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Published December 20, 2014 | Published
Journal Article Open

Constraining the Exozodiacal Luminosity Function of Main-sequence Stars: Complete Results from the Keck Nuller Mid-infrared Surveys


Forty-seven nearby main-sequence stars were surveyed with the Keck Interferometer mid-infrared Nulling instrument (KIN) between 2008 and 2011, searching for faint resolved emission from exozodiacal dust. Observations of a subset of the sample have already been reported, focusing essentially on stars with no previously known dust. Here we extend this previous analysis to the whole KIN sample, including 22 more stars with known near-and/or far-infrared excesses. In addition to an analysis similar to that of the first paper of this series, which was restricted to the 8-9 µm spectral region, we present measurements obtained in all 10 spectral channels covering the 8-13 µm instrumental bandwidth. Based on the 8-9 µm data alone, which provide the highest signal-to-noise measurements, only one star shows a large excess imputable to dust emission (η Crv), while four more show a significant (> 3σ) excess: β Leo, β UMa, ζ Lep, and y Oph. Overall, excesses detected by KIN are more frequent around A-type stars than later spectral types. Astatistical analysis of the measurements further indicates that stars with known far-infrared (y ≥ 70 µm) excesses have higher exozodiacal emission levels than stars with no previous indication of a cold outer disk. This statistical trend is observed regardless of spectral type and points to a dynamical connection between the inner (zodi-like) and outer (Kuiper-Belt-like) dust populations. The measured levels for such stars are clustering close to the KIN detection limit of a few hundred zodis and are indeed consistent with those expected from a population of dust that migrated in from the outer belt by Poynting-Robertson drag. Conversely, no significant mid-ilinfrared excess is found around sources with previously reported near-infrared resolved excesses, which typically have levels of the order of 1% over the photospheric flux. If dust emission is really at play in these near-infrared detections, the absence of a strong mid-infrared counterpart points to populations of very hot and small (submicron) grains piling up very close to the sublimation radius. For solar-type stars with no known infrared excess, likely to be the most relevant targets for a future exo-Earth direct imaging mission, we find that their median zodi level is 12±24 zodis and lower than 60 (90) zodis with 95% (99%) confidence, if a lognormal zodi luminosity distribution is assumed.

Additional Information

© 2014 American Astronomical Society. Received 2014 July 5; accepted 2014 October 1; published 2014 December 8. The Keck Interferometer was funded by the National Aeronautics and Space Administration (NASA). Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, and at the NASA Exoplanet Science Center (NExScI), under contract with NASA. The Keck Observatory was made possible through the generous financial support of the W. M. Keck Foundation. This research has made use of the Washington Double Star Catalog maintained at the U.S. Naval Observatory. M.C.W. is grateful for support from the European Union through ERC grant number 279973.

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