  This paper shows how the dips and secular dimming in the KIC8462852 light
curve can originate in circumstellar material distributed around a single
elliptical orbit (e.g., exocomets). The expected thermal emission and
wavelength dependent dimming is derived for different orbital parameters and
geometries, including dust that is optically thick to stellar radiation, and
for a size distribution of dust with realistic optical properties. We first
consider dust distributed evenly around the orbit, then show how to derive its
uneven distribution from the optical light curve and to predict light curves at
different wavelengths. The fractional luminosity of an even distribution is
approximately the level of dimming times stellar radius divided by distance
from the star at transit. Non-detection of dust thermal emission for KIC8462852
thus provides a lower limit on the transit distance to complement the 0.6au
upper limit imposed by 0.4day dips. Unless the dust distribution is optically
thick, the putative 16% century-long secular dimming must have disappeared
before the WISE 12micron measurement in 2010, and subsequent 4.5micron
observations require transits at >0.05au. However, self-absorption of thermal
emission removes these constraints for opaque dust distributions. The passage
of dust clumps through pericentre is predicted to cause infrared brightening
lasting 10s of days and dimming during transit, such that total flux received
decreases at wavelengths <5micron, but increases to potentially detectable
levels at longer wavelengths. We suggest that lower dimming levels than seen
for KIC8462852 are more common in the Galactic population and may be detected
in future transit surveys.
