78 research outputs found
High resolution imaging at large telescopes
Image recovery at a resolution limited only by diffraction is now possible at large telescopes. The theory of speckle image reconstruction is explained and the current status of a video recording and digitization system for the reconstruction procedure is described. Potential applications of the process when used with very large telescopes are discussed. The constraints on telescope design imposed by these techniques are listed
High resolution imaging with Fresnel interferometric arrays: suitability for exoplanet detection
We propose a new kind of interferometric array that yields images of high
dynamic range and large field. The numerous individual apertures in this array
form a pattern related to a Fresnel zone plate. This array can be used for
astrophysical imaging over a broad spectral bandwidth spanning from the U.V.
(50 nanometers) to the I.R. (20 microns). Due to the long focal lengths
involved, this instrument requires formation-flying of two space borne vessels.
We present the concept and study the S/N ratio in different situations, then
apply these results to probe the suitability of this concept to detect
exoplanets.Comment: 12 pages, 19 figures, to be published in A&
Calculation of an optimized telescope apodizer for Terrestrial Planet Finder coronagraphic telescope
One of two approaches to implementing NASA's Terrestrial Planet Finder is to
build a space telescope that utilizes the techniques of coronagraphy and
apodization to suppress diffraction and image exo-planets. We present a method
for calculation of a telescope's apodizer which suppresses the side lobes of
the image of a star so as to optimally detect an Earth-like planet. Given the
shape of a telescope's aperture and given a search region for a detector, we
solve an integral equation to determine an amplitude modulation (an apodizer)
which suppresses the star's energy in the focal plane search region. The method
is quite general and yields as special cases the product apodizer reported by
Nisenson and Papaliolios (2001) and the Prolate spheroidal apodizer of Kasdin
et al (2002), and Aime et al (2002). We show computer simulations of the
apodizers and the corresponding point spread functions for various
aperture-detector configurations.Comment: 16 Pages, 9 figures, Accepted for publication in June issue of PAS
Detection of Earth-like Planets Using Apodized Telescopes
The mission of NASA's Terrestrial Planet Finder (TPF) is to find Earth-like
planets orbiting other stars and characterize the atmospheres of these planets
using spectroscopy. Because of the enormous brightness ratio between the star
and the reflected light from the planet, techniques must be found to reduce the
brightness of the star. The current favorite approach to doing this is with
interferometry: interfering the light from two or more separated telescopes
with a phase shift, nulling out the starlight. While this technique can,
in principle, achieve the required dynamic range, building a space
interferometer that has the necessary characteristics poses immense technical
difficulties. In this paper, we suggest a much simpler approach to achieving
the required dynamic range. By simply adjusting the transmissive shape of a
telescope aperture, the intensity in large regions around the stellar image can
be reduced nearly to zero. This approach could lead to construction of a TPF
using conventional technologies, requiring space optics on a much smaller scale
than the current TPF approach.Comment: Accepted for publication in ApJ Letters, 9 pages, 6 figure
Exoplanets or Dynamic Atmospheres? The Radial Velocity and Line Shape Variations of 51 Pegasi and Tau Bootis
Because of our relatively low spectral resolution, we compare our
observations with Gray's line bisector data by fitting observed line profiles
to an expansion in terms of orthogonal (Hermite) functions. To obtain an
accurate comparison, we model the emergent line profiles from rotating and
pulsating stars, taking the instrumental point spread function into account. We
describe this modeling process in detail.
We find no evidence for line profile or strength variations at the radial
velocity period in either 51 Peg or in Tau Boo. For 51 Peg, our upper limit for
line shape variations with 4.23-day periodicity is small enough to exclude with
10 sigma confidence the bisector curvature signal reported by Gray & Hatzes;
the bisector span and relative line depth signals reported by Gray (1997) are
also not seen, but in this case with marginal (2 sigma) confidence. We cannot,
however, exclude pulsations as the source of 51 Peg's radial velocity
variation, because our models imply that line shape variations associated with
pulsations should be much smaller than those computed by Gray & Hatzes; these
smaller signals are below the detection limits both for Gray & Hatzes' data and
for our own.
Tau Boo's large radial velocity amplitude and v*sin(i) make it easier to test
for pulsations in this star. Again we find no evidence for periodic line-shape
changes, at a level that rules out pulsations as the source of the radial
velocity variability. We conclude that the planet hypothesis remains the most
likely explanation for the existing data.Comment: 44 pages, 19 figures, plain TeX, accepted to ApJS (companion to
letter astro-ph/9712279
A High-Eccentricity Low-Mass Companion to HD 89744
HD 89744 is an F7 V star with mass 1.4 M, effective temperature 6166 K, age
2.0 Gy and metallicity [Fe/H]= 0.18. The radial velocity of the star has been
monitored with the AFOE spectrograph at the Whipple Observatory since 1996, and
evidence has been found for a low mass companion. The data were complemented by
additional data from the Hamilton spectrograph at Lick Observatory during the
companion's periastron passage in fall 1999. As a result, we have determined
the star's orbital wobble to have period P = 256 d, orbital amplitude K = 257
m/s, and eccentricity e = 0.7. From the stellar mass we infer that the
companion has minimum mass m2 sin i = 7.2 MJup in an orbit with semi-major axis
a2 = 0.88 AU. The eccentricity of the orbit, among the highest known for
extra-solar planets, continues the trend that extra-solar planets with
semi-major axes greater than about 0.15 AU tend to have much higher
eccentricities than are found in our solar system. The high metallicity of the
parent star reinforces the trend that parent stars of extra-solar planets tend
to have high metallicityComment: AASTEX-LateX v5.0, 7 pages w/ 3 figures, to be published in ApJ
On mesogranulation, network formation and supergranulation
We present arguments which show that in all likelihood mesogranulation is not
a true scale of solar convection but the combination of the effects of both
highly energetic granules, which give birth to strong positive divergences
(SPDs) among which we find exploders, and averaging effects of data processing.
The important role played by SPDs in horizontal velocity fields appears in the
spectra of these fields where the scale 4 Mm is most energetic; we
illustrate the effect of averaging with a one-dimensional toy model which shows
how two independent non-moving (but evolving) structures can be transformed
into a single moving structure when time and space resolution are degraded.
The role of SPDs in the formation of the photospheric network is shown by
computing the advection of floating corks by the granular flow. The coincidence
of the network bright points distribution and that of the corks is remarkable.
We conclude with the possibility that supergranulation is not a proper scale of
convection but the result of a large-scale instability of the granular flow,
which manifests itself through a correlation of the flows generated by SPDs.Comment: 10 pages, 11 figures, to appear in Astronomy and Astrophysic
Automated Detection and Tracking of Solar Magnetic Bright Points
Magnetic Bright Points (MBPs) in the internetwork are among the smallest
objects in the solar photosphere and appear bright against the ambient
environment. An algorithm is presented that can be used for the automated
detection of the MBPs in the spatial and temporal domains. The algorithm works
by mapping the lanes through intensity thresholding. A compass search, combined
with a study of the intensity gradient across the detected objects, allows the
disentanglement of MBPs from bright pixels within the granules. Object growing
is implemented to account for any pixels that might have been removed when
mapping the lanes. The images are stabilized by locating long-lived objects
that may have been missed due to variable light levels and seeing quality.
Tests of the algorithm employing data taken with the Swedish Solar Telescope
(SST), reveal that ~90% of MBPs within a 75"x 75" field of view are detected
Characterization of extrasolar terrestrial planets from diurnal photometric variability
The detection of massive planets orbiting nearby stars has become almost
routine, but current techniques are as yet unable to detect terrestrial planets
with masses comparable to the Earth's. Future space-based observatories to
detect Earth-like planets are being planned. Terrestrial planets orbiting in
the habitable zones of stars-where planetary surface conditions are compatible
with the presence of liquid water-are of enormous interest because they might
have global environments similar to Earth's and even harbor life. The light
scattered by such a planet will vary in intensity and colour as the planet
rotates; the resulting light curve will contain information about the planet's
properties. Here we report a model that predicts features that should be
discernible in light curves obtained by low-precision photometry. For
extrasolar planets similar to Earth we expect daily flux variations up to
hundreds of percent, depending sensitively on ice and cloud cover. Qualitative
changes in surface or climate generate significant changes in the predicted
light curves. This work suggests that the meteorological variability and the
rotation period of an Earth-like planet could be derived from photometric
observations. Other properties such as the composition of the surface (e.g.,
ocean versus land fraction), climate indicators (for example ice and cloud
cover), and perhaps even signatures of Earth-like plant life could be
constrained or possibly, with further study, even uniquely determined.Comment: Published in Nature. 9 pages including 3 figure
Photometric properties of resolved and unresolved magnetic elements
We investigate the photometric signature of magnetic flux tubes in the solar
photosphere. We developed two dimensional, static numerical models of isolated
and clustered magnetic flux tubes. We investigated the emergent intensity
profiles at different lines-of-sight for various spatial resolutions and
opacity models. We found that both geometric and photometric properties of
bright magnetic features are determined not only by the physical properties of
the tube and its surroundings, but also by the particularities of the
observations, including the line/continuum formation height, the spatial
resolution and the image analyses techniques applied. We show that some
observational results presented in the literature can be interpreted by
considering bright magnetic features to be clusters of smaller elements, rather
than a monolithic flux tube.Comment: 12 page
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