EXOZODIACAL DUST LEVELS FOR NEARBY MAIN-SEQUENCE STARS: A SURVEY WITH THE KECK INTERFEROMETER NULLER

Abstract

The Keck Interferometer Nuller (KIN) was used to survey 25 nearby main\nsequence stars in the mid-infrared, in order to assess the prevalence of warm\ncircumstellar (exozodiacal) dust around nearby solar-type stars. The KIN\nmeasures circumstellar emission by spatially blocking the star but transmitting\nthe circumstellar flux in a region typically 0.1 - 4 AU from the star. We find\none significant detection (eta Crv), two marginal detections (gamma Oph and\nalpha Aql), and 22 clear non-detections. Using a model of our own Solar\nSystem's zodiacal cloud, scaled to the luminosity of each target star, we\nestimate the equivalent number of target zodis needed to match our\nobservations. Our three zodi detections are eta Crv (1250 +/- 260), gamma Oph\n(200 +/- 80) and alpha Aql (600 +/- 200), where the uncertainties are 1-sigma.\nThe 22 non-detected targets have an ensemble weighted average consistent with\nzero, with an average individual uncertainty of 160 zodis (1-sigma). These\nmeasurements represent the best limits to date on exozodi levels for a sample\nof nearby main sequence stars. A statistical analysis of the population of 23\nstars not previously known to contain circumstellar dust (excluding eta Crv and\ngamma Oph) suggests that, if the measurement errors are uncorrelated (for which\nwe provide evidence) and if these 23 stars are representative of a single class\nwith respect to the level of exozodi brightness, the mean exozodi level for the\nclass is <150 zodis (3-sigma upper-limit, corresponding to 99% confidence under\nthe additional assumption that the measurement errors are Gaussian). We also\ndemonstrate that this conclusion is largely independent of the shape and mean\nlevel of the (unknown) true underlying exozodi distribution.\n