The Antarctic planet interferometer
Mark R. Swain, Christopher K. Walker, Wesley A. Traub, John W. Storey, Vincent Coude du Foresto, Eric Fossat, Farrokh Vakili, Anthony A. Stark, James P. Lloyd, Peter R. Lawson, Adam S. Burrows, Michael Ireland
(+6 others)
2004
New Frontiers in Stellar Interferometry
The Antarctic Planet Interferometer is an instrument concept designed to detect and characterize extrasolar planets by exploiting the unique potential of the best accessible site on earth for thermal infrared interferometry. High-precision interferometric techniques under development for extrasolar planet detection and characterization (differential phase, nulling and astrometry) all benefit substantially from the slow, low-altitude turbulence, low water vapor content, and low temperature found
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... on the Antarctic plateau. At the best of these locations, such as the Concordia base being developed at Dome C, an interferometer with two-meter diameter class apertures has the potential to deliver unique science for a variety of topics, including extrasolar planets, active galactic nuclei, young stellar objects, and protoplanetary disks. sites on the Antarctic plateau enable infrared interferometer measurements rivaling space experiments in terms of sensitivity and unique discovery space science potential. Key Question: What is the nature of exoplanet atmospheres? Measurement of the spectral energy distribution (SED) of a hot Jovian exoplanet is probably the most important near-term objective in observational exoplanet astronomy. Theoretical models for the SED of hot Jovian class exoplanets make predictions that could be directly tested by observations; however, the observations are difficult with single aperture telescopes 5,6,7 . Extensive attempts to observe an exoplanet SED have resulted in only one direct measurement 8 . With API phase 1, we could directly test the 4 µm "bump" predicted in the SED of hot Jovian exoplanets 9,10 and the dependence of the SED on orbital phase angle 11,12 . With API phase 1, we could study exoplanet weather by measuring intrinsic SED variations due to circulation induced atmosphere variation 13,14 . With API phase 2, we could extend these measurements from 1.2 to 28 µm and include ~300 K Jovian-class exoplanets in the habitable zone. Thus, API phase 2 could begin the observation and study of other worlds where life could exist. Key Question: What is the frequency of Earth-like planets? Measurement of the frequency of Earth-like planets around solar-type stars, ? Earth would constitute a major scientific achievement and would provide crucial information for the sensitivity requirements for the Terrestrial Planet Finder (TPF) and Darwin missions. Using high-contrast modes such as differential closure phase, capable of 1:10 6 contrast ratios at Antarctic Dome C, API phase 2 could detect Earthmass planets in ~0.25 AU orbits. To infer the frequency of occurrence of Earth-like planets (Earth-mass planets occurring in the habitable zone) we would invoke the premise which is true in our solar system, namely that ? hot Earth = ? Earth . Key question: What is the proper star-disk flux decomposition? Until true direct images are obtained, all interferometer measurements rely on model-fitting techniques for data interpretation. For composite objects such as a young star plus extended disk, one free parameter is the relative contribution to the total flux by each component. All the YSO measurements to date depend on estimating the stellar contribution to the total flux by decomposing the measured spectral energy distribution, a procedure that has never been validated empirically. The initial baseline (~100 m) of API phase 1 will partially resolve the extended disk component of most systems, while the 400~m baseline will completely resolve this component, exposing a baseline-independent fringe amplitude that equals Fstar/Ftotal, directly measurable to the accuracy of API phase 1 visibility amplitude calibration (sub-percent). Another ambiguity arises in the interpretation of sparsely sampled visibility amplitudes, in that if the star-disk system contains an additional large (over-resolved) emitting region, such as from scattering in the nebulosity known to exist around most YSOs, the incoherent flux decreases and, if not accounted for, the effect is indistinguishable from a larger disk. All current measurements are susceptible to this interpretation error, and thus far only indirect arguments have been offered to quantify the plausible magnitudes of a scattering component. The degeneracy can, however, be lifted if measurements at more than one wavelength are available, such as the HKLM API phase 1 capabilities, since the resolved emission will be redder for thermal emission than for scattered emission. Key question: Can planet signatures be detected? A growing planet is believed to open a gap in the circumstellar disk in which it is forming 29 . Such gaps would be undetectable in the spectral energy distributions but are apparent as lowlevel features in visibility amplitude curves. These signatures could be extracted by exploiting the high-accuracy (sub-1%) mode of API phase 1, made possible by the extreme seeing conditions and the photometric calibration capability of the L/M beam combiner. As an extreme example, the putative inner hole at 4 AU in the 10 Myr old system TW~Hya is believed to have been carved by a growing planet 30 and results in few-% excess emission at short IR wavelengths. Although easily resolvable, such a gap is apparent only via structure in the visibility response at sub-% levels, beyond the reach of current instruments but accessible to API phase 1 high-accuracy mode. In fact, it has been shown 31 that, in the thermal IR, visibility curves calculated from 3D hydrodynamical/radiative transfer disk simulations differ, with and without a gap carved by a Jupiter-sized planet, by as much as 5%. Active Galactic Nuclei The origin of the nuclear infrared emission in AGN remains an unsolved question after more than 30 years of observations; infrared interferometry is the only way to obtain the necessary angular resolution to definitively understand the origin of infrared emission and thus the physics of AGN on the scale of the broad-line region (BLR). Key Question: What is the location and structure of the inner edge of the dust tori? The prevailing interpretation of unresolved nuclear near-infrared emission is that it arises from thermal dust that reprocesses UV and optical emission from the central accretion disk. Mid-infrared (10 µm band) observations show compact features at the location of the nucleus 32, 33, 34 . This is interpreted as warm dust (T~300 K), and this dust is thought to extend inward towards the nucleus
doi:10.1117/12.552221
fatcat:lcmyqiky5jeepnjrr5t6gdbrt4