2018MNRAS.481.2235S

The star KIC 8462852 (Boyajian's Star) displays both fast dips of up to 20 per cent on time-scales of days and long-term secular fading by up to 19 per cent on time-scales from a year to a century. We report on CCD photometry of KIC 8462852 from 2015.75 to 2018.18, with 19 176 images making for 1866 nightly magnitudes in BVRI. Our light curves show a continuing secular decline (by 0.023 ± 0.003 mags in the B band) with three superposed dips with duration 120-180 d. This demonstrates that there is a continuum of dip durations from a day to a century, so the secular fading is seen to be by the same physical mechanism as the short-duration Kepler dips. The BVRI light curves all have the same shape, with the slopes and amplitudes for VRI being systematically smaller than in the B band by factors of 0.77 ± 0.05, 0.50 ± 0.05, and 0.31 ± 0.05. We rule out any hypothesis involving occultation of the primary star by any star, planet, solid body, or optically thick cloud. But these ratios are the same as that expected for ordinary extinction by dust clouds. This chromatic extinction implies dust particle sizes going down to ∼0.1 micron, suggesting that this dust will be rapidly blown away by stellar radiation pressure, so the dust clouds must have formed within months. The modern infrared observations were taken at a time when there was at least 12.4 per cent ±1.3per cent dust coverage (as part of the secular dimming), and this is consistent with dimming originating in circumstellar dust.