59,588 research outputs found
The binary gravitational lens and its extreme cases
The transition of the binary gravitational lens from the equal mass case to
small (planetary) mass ratios q is studied. It is shown how the limit of a
(pure shear) Chang-Refsdal lens is approached, under what conditions the
Chang-Refsdal approximation is valid, and how the 3 different topologies of the
critical curves and caustics for a binary lens are mapped onto the 2 different
topologies for a Chang-Refsdal lens with pure shear. It is shown that for wide
binaries, the lensing in the vicinity of both lens objects can be described by
a Taylor-expansion of the deflection term due to the other object, where the
Chang-Refsdal approximation corresponds to a truncation of this series. For
close binaries, only the vicinity of the secondary, less massive, object can be
described in this way. However, for image distances much larger than the
separation of the lens objects, any binary lens can be approximated by means of
multipole expansion, where the first non-trivial term is the quadrupole term.
It is shown that an ambiguity exists between wide and close binary lenses,
where the shear at one of the objects due to the other object for the wide
binary is equal to the absolute value of the eigenvalues of the quadrupole
moment for the close binary. This analysis provides the basis for a
classification of binary lens microlensing events, especially of planetary
events, and an understanding of present ambiguities.Comment: 20 pages in LaTeX2e format with 9 embedded PostScript figures;
figures modified and embedded; accepted for publication in A&
Speckle observations of binary stars with a 0.5 m telescope
We present 36 observations of 17 visual binaries of moderate separation
(range from 0.15'' to 0.79'') made with the 50 cm Cassegrain telescope of the
Jagiellonian University in Cracow. The speckle interferometry technique was
combined with modest optical hardware and a standard photometric CCD camera. We
used broad-band V,R,I filters without a Risley prism to reduce differential
colour refraction, so we performed model analysis to investigate the influence
of this effect on the results of measurements. For binary components of
spectral type O-F, the difference of three spectral classes between them should
bias their relative positions by no more than a couple of tens of
milliarcseconds (mas) for moderate zenith distances. The statistical analysis
of our results confirmed this conclusion. A cross-spectrum approach was applied
to resolve the quadrant ambiguity. Our separations have RMS deviations of
0.012'' and our position angles have RMS deviations of 1.8 deg. Relative
photometry in V, R and I filters appeared to be the less accurately determined
parameter. We discuss our errors in detail and compare them to other speckle
data. This comparison clearly shows the high value of our measurements. We also
present an example of the enhancement of image resolution for an extended
object of angular size greater than the atmospheric coherence patch using
speckle interferometry techniques.Comment: Submitted to Astronomy & Astrophysic
Astrometric and Photometric Measurements of Binary Stars with Adaptive Optics: Observations from 2002
The adaptive optics system at the 3.6 m AEOS telescope was used to measure
the astrometry and differential magnitude in I-band of 56 binary stars in 2002.
The astrometric measurements will be of use for future orbital determination,
and the photometric measurements will be of use in estimating the spectral
types of the component stars. Two candidate companions were detected, but
neither is likely to be gravitationally bound. Nine systems had not been
observed in over 40 years. Eight of these are shown to share common proper
motion, while HD 182352 is shown to be a background star. One of the two
components of the HD 114378 (Alpha Com) is shown to be a variable star of
unknown type. In addition, 86 stars were unresolved and the full-width half
maxima of the images are presented.Comment: 9 pages, 1 figure, 3 Table
Coplanar Circumbinary Debris Disks
We present resolved Herschel images of circumbinary debris disks in the alpha
CrB (HD139006) and beta Tri (HD13161) systems. We find that both disks are
consistent with being aligned with the binary orbital planes. Though secular
perturbations from the binary can align the disk, in both cases the alignment
time at the distances at which the disk is resolved is greater than the stellar
age, so we conclude that the coplanarity was primordial. Neither disk can be
modelled as a narrow ring, requiring extended radial distributions. To satisfy
both the Herschel and mid-IR images of the alpha CrB disk, we construct a model
that extends from 1-300AU, whose radial profile is broadly consistent with a
picture where planetesimal collisions are excited by secular perturbations from
the binary. However, this model is also consistent with stirring by other
mechanisms, such as the formation of Pluto-sized objects. The beta Tri disk
model extends from 50-400AU. A model with depleted (rather than empty) inner
regions also reproduces the observations and is consistent with binary and
other stirring mechanisms. As part of the modelling process, we find that the
Herschel PACS beam varies by as much as 10% at 70um and a few % at 100um. The
70um variation can therefore hinder image interpretation, particularly for
poorly resolved objects. The number of systems in which circumbinary debris
disk orientations have been compared with the binary plane is now four. More
systems are needed, but a picture in which disks around very close binaries
(alpha CrB, beta Tri, and HD 98800, with periods of a few weeks to a year) are
aligned, and disks around wider binaries (99 Her, with a 50 yr period) are
misaligned, may be emerging. This picture is qualitatively consistent with the
expectation that the protoplanetary disks from which the debris emerged are
more likely to be aligned if their binaries have shorter periods.Comment: accepted to MNRA
Hubble Space Telescope High Resolution Imaging of Kepler Small and Cool Exoplanet Host Stars
High resolution imaging is an important tool for follow-up study of exoplanet
candidates found via transit detection with the Kepler Mission. We discuss here
HST imaging with the WFC3 of 23 stars that host particularly interesting Kepler
planet candidates based on their small size and cool equilibrium temperature
estimates. Results include detections, exclusion of background stars that could
be a source of false positives for the transits, and detection of
physically-associated companions in a number of cases providing dilution
measures necessary for planet parameter refinement. For six KOIs, we find that
there is ambiguity in which star hosts the transiting planet(s), with
potentially strong implications for planetary characteristics. Our sample is
evenly distributed in G, K, and M spectral types. Albeit with a small sample
size, we find that physically-associated binaries are more common than expected
at each spectral type, reaching a factor of 10 frequency excess at M. We
document the program detection sensitivities, detections, and deliverables to
the Kepler follow-up program archive.Comment: Accepted for the Astronomical Journal; 13 pages with 9 figure
Visual Pleasure and Racial Ambiguity
I struggle to present work that reflects a psychological expressivity which at the same time conveys intellectual concepts that are of concern to me. It seems that the fluidity of an image can communicate a certain pathos, and correspond to the fluid nature of one’s identity. Drippy paint, distorted bodies, and vertiginous video clips can give an indication about what a body feels like from within. Depictions of these bodily feelings help to communicate ideas about what it means to be alive in general, and a mixed race woman, in particular
Visual Pleasure and Racial Ambiguity
I struggle to present work that reflects a psychological expressivity which at the same time conveys intellectual concepts that are of concern to me. It seems that the fluidity of an image can communicate a certain pathos, and correspond to the fluid nature of one’s identity. Drippy paint, distorted bodies, and vertiginous video clips can give an indication about what a body feels like from within. Depictions of these bodily feelings help to communicate ideas about what it means to be alive in general, and a mixed race woman, in particular
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