3,764 research outputs found
Orbiting Space Interferometer (OSI): A first generation space interferometer
The technical requirements and performance of a first generation space interferometer is discussed. The performance of an interferometer, sensitivity, field of view, dynamic range, astrometric accuracy, etc, in space is set by what cannot be achieved for a ground-based instrument. For the Orbiting Space Interferometer (OSI), the nominal performance parameters are 20 mag sensitivity, field of view of approximately 500*500 pixels, a 1000:1 dynamic range in the image with one milliarcsec resolution, and an astrometric accuracy of 0.1 milliarcsec for wide angle astrometry and 10 microarcsec accuracy for narrow field astrometry (few degrees). OSI is a fully phased interferometer where all critical optical paths are controlled to 0.05 wavelengths. The instrument uses two guide interferometers locked on bright stars several degrees away to provide the spacecraft attitude information needed to keep the fringes from the faint science object stable on the detector
Exo-Earth/Super-Earth Yield of JWST plus a Starshade External Occulter
We examine the scientific viability of an imaging mission to find exo-Earths
combining the James Webb Space Telescope (JWST) with a starshade external
occulter under a realistic set of astrophysical assumptions. We define an
exo-Earth as a planet of 1 to 10 Earth masses orbiting in the habitable zone
(HZ) of a solar-type star. We show that for a survey strategy that relies on a
single image to detect an exo-Earth, roughly half of all exo-Earth detections
will be false alarms. Here, a false alarm is a mistaken identification of a
planet as an exo-Earth. We consider two survey strategies designed to mitigate
the false alarm problem. The first is to require that for each potential
exo-Earth, a sufficient number of detections are made to measure the orbit.
When the orbit is known we can determine if the planet is in the habitable
zone. With this strategy, we find that the number of exo-Earths found is on
average 0.9, 1.9 and 2.7 for {\eta}_Earth = 0.1, 0.2 and 0.3. Here,
{\eta}_Earth is the frequency of exo-Earths orbiting solar-type stars. There is
a ~40% probability of finding zero exo-Earths for {\eta}_Earth = 0.1. A second
strategy can be employed if a space astrometry mission has identified and
measured the orbits and masses of the planets orbiting nearby stars. We find
that with prior space-based astrometry from a survey of 60 nearby stars, JWST
plus an external occulter can obtain orbital solutions for the majority (70% to
80%) of the exo-Earths orbiting these 60 stars. The exo-Earth yield is
approximately five times higher than the yield for the JWST plus occulter
mission without prior astrometry. With prior astrometry, the probability that
an imaging mission will find zero exo-Earths is reduced to below 1% for the
case of {\eta}_Earth = 0.1.Comment: Accepted by PASP. To appear in February 2010 issue. 15 pages, 2
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The Synergy of Direct Imaging and Astrometry for Orbit Determination of exo-Earths
The holy grail of exoplanet searches is an exo-Earth, an Earth mass planet in
the habitable zone around a nearby star. Mass is the most important parameter
of a planet and can only be measured by observing the motion of the star around
the planet-star center of mass. A single image of a planet, however, does not
provide evidence that the planet is Earth mass or that it is in a habitable
zone orbit. The planet's orbit, however, can be measured either by imaging the
planet at multiple epochs or by measuring the position of the star at multiple
epochs by space-based astrometry. The measurement of an exo-planet's orbit by
direct imaging is complicated by a number of factors: (1) the inner working
angle (IWA); (2) the apparent brightness of the planet depending on the orbital
phase; (3) confusion arising from the presence of multiple planets; and (4) the
planet-star contrast. In this paper we address the question: "Can a prior
astrometric mission that can identify which stars have Earthlike planets
significantly improve the science yield of a mission to image exo-Earths?" We
find that the Occulting Ozone Observatory (a small external occulter mission
that cannot measure spectra) could confirm the orbits of ~4 to ~5 times as many
exo-Earths if an astrometric mission preceded it to identify which stars had
such planets. We find that in the case of an internal coronagraph, a survey of
the nearest ~60 stars could be done with a telescope of half the size if an
astrometric mission had first identified the presence of Earth-like planets in
the habitable zone and measured their orbital parameters.Comment: ApJ, in press; 28 pages, 8 figure
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