Shallow Ultraviolet Transits of WD 1145+017

WD 1145+017 is a unique white dwarfsystem that has a heavily polluted atmosphere, an infrared excessfrom a dust disk, numerous broad absorption lines from circumstellar gas, and changing transitfeatures, likely from fragments of an actively disintegrating asteroid. Here, we present results from a large photometric and spectroscopic campaign with Hubble, Keck , VLT, Spitzer, and many other smaller telescopes from 2015 to 2018. Somewhat surprisingly, but consistent with previous observations in the u' band, the UV transitdepths are always shallower than those in the optical. We develop a model that can quantitatively explain the observed " bluing" and the main findings are: I. the transitingobjects, circumstellar gas, and white dwarfare all aligned along our line of sight; II. the transitingobject is blocking a larger fraction of the circumstellar gas than of the white dwarfitself. Because most circumstellar lines are concentrated in the UV, the UV flux appears to be less blocked compared to the optical during a transit, leading to a shallower UV transit. This scenario is further supported by the strong anti-correlation between optical transitdepth and circumstellar line strength. We have yet to detect any wavelength-dependent transitscaused by the transitingmaterial around WD 1145+017.