Simultaneous infrared and optical observations of the transiting debris cloud around WD 1145+017

We present multi-wavelength photometric monitoring of WD 1145+017, a white dwarf exhibiting periodic dimming events interpreted to be the transits of orbiting, disintegrating planetesimals. Our observations include the rst set of near-infrared light curves for the object, obtained on multiple nights over the span of one month, and recorded multiple transit events with depths varying between 20 to 50 per cent. Simultaneous near-infrared and optical observations of the deepest and longest duration transit event were obtained on two epochs with the Anglo-Australian Telescope and three optical facilities, over the wavelength range of 0.5 to 1:2 m. These observations revealed no measurable dierence in transit depths for multiple photometric pass bands, allowing us to place a 2 lower limit of 0:8 m on the grain size in the putative transiting debris cloud. This conclusion is consistent with the spectral energy distribution of the system, which can be t with an optically thin debris disc with minimum particle sizes of 10 +5 3 m.