The HOSTS survey for exo-zodiacal dust: preliminary results and future prospects

The presence of large amounts of dust in the habitable zones of nearby starsis a significant obstacle for future exo-Earth imaging missions. We executed the HOSTS (Hunt for Observable Signatures of Terrestrial Systems) survey to determine the typical amount of such exozodiacal dustaround a sample of nearby main sequence stars. The majority of the data have been analyzed and we present here an update of our ongoing work. Nulling interferometry in N band was used to suppress the bright stellar light and to detect faint, extended circumstellar dustemission. We present an overview of the latest results from our ongoing work. We find seven new N band excesses in addition to the high confidence confirmation of three that were previously known. We find the first detections around Sun-like starsand around starswithout previously known circumstellar dust. Our overall detection rate is 23%. The inferred occurrence rate is comparable for early type and Sun-like stars, but decreases from 71 +11 -20 % for starswith previously detected mid- to far- infrared excessto 11 +9 -4 % for starswithout such excess, confirming earlier results at high confidence. For completed observations on individual stars, our sensitivity is five to ten times better than previous results. Assuming a lognormal luminosity functionof the dust, we find upper limits on the median dust level around all starswithout previously known mid to far infrared excessof 11.5 zodis at 95% confidence level. The corresponding upper limit for Sun-like starsis 16 zodis. An LBTI vetted target list of Sun-like starsfor exo-Earth imaging would have a corresponding limit of 7.5 zodis. We provide important new insights into the occurrence rate and typical levels of habitable zone dust around main sequence stars. Exploiting the full range of capabilities of the LBTI provides a critical opportunity for the detailed characterization of a sample of exozodiacal dustdisks to understand the origin, distribution, and properties of the dust.