309 research outputs found

### Helioseismic determination of the solar gravitational quadrupole moment

One of the most well-known tests of General Relativity (GR) results from
combining measurements of the anomalous precession of the orbit of Mercury with
a determination of the gravitational quadrupole moment of the Sun J_2. The
latter can be done by inference from an integral relation between J_2 and the
solar internal rotation. New observational data of high quality obtained from
the Solar Heliospheric Satellite (SoHO) and from the Global Oscillations
Network Group (GONG), allow the determination of the internal rotation velocity
of the Sun as a function of radius and latitude with unprecedented spatial
resolution and accuracy. As a consequence, a number of global properties of the
Sun can also be determined with much higher accuracy, notably the gravitational
quadrupole moment of the Sun. The anomalous precession of the orbit of Mercury
is primarily due to GR effects but there are classical corrections the largest
of which is that due to J_2. It is shown here that the data are currently
consistent with the predictions of GR.Comment: 5 pages, 1 figure, plain TeX uses epsf.tex, mn.tex, accepted for
MNRA

### Unbiased image reconstruction as an inverse problem

An unbiased method for improving the resolution of astronomical images is
presented. The strategy at the core of this method is to establish a linear
transformation between the recorded image and an improved image at some
desirable resolution. In order to establish this transformation only the actual
point spread function and a desired point spread function need be known. Any
image actually recorded is not used in establishing the linear transformation
between the recorded and improved image. This method has a number of advantages
over other methods currently in use. It is not iterative which means it is not
necessary to impose any criteria, objective or otherwise, to stop the
iterations. The method does not require an artificial separation of the image
into ``smooth'' and ``point-like'' components, and thus is unbiased with
respect to the character of structures present in the image. The method
produces a linear transformation between the recorded image and the deconvolved
image and therefore the propagation of pixel-by-pixel flux error estimates into
the deconvolved image is trivial. It is explicitly constrained to preserve
photometry.Comment: 11 pages, TeX, uses mn.tex epsf.tex, accepted for publication in
MNRA

### A modified R1 X R1 method for helioseismic rotation inversions

We present an efficient method for two dimensional inversions for the solar
rotation rate using the Subtractive Optimally Localized Averages (SOLA) method
and a modification of the R1 X R1 technique proposed by Sekii (1993). The SOLA
method is based on explicit construction of averaging kernels similar to the
Backus-Gilbert method. The versatility and reliability of the SOLA method in
reproducing a target form for the averaging kernel, in combination with the
idea of the R1 X R1 decomposition, results in a computationally very efficient
inversion algorithm. This is particularly important for full 2-D inversions of
helioseismic data in which the number of modes runs into at least tens of
thousands.Comment: 12 pages, Plain TeX + epsf.tex + mn.te

### Selection criteria for targets of asteroseismic campaigns

Various dedicated satellite projects are underway or in advanced stages of planning to perform high-precision, long duration time series photometry of stars, with the purpose of using the frequencies of stellar oscillations to put new constraints on the internal structure of stars. It is known (cf. Brown, et al. 1994) that the effectiveness of oscillation frequencies in constraining stellar model parameters is significantly higher if classical parameters such as effective temperature, and luminosity are known with high precision. In order to optimize asteroseismic campaigns it is therefore useful to select targets from among candidates for which good spectroscopic and astrometric data already exists. This paper presents selection criteria, as well as redeterminations of stellar luminosity and reddening for stars satisfying these criteria

### Structure of the near-surface layers of the Sun: asphericity and time variation

We present results on the structure of the near-surface layers of the Sun
obtained by inverting frequencies of high-degree solar modes from "ring
diagrams". We have results for eight epochs between June 1996 and October 2003.
The frequencies for each epoch were obtained from ring diagrams constructed
from MDI Dopplergrams spanning complete Carrington rotations. We find that
there is a substantial latitudinal variation of both sound speed and the
adiabatic index Gamma_1 in the outer 2% of the Sun. We find that both the
sound-speed and Gamma_1 profiles change with changes in the level of solar
activity. In addition, we also study differences between the northern and
southern hemispheres of the Sun and find a small asymmetry that appears to
reflect the difference in magnetic activity between the two hemispheres.Comment: To appear in ApJ (January 2007

### An optical time-delay estimate for the double gravitational lens system B1600+434

We present optical I-band light curves of the gravitationally lensed double
QSO B1600+434 from observations obtained at the Nordic Optical Telescope (NOT)
between April 1998 and November 1999. The photometry has been performed by
simultaneous deconvolution of all the data frames, involving a numerical lens
galaxy model. Four methods have been applied to determine the time delay
between the two QSO components, giving a mean estimate of \Delta_t = 51+/-4
days (95% confidence level). This is the fourth optical time delay ever
measured. Adopting a Omega=0.3, Lambda=0 Universe and using the mass model of
Maller et al. (2000), this time-delay estimate yields a Hubble parameter of
H_0=52 (+14, -8) km s^-1 Mpc^-1 (95% confidence level) where the errors include
time-delay as well as model uncertainties. There are time-dependent offsets
between the two (appropriately shifted) light curves that indicate the presence
of external variations due to microlensing.Comment: 15 pages, 4 figures, accepted for publication in Ap

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