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This paper shows how the dips and secular dimming in the KIC8462852 lightcurve can originate in circumstellar material distributed around a singleelliptical orbit (e.g., exocomets). The expected thermal emission andwavelength dependent dimming is derived for different orbital parameters andgeometries, including dust that is optically thick to stellar radiation, andfor a size distribution of dust with realistic optical properties. We firstconsider dust distributed evenly around the orbit, then show how to derive itsuneven distribution from the optical light curve and to predict light curves atdifferent wavelengths. The fractional luminosity of an even distribution isapproximately the level of dimming times stellar radius divided by distancefrom the star at transit. Non-detection of dust thermal emission for KIC8462852thus provides a lower limit on the transit distance to complement the 0.6auupper limit imposed by 0.4day dips. Unless the dust distribution is opticallythick, the putative 16% century-long secular dimming must have disappearedbefore the WISE 12micron measurement in 2010, and subsequent 4.5micronobservations require transits at >0.05au. However, self-absorption of thermalemission removes these constraints for opaque dust distributions. The passageof dust clumps through pericentre is predicted to cause infrared brighteninglasting 10s of days and dimming during transit, such that total flux receiveddecreases at wavelengths <5micron, but increases to potentially detectablelevels at longer wavelengths. We suggest that lower dimming levels than seenfor KIC8462852 are more common in the Galactic population and may be detectedin future transit surveys.

Modelling the KIC8462852 light curves: compatibility of the dips and secular dimming with an exocomet interpretation