@article{99201e599fba4d0c99bff4da7d1ff45a,
title = "Once in a blue moon: Detection of 'bluing' during debris transits in the white dwarf WD 1145+017",
abstract = "The first transiting planetesimal orbiting a white dwarf was recently detected in K2 data of WD 1145+017 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, mostly in the u' band, and a dust disc, indicated by an infrared excess. Here we present the first detection of a change in colour of WD 1145+017 during transits, using simultaneous multiband fastphotometry ULTRACAM measurements over the u'g'r'i' bands. The observations reveal what appears to be 'bluing' during transits; transits are deeper in the redder bands, with a u' - r' colour difference of up to ~-0.05 mag. We explore 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 colour 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, as would be expected if the gas, the transiting debris and the dust emitting the infrared excess are part of the same general disc structure (although possibly at different radii). In addition, we present the results of a week-long monitoring campaign of the system using a global network of telescopes.",
keywords = "Asteroids: general, Eclipses, Minor planets, Stars: individual: WD 1145+017, Techniques: photometric, White dwarfs",
author = "N. Hallakoun and S. Xu and D. Maoz and Marsh, {T. R.} and Ivanov, {V. D.} and Dhillon, {V. S.} and Bours, {M. C.P.} and Parsons, {S. G.} and P. Kerry and S. Sharma and K. Su and S. Rengaswamy and P. Pravec and P. Ku{\v s}nir{\'a}k and H. Ku{\v c}{\'a}kov{\'a} and Armstrong, {J. D.} and C. Arnold and N. Gerard and L. Vanzi",
note = "Funding Information: We are indebted to Bruce L. Gary, Saul Rappaport, Tom Kaye, Roi Alonso and Josch Hambsch for providing their WD 1145+017 light curves and dip fit results. We thank the PIs, Michael Jura (U019), John Debes (U124D), and Ben Zuckerman (U067E), for providing the Keck spectra of WD 1145+017 and sharing the results of the preliminary analysis. We thank the anonymous referee for valuable comments that improved this paper. We thank Bruce L. Gary, Saul Rappaport, Ferdinando Patat, Tsevi Mazeh, Alexandros Gianninas, Sahar Shahaf, Yakov Faerman, Stefan Jordan and Andreas Quir-renbach for useful discussions, the members of the VBO Steering Committee and the Time Allocation Committee (TAC), and the observing assistants: Mr G. Selvakumar, Mr S. Venkatesh, Mr S. Parthiban and Mr R. Rajini Rao. NH thanks Tamar Faran for moral support. This work was supported in part by Grant 1829/12 of the Israeli Centers for Research Excellence (I-CORE) programme of the Planning and Budgeting Committee (PBC) and the Israel Science Foundation (ISF) and by Grant 648/12 by the ISF (DM). The ULTRACAM team acknowledges the support of the Science and Technology Facilities Council (STFC). Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 097.C-0829, 296.C-5024 and 296.C-5014. This work makes use of observations from the LCO global telescope network. UKIRT is supported by NASA and operated under an agreement among the University of Hawaii, the University of Arizona and Lockheed Martin Advanced Technology Center; operations are enabled through the cooperation of the East Asian Observatory. The UC Observatory Santa Martina was part of the photometric monitoring campaign presented in this work. This work used the astronomy and astrophysics package for MATLAB (Ofek 2014). Publisher Copyright: {\textcopyright} 2017 The Authors.",
year = "2017",
month = aug,
day = "11",
doi = "10.1093/mnras/stx924",
language = "English (US)",
volume = "469",
pages = "3213--3224",
journal = "Monthly Notices of the Royal Astronomical Society",
issn = "0035-8711",
publisher = "Oxford University Press",
number = "3",
}