24,131 research outputs found
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
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
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
Real root finding for equivariant semi-algebraic systems
Let be a real closed field. We consider basic semi-algebraic sets defined
by -variate equations/inequalities of symmetric polynomials and an
equivariant family of polynomials, all of them of degree bounded by .
Such a semi-algebraic set is invariant by the action of the symmetric group. We
show that such a set is either empty or it contains a point with at most
distinct coordinates. Combining this geometric result with efficient algorithms
for real root finding (based on the critical point method), one can decide the
emptiness of basic semi-algebraic sets defined by polynomials of degree
in time . This improves the state-of-the-art which is exponential
in . When the variables are quantified and the
coefficients of the input system depend on parameters , one
also demonstrates that the corresponding one-block quantifier elimination
problem can be solved in time
Self-regulated gravitational accretion in protostellar discs
We present a numerical model for the evolution of a protostellar disc that
has formed self-consistently from the collapse of a molecular cloud core. The
global evolution of the disc is followed for several million years after its
formation. The capture of a wide range of spatial and temporal scales is made
possible by use of the thin-disc approximation. We focus on the role of
gravitational torques in transporting mass inward and angular momentum outward
during different evolutionary phases of a protostellar disc with disc-to-star
mass ratio of order 0.1. In the early phase, when the infall of matter from the
surrounding envelope is substantial, mass is transported inward by the
gravitational torques from spiral arms that are a manifestation of the
envelope-induced gravitational instability in the disc. In the late phase, when
the gas reservoir of the envelope is depleted, the distinct spiral structure is
replaced by ongoing irregular nonaxisymmetric density perturbations. The
amplitude of these density perturbations decreases with time, though this
process is moderated by swing amplification aided by the existence of the
disc's sharp outer edge. Our global modelling of the protostellar disc reveals
that there is typically a residual nonzero gravitational torque from these
density perturbations, i.e. their effects do not exactly cancel out in each
region. In particular, the net gravitational torque in the inner disc tends to
be negative during first several million years of the evolution, while the
outer disc has a net positive gravitational torque. Our global model of a
self-consistently formed disc shows that it is also self-regulated in the late
phase, so that it is near the Toomre stability limit, with a near-uniform
Toomre parameter Q\approx 1.5-2.0. (Abstract abridged).Comment: 9 pages, 9 figures, accepted for publication in MNRA
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