Once in a blue moon: detection of ‘bluing’ during debris transits in the white dwarf WD 1145+017

WD 1145+017 color difference
Demonstration of the circumstellar gas-induced bluing using VLT/X-SHOOTER spectra: photometric (error bars) and ‘spectral’ (star- shape symbols) color differences in and out of transit, for various color indices. The error bars are color-coded by the corresponding mean transit depth (the bluer the deeper). The lower panels show the g'-band ULTRACAM light curves from 2016 April 21 (middle) and 2016 April 26 (bottom). The highlighted areas correspond to the integrated time intervals, with colors matching those of the top panel error bars. The photometry derived from the spectra broadly reproduces the bluing and the trends seen in the photometric transit data (Hallakoun et al. 2017).

The first transiting planetesimal orbiting a WD was detected in K2 data of WD 1145+017 by Vanderburg et al. 2015 and has been followed up intensively. The multiple, long and variable transits suggest the transiting objects are dust clouds, probably produced by a disintegrating asteroid. In addition, the system contains circumstellar gas, evident by broad absorption lines, and a dust disc, indicated by an infrared excess. Using simultaneous multiband fast-photometry ULTRACAM measurements over the u'g'r'i' bands, we detected for the first time a change in the color of WD 1145+017 during transits. The observations reveal what appears to be ‘bluing’ during transits; transits are deeper in the redder bands, with a u'−r' color difference of up to ∼−0.05 mag. We explored various possible explanations for the bluing, including limb darkening or peculiar dust properties. ‘Spectral’ photometry obtained by integrating over bandpasses in the spectroscopic data in and out of transit, compared to the photometric data, shows that the observed color difference is most likely the result of reduced circumstellar absorption in the spectrum during transits. This indicates that the transiting objects and the gas share the same line of sight and that the gas covers the white dwarf only partially.

For further reading:

  • N. Hallakoun, S. Xu, D. Maoz, T. R. Marsh, V. D. Ivanov, V. S. Dhillon, M. C. P. Bours, S. G. Parsons, P. Kerry, S. Sharma, K. Su, S. Rengaswamy, P. Pravec, P. Kušnirák, H. Kučáková, J. D. Armstrong, C. Arnold, N. Gerard, and L. Vanzi
    "Once in a blue moon: detection of ‘bluing’ during debris transits in the white dwarf WD 1145+017"
    2017, Monthly Notices of the Royal Astronomical Society, 469, 3213
  • Siyi Xu, Na'ama Hallakoun, Bruce Gary, Paul A. Dalba, John Debes, Patrick Dufour, Maude Fortin-Archambault, Akihiko Fukui, Michael A. Jura, Beth Klein, Nobuhiko Kusakabe, Philip S. Muirhead, Norio Narita, Amy Steele, Kate Y. L. Su, Andrew Vanderburg, Noriharu Watanabe, Zhuchang Zhan, and Ben Zuckerman
    "Shallow Ultraviolet Transits of WD 1145+017"
    2019, The Astronomical Journal, 157, 255