TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 2
Figure 2-12. Models of the evolution of total exo-zodiacal emission as a function of time as a function
of total disk mass (left) and disk location (right; Wyatt et al. 2006) show that after a few Gyr solar-type
stars with disks interior to 10 AU reach zodiacal levels comparable to our own, f = L disk /L = 10 -7 .
case corresponds to the location of our asteroid belt and reproduces almost exactly the level of emission
in our own Solar System, L disk /L * , a few × 10 -7 . The 10- and 30-AU cases predict a higher level of zodiacal
emission than is presently seen in the Solar System; however, this model ignores the clearing action of the
Jupiter and Saturn which would either have incorporated much of the planetesimal material into a solid
core, or ejected the material. While our theoretical understanding is far from complete, curves such as
these, validated by present and future observations of disks, should give us confidence that the expected
level of emission will be at or below the desired ~10–20 EZ level needed for the detection of terrestrial
planets around many nearby stars.
2.6.5 Prospects for Future Observations
It will take observations with facilities other than Spitzer to push to lower levels of zodiacal emission. The
Herschel telescope will measure cold Kuiper Belt disks to Solar System levels while ground-based
interferometers, such as the Keck Interferometer (KI) and the Large Binocular Telescope Interferometers
(LBTI), will spatially suppress the stellar component to measure definitively the 10-μm exozodiacal
emission that arises in the habitable zone and that might cause problems for TPF.
The Keck Interferometer (KI; Colavita et al. 2006) is currently implementing a nulling interferometry
mode at 10 μm specifically targeted at observations of exozodiacal emission around nearby mainsequence
stars. In this mode, the central star is placed on a destructive fringe, allowing detection of the
much fainter surrounding emission while rejecting intense photospheric emission. The size scales probed
by 85-meter baseline in this mode are 25 to 200 mas, corresponding to the habitable zone for many
nearby main-sequence stars. Initial observations using this mode have been made, and the final detection
level is expected to be 100 times the level of our Solar System with a sensitivity limit of 2 Jy for the
target star. At this sensitivity limit and within the declination range of the telescope, there are 53 mainsequence
stars with A through K spectral types, which can be observed with KI, including: 10 A stars, 18
F stars, 10 G stars, and 6 K stars. The KI observations will be sensitive to dust in the habitable zone at a
factor of 10 lower levels than the Spitzer observations. The sample of available stars will determine the
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