Shallow Ultraviolet Transits of WD 1145+017

• Published in: The Astronomical journal Vol. 157; no. 6; pp. 255 - 266
• Main Authors: Xu, Siyi, Hallakoun, Na'ama, Gary, Bruce, Dalba, Paul A., Debes, John, Dufour, Patrick, Fortin-Archambault, Maude, Fukui, Akihiko, Jura, Michael A., Klein, Beth, Kusakabe, Nobuhiko, Muirhead, Philip S., Narita, Norio, Steele, Amy, Su, Kate Y. L., Vanderburg, Andrew, Watanabe, Noriharu, Zhan, Zhuchang, Zuckerman, Ben
• Format: Journal Article
• Language: English
• Published: Madison The American Astronomical Society 01.06.2019; IOP Publishing
• Subjects: ABSORPTION; Air pollution; ASTEROIDS; Astronomy; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ATMOSPHERES; Bluing; circumstellar matter; COMPARATIVE EVALUATIONS; COSMIC DUST; Disintegration; Hubble Space Telescope; minor planets, asteroids: general; PLANETS; SPACE; Space telescopes; stars: individual (WD 1145+017); TELESCOPES; Transit; Transits; ULTRAVIOLET RADIATION; Very Large Telescope; WHITE DWARF STARS; white dwarfs
• ISSN: 0004-6256; 1538-3881; 1538-3881
• DOI: 10.3847/1538-3881/ab1b36

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 Space Telescope, Keck, Very Large Telescope (VLT), Spitzer, and many other smaller telescopes from 2015 to 2018. Somewhat surprisingly the ultraviolet (UV) transit depths are always shallower than those in the optical. We develop a model that can quantitatively explain the observed "bluing" and confirm the previous finding that: (1) the transiting objects, circumstellar gas, and white dwarf are all aligned along our line of sight; (2) 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 anticorrelation 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.