# Shallow Ultraviolet Transits of WD 1145+017

## TL;DR

This study presents multi-wavelength observations of WD 1145+017, revealing that UV transits are shallower than optical ones due to the alignment of transiting objects with circumstellar gas, supporting a model of gas and debris interaction.

## Contribution

The paper introduces a quantitative model explaining the shallower UV transits and the alignment of transiting objects, circumstellar gas, and the white dwarf in WD 1145+017.

## Key findings

- UV transit depths are shallower than optical.
- Transiting objects, gas, and star are aligned along our line of sight.
- UV flux is less blocked during transits due to circumstellar gas absorption.

## Abstract

WD 1145+017 is a unique white dwarf system that has a heavily polluted atmosphere, an infrared excess from a dust disk, numerous broad absorption lines from circumstellar gas, and changing transit features, 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 transit depths 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 transiting objects, circumstellar gas, and white dwarf are all aligned along our line of sight; II. the transiting object is blocking a larger fraction of the circumstellar gas than of the white dwarf itself. 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 transit depth and circumstellar line strength. We have yet to detect any wavelength-dependent transits caused by the transiting material around WD 1145+017.