrum in terms
of the relative locations of eigenmodes and troughs in the spectral response.
It is suggested that the difference in measured phase function between the
modes and the troughs in the spectrum will provide a key diagnostic of the
source of the oscillations. I also suggest a form for the asymmetric line
profiles to be used in the fitting of solar power spectra.
Finally I present a comparison between the numerical and asymptotic
descriptions of the oscillations. The numerical results bear out the
qualitative features suggested by the asymptotic analysis but suggest that
numerical calculations of the locations of the troughs will be necessary for a
quantitative comparison with the observations.Comment: 18 pages + 8 separate figures. To appear in Ap
What Causes P-mode Asymmetry Reversal?
The solar acoustic p-mode line profiles are asymmetric. Velocity spectra have
more power on the low-frequency sides, whereas intensity profiles show the
opposite sense of asymmetry. Numerical simulations of the upper convection zone
have resonant p-modes with the same asymmetries and asymmetry reversal as the
observed modes. The temperature and velocity power spectra at optical depth
have the opposite asymmetry as is observed for the
intensity and velocity spectra. At a fixed geometrical depth, corresponding to
, however, the temperature and velocity spectra have the
same asymmetry. This indicates that the asymmetry reversal is produced by
radiative transfer effects and not by correlated noise.Comment: 16 pages, 10 figures, submitted to Astrophysical Journa
Empirical Constraints on Convective Core Overshoot
In stellar evolution calculations, the local pressure scale height is often
used to empirically constrain the amount of convective core overshoot. However,
this method brings unsatisfactory results for low-mass stars (< 1.1 -1.2 Mo for
Z= \Zs) which have very small cores or no convective core at all. Following
Roxburgh's integral constraint,we implemented an upper limit of overshoot
within the conventional method of alpha parameterization in order to remove an
overly large overshoot effect on low-mass stars. The erroneously large effect
of core overshoot due to the failure of alpha parameterization can be
effectively corrected by limiting the amount of overshoot to < 15 % of the core
radius. 15 % of the core radius would be a proper limit of overshoot which can
be implemented in a stellar evolution code for intermediate to low mass stars.
The temperature structure of the overshoot region does not play a crucial role
in stellar evolution since this transition region is very thin.Comment: 15 pages, 5 figures, accepted for A
Global asteroseismic properties of solar-like oscillations observed by Kepler : A comparison of complementary analysis methods
We present the asteroseismic analysis of 1948 F-, G- and K-type main-sequence
and subgiant stars observed by the NASA {\em Kepler Mission}. We detect and
characterise solar-like oscillations in 642 of these stars. This represents the
largest cohort of main-sequence and subgiant solar-like oscillators observed to
date. The photometric observations are analysed using the methods developed by
nine independent research teams. The results are combined to validate the
determined global asteroseismic parameters and calculate the relative precision
by which the parameters can be obtained. We correlate the relative number of
detected solar-like oscillators with stellar parameters from the {\em Kepler
Input Catalog} and find a deficiency for stars with effective temperatures in
the range \,K and a drop-off in
detected oscillations in stars approaching the red edge of the classical
instability strip. We compare the power-law relationships between the frequency
of peak power, , the mean large frequency separation,
, and the maximum mode amplitude, , and show that
there are significant method-dependent differences in the results obtained.
This illustrates the need for multiple complementary analysis methods to be
used to assess the robustness and reproducibility of results derived from
global asteroseismic parameters.Comment: 14 pages, 9 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
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