18,670 research outputs found
The discrepancy between solar abundances and helioseismology
There have been recent downward revisions of the solar photospheric
abundances of Oxygen and other heavy elements. These revised abundances along
with OPAL opacities are not consistent with seismic constraints. In this work
we show that the recently released OP opacity tables cannot resolve this
discrepancy either. While the revision in opacities does not seem to resolve
this conflict, an upward revision of Neon abundance in solar photosphere offers
a possible solution to this problem.Comment: To appear in ApJ Letter
Does solar structure vary with solar magnetic activity?
We present evidence that solar structure changes with changes in solar
activity. We find that the adiabatic index, Gamma_1, changes near the second
helium ionization, i.e., at a depth of about 0.98 R_sun. We believe that this
change is a result of the change in the effective equation of state caused by
magnetic fields. Inversions should be able to detect the changes in Gamma_1 if
mode sets with reliable and precise high-degree modes are available.Comment: To appear in ApJ Letter
Temporal variations of the rotation rate in the solar interior
The temporal variations of the rotation rate in the solar interior are
studied using frequency splittings from Global Oscillations Network Group
(GONG) data obtained during the period 1995-99. We find alternating latitudinal
bands of faster and slower rotation which appear to move towards the equator
with time - similar to the torsional oscillations seen at the solar surface.
This flow pattern appears to persist to a depth of about 0.1R_sun and in this
region its magnitude is well correlated with solar activity indices. We do not
find any periodic or systematic changes in the rotation rate near the base of
the convection zone.Comment: To appear in Ap
Determining solar abundances using helioseismology
The recent downward revision of solar photospheric abundances of Oxygen and
other heavy elements has resulted in serious discrepancies between solar models
and solar structure as determined through helioseismology. In this work we
investigate the possibility of determining the solar heavy-element abundance
without reference to spectroscopy by using helioseismic data. Using the
dimensionless sound-speed derivative in the solar convection zone, we find that
the heavy element abundance, Z, of 0.0172 +/- 0.002, which is closer to the
older, higher value of the abundances.Comment: To appear in Ap
GRB011211: An alternative interpretation of the optical and X-ray spectra in terms of blueshifts
The redshifts of the gamma ray burst (GRB) GRB 011211 has been determined as
2.14 from several absorption lines seen in the spectrum of its optical
afterglow. The spectrum of its X-ray afterglow exhibited several emission
lines,and their identification led to a mean redshift 1.862. A supernova model
has been proposed based on the redshift of the GRB as 2.141. It is shown here
that the redshift interpretation cannot explain the observed spectra, as some
serious inconsistencies exist in the process of redshift determinations in
spectra of both optical and X-ray afterglows. In view of that, an alternative
interpretation of the spectra is presented in terms of blueshifts. Ejection
mechanism is proposed as a possible scenario to explain the blueshifted
spectrum.Comment: 26 pages, one table; in Canadian Journal of Physics, June 200
Solar Models with Revised Abundances and Opacities
Using reconstructed opacities, we construct solar models with low
heavy-element abundance. Rotational mixing and enhanced diffusion of helium and
heavy elements are used to reconcile the recently observed abundances with
helioseismology. The sound speed and density of models where the relative and
absolute diffusion coefficients for helium and heavy elements have been
increased agree with seismically inferred values at better than the 0.005 and
0.02 fractional level respectively. However, the surface helium abundance of
the enhanced diffusion model is too low. The low helium problem in the enhanced
diffusion model can be solved to a great extent by rotational mixing. The
surface helium and the convection zone depth of rotating model M04R3, which has
a surface Z of 0.0154, agree with the seismic results at the levels of 1
and 3 respectively. M04R3 is almost as good as the standard
model M98. Some discrepancies between the models constructed in accord with the
new element abundances and seismic constraints can be solved individually, but
it seems difficult to resolve them as a whole scenario.Comment: 10 pages, 1 figur
Solar internal rotation rate and the latitudinal variation of the tachocline
A new set of accurately measured frequencies of solar oscillations are used
to infer the rotation rate inside the Sun, as a function of radial distance as
well as latitude. We have adopted a regularized least squares technique with
iterative refinement for both 1.5D inversion using the splitting coefficients
and 2D inversion using individual m splittings. The inferred rotation rate
agrees well with earlier estimates showing a shear layer just below the surface
and another one around the base of the convection zone. The tachocline or the
transition layer where the rotation rate changes from differential rotation in
the convection zone to almost latitudinally independent rotation rate in the
radiative interior is studied in detail. No compelling evidence for any
latitudinal variation in position and width of tachocline is found though it
appears that the tachocline probably shifts to slightly larger radial distance
at higher latitudes and possibly becomes thicker also. However, these
variations are within the estimated errors and more accurate data would be
needed to make a definitive statement about latitudinal variations.Comment: 15 pages, MNRAS-TeX, 15 figure
The structure of the infinite models in integer programming
The infinite models in integer programming can be described as the convex
hull of some points or as the intersection of halfspaces derived from valid
functions. In this paper we study the relationships between these two
descriptions. Our results have implications for corner polyhedra. One
consequence is that nonnegative, continuous valid functions suffice to describe
corner polyhedra (with or without rational data)
Ring diagram analysis of near-surface flows in the Sun
Ring diagram analysis of solar oscillation power spectra obtained from MDI
data is carried out to study the velocity fields in the outer part of the solar
convection zone. The three dimensional power spectra are fitted to a model
which has a Lorentzian profile in frequency and which includes the advection of
the wave front by horizontal flows, to obtain the two components of the
sub-surface flows as a function of the horizontal wave number and radial order
of the oscillation modes. This information is then inverted using OLA and RLS
methods to infer the variation in horizontal flow velocity with depth. The
average rotation velocity at different latitudes obtained by this technique
agrees reasonably with helioseismic estimates made using frequency splitting
data. The shear layer just below the solar surface appears to consist of two
parts with the outer part up to a depth of 4 Mm, where the velocity gradient
does not show any reversal up to a latitude of 60 degrees. In the deeper part
the velocity gradient shows reversal in sign around a latitude of 55 degrees.
The zonal flow velocities inferred in the outermost layers appears to be
similar to those obtained by other measurements. A meridional flow from equator
polewards is found. It has a maximum amplitude of about 30 m/s near the surface
and the amplitude is nearly constant in the outer shear layer.Comment: aastex, 12 figures, to appear in Ap.
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