9,219 research outputs found
Inversion of polarimetric data from eclipsing binaries
We describe a method for determining the limb polarization and limb darkening
of stars in eclipsing binary systems, by inverting photometric and polarimetric
light curves.
Because of the ill-conditioning of the problem, we use the Backus-Gilbert
method to control the resolution and stability of the recovered solution, and
to make quantitative estimates of the maximum accuracy possible. Using this
method we confirm that the limb polarization can indeed be recovered, and
demonstrate this with simulated data, thus determining the level of
observational accuracy required to achieve a given accuracy of reconstruction.
This allows us to set out an optimal observational strategy, and to critcally
assess the claimed detection of limb polarization in the Algol system.
The use of polarization in stars has been proposed as a diagnostic tool in
microlensing surveys by Simmons et al. (1995), and we discuss the extension of
this work to the case of microlensing of extended sources.Comment: 10pp, 5 figures. To appear in A&
Probing the Atmospheres of Planets Orbiting Microlensed Stars via Polarization Variability
We present a new method to identify and probe planetary companions of stars
in the Galactic Bulge and Magellanic Clouds using gravitational microlensing.
While spectroscopic studies of these planets is well beyond current
observational techniques, monitoring polarization fluctuations during high
magnification events induced by binary microlensing events will probe the
composition of the planetary atmospheres, an observation which otherwise is
currently unattainable even for nearby planetary systems.Comment: 7 pages, 2 figures. To appear in Astrophysical Journal Letter
Polarimetric variations of binary stars. II. Numerical simulations for circular and eccentric binaries in Mie scattering envelopes
We present numerical simulations of the periodic polarimetric variations
produced by a binary star placed at the center of an empty spherical cavity
inside a circumbinary ellipsoidal and optically thin envelope made of dust
grains. Mie single-scattering is considered along with pre- and post-scattering
extinction factors which produce a time-varying optical depth and affect the
morphology of the periodic variations. We are interested in the effects that
various parameters will have on the average polarization, the amplitude of the
polarimetric variations, and the morphology of the variability. We show that
the absolute amplitudes of the variations are smaller for Mie scattering than
for Thomson scattering. Among the four grain types that we have studied, the
highest polarizations are produced by grains with sizes in the range 0.1-0.2
micron. In general, the variations are seen twice per orbit. In some cases,
because spherical dust grains have an asymmetric scattering function, the
polarimetric curves produced also show variations seen once per orbit.
Circumstellar disks produce polarimetric variations of greater amplitude than
circumbinary envelopes.
Another goal of these simulations is to see if the 1978 BME (Brown, McLean, &
Emslie, ApJ, 68, 415) formalism, which uses a Fourier analysis of the
polarimetric variations to find the orbital inclination for Thomson-scattering
envelopes, can still be used for Mie scattering. We find that this is the case,
if the amplitude of the variations is sufficient and the true inclinations is
i_true > 45 deg. For eccentric orbits, the first-order coefficients of the
Fourier fit, instead of second-order ones, can be used to find almost all
inclinations.Comment: 23 pages, 5 figures, to be published in Astronomical Journa
Placing Confidence Limits on Polarization Measurements
The determination of the true source polarization given a set of measurements
is complicated by the requirement that the polarization always be positive.
This positive bias also hinders construction of upper limits, uncertainties,
and confidence regions, especially at low signal-to-noise levels. We generate
the likelihood function for linear polarization measurements and use it to
create confidence regions and upper limits. This is accomplished by integrating
the likelihood function over the true polarization (parameter space), rather
than the measured polarization (data space). These regions are valid for both
low and high signal-to-noise measurements.Comment: 8 pages, 3 figures, 1 table, submitted to PAS
Microlensing of Extended Stellar Sources
We investigate the feasibility of reconstructing the radial intensity profile
of extended stellar sources by inverting their microlensed light curves. Using
a simple, linear, limb darkening law as an illustration, we show that the
intensity profile can be accurately determined, at least over the outer part of
the stellar disc, with realistic light curve sampling and photometric errors.
The principal requirement is that the impact parameter of the lens be less than
or equal to the stellar radius. Thus, the analysis of microlensing events
provides a powerful method for testing stellar atmosphere models.Comment: 4 pages LaTeX, to appear in New Astronomy Reviews - proceedings of
the Oxford Workshop `Gravitational Lensing: Nature's Own Weighing Scales'.
Uses elsart.cls. Paper also available at
ftp://info.astro.gla.ac.uk/pub/martin/extended.p
The optical polarization of spiral galaxies
Scattering of starlight by dust, molecules and electrons in spiral galaxies
will produce a modification of the direct intensity and a polarization in the
observed light. We treat the case where the distribution of scatterers can be
considered to be optically thin, and derive semi-analytic expressions for the
resolved intensity and polarized intensity for Thomson, Rayleigh, and more
general scattering mechanisms. These expressions are applied to a parametric
model spiral galaxies. It is further shown that in the case of Thomson and
Rayleigh scattering, and when scatterers and stars are distributed with
rotational symmetry, the total polarized flux depends on the inclination, ,
of the galactic axis to the line of sight according to a simple
law. This generalises the well known result for pointlike and spherical light
sources. By using a method based on spherical harmonics, we generalise this law
for more general mechanisms, and show that to good approximation, the law still holds for the class of models considered
Photoluminescence Detected Doublet Structure in the Integer and Fractional Quantum Hall Regime
We present here the results of polarized magneto-photoluminescence
measurements on a high mobility single-heterojunction. The presence of a
doublet structure over a large magnetic field range (2>nu>1/6) is interpreted
as possible evidence for the existence of a magneto-roton minima of the charged
density waves. This is understood as an indication of strong electronic
correlation even in the case of the IQHE limit.Comment: submitted to Solid State Communication
Detecting Stellar Spots by Gravitational Microlensing
During microlensing events with a small impact parameter, the amplification
of the source flux is sensitive to the surface brightness distribution of the
source star. Such events provide a means for studying the surface structure of
target stars in the ongoing microlensing surveys, most efficiently for giants
in the Galactic bulge. In this work we demonstrate the sensitivity of
point-mass microlensing to small spots with radii source
radii. We compute the amplification deviation from the light curve of a
spotless source and explore its dependence on lensing and spot parameters.
During source-transit events spots can cause deviations larger than 2%, and
thus be in principle detectable. Maximum relative deviation usually occurs when
the lens directly crosses the spot. Its numerical value for a dark spot with
sufficient contrast is found to be roughly equal to the fractional radius of
the spot, i.e., up to 20% in this study. Spots can also be efficiently detected
by the changes in sensitive spectral lines during the event. Notably, the
presence of a spot can mimic the effect of a low-mass companion of the lens in
some events.Comment: 18 pages with 7 Postscript figures, to appear in ApJ, January 2000;
discussion expanded, references added, minor revisions in tex
The Location of the Nucleus of NGC 1068 and the Three-dimensional Structure of Its Nuclear Region
The HST archival UV imaging polarimetry data of NGC 1068 is re-examined.
Through an extensive estimation of the observational errors, we discuss whether
the distribution of the position angles (PAs) of polarization is simply
centrosymmetric or not. Taking into account the effect of a bad focus at the
time of the observation, we conclude that, within the accuracy of HST/FOC
polarimetry, the PA distribution is completely centrosymmetric. This means that
the UV polarization originates only from scattering of the radiation from a
central point-like source.
However, our analysis shows that the most probable location of the nucleus is
only ~0.''08 (~6pc) south from the brightest cloud called ``cloud B''. The
error circle of 99% confidence level extends to cloud B and to ``cloud A''
which is about 0.''2 south of cloud B. By this FOC observation, Cloud B is only
marginally rejected as the nucleus.
Assuming that the UV flux is dominated by electron-scattered light, we have
also derived a three-dimensional structure of the nuclear region. The inferred
distribution suggests a linear structure which could be related to the radio
jet.Comment: 19 pages, 14 figures, to be published in the Astrophysical Journa
- …