Exozodiacal characterization
The long-range goal of the Origins program is
to discover and characterize Earth-like planets. While this very difficult task ultimately requires space-based
interferometers like the Space Interferometry Mission (SIM)
and the Terrestrial Planet Finder (TPF), the Keck Interferometer
plays a key role. Like our solar system, other planetary systems are expected to have some amount of zodiacal
dust (dust in the plane of the solar system) around the central star. Large amounts of this exozodiacal dust can obscure the signature of a planet,
making detection more challenging. Characterizing the exozodiacal dust around potential targets for TPF will
help optimize the mission design, observational scenario, and target list.
The Keck Interferometer will use a nulling technique at a wavelength a 10 microns to null the starlight so that the emission
of the dust around a star can be detected.
The measurement of exozodiacal dust is challenging. The features of the problem that drive the measurement
technique are the strong light from the central star and the relatively weak exozodiacal signal. However, as
the disk is best detected in the mid-IR, the measurement technique needs to accommodate the strong thermal
background. The planned approach uses interferometric nulling to cancel light from the central star to reduce
the required dynamic range, and fast modulation to accommodate the mid-IR background.
The scales of the problem match those of the instrument: at a distance of 10 pc, the diameter of the central
star is 1 mas, while the diameter of a (1 AU radius) dust disk is approximately 200 mas; thus the light from the
target will be collected in the Lambda/D = 200 mas beam of a single Keck, while the interferometer resolution
Lambda/B at 10 µm of 25 mas allows exploitation of interferometric methods. The measurement technique
is shown schematically in the figure at the right. Each Keck aperture is split into two halves, each of which
is sent separately to the beam combining lab. Interferometer nullers cancel most of the light from the central
star on two 85-m baselines. The outputs of these nullers are then combined in a conventional beam combiner which
uses fast fringe scanning to demodulate the exozodiacal signature in the presence of the 10 µm background.