Constraining the Exozodiacal Luminosity Function of Main-Sequence Stars: Complete Results From the Keck Nuller Mid-Infrared Surveys

Abstract

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 γ Oph. Overall, excesses detected by KIN are more frequent around A-type stars than later spectral types. A statistical analysis of the measurements further indicates that stars with known far-infrared (λ 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 midinfrared 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.

Exozodiacal clouds: hot and warm dust around main sequence stars

“…This favours PR-drag as a good explanation as it is relatively insensitive to the properties of the parent belt. In addition, Mennesson et al [76] show that the observed mid-IR excesses are compatible with PR-drag levels expected from Wyatt [121]. Figure 12 shows the vertical optical depth as a function of the radial distance to the star that is expected from the interplay between PR-drag and collisions from an outer belt located at different initial positions (different colours) and for two different stellar masses (dashed and solid lines).…”

“…Regarding the occurrence of warm exozodis around stars with no previously detected far infrared excess (i.e. no outer dust reservoir), Mennesson et al [76] derived that the median dust level has to be below 60 times the solar value with high confidence (95%, assuming a log-normal luminosity distribution). To go beyond this state-of-the-art exozodi sensitivity, Downloaded by [University of Liege] at 05: 23 25 October 2017 NASA has funded the Large Binocular Telescope Interferometer (LBTI) to carry out a survey in the N' band (9.81-12.41 µm) on 40-50 carefully chosen nearby main-sequence stars [96].…”

A near-infrared interferometric survey of debris-disc stars

“…Mid-infrared (mid-IR) nulling observations reveal no correlation between near-IR and mid-IR excesses (Mennesson et al 2014) and follow-up observations of the near-IR excess stars, A&A 595, A44 (2016) attempting to detect polarized scattered light emission from the circumstellar dust, did not result in significant detections (Marshall et al 2016). When combined with the near-IR detections, these data provide strong and valuable constraints -even in the case of upper limits -on the emission at different wavelengths and on different spatial scales, and thus on the origin of the excesses (Lebreton et al 2013).…”

Making high-accuracy null depth measurements for the LBTI exozodi survey

“…Assuming for instance that 36 stars are observed by LBTI with an individual measurement uncertainty of 12 zodis, the final uncertainty on the sample median would be of the order of that 2 zodis limit. In comparison, the current best estimate for the median exozodi level of solar-type stars is 12 +/-24 zodis, as derived from the Keck Interferometer observations of 20 solar-type stars between 2008 and 2011, at a typical accuracy level of 100 to 150 zodis per star 7 . This shows that a tenfold improvement over the state of the art is required of LBTI measurements to properly assess the risk of confusing exo-Earths in the habitable zone with bright exozodi clumps.…”