High quality fast photometry of a highly irradiated transiting exoplanet WASP-12 b J. J. Bochinski1, C. A. Haswell1, L. Fossati1,2, V. S. Dhillon3, T. R. Marsh4, E. Nickson1, P. J. Wheatley4 The Open University, 2Argelander Institute, University of Bonn, 3University of Sheffield, 4University of Warwick
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Rationale
Close-in hot Jupiters are surrounded by extended clouds of diffuse exospheric absorbing gas (Vidal-Madjar et al 2003, Lecavelier des Etangs et al 2012). WASP-12 b is one of the most irradiated exoplanets and is likely to be undergoing significant mass loss. Figure 1: NUV observations of WASP-12 b transit by Haswell et al (2012) Near-UV observations with Hubble Space Telescope (Fossati et al 2010; Haswell et al 2012) showed early ingresses and excess transit depths, revealing absorbing material in the WASP-12 system (Fig. 1). As HST time is a scarce resource, we undertook sensitive optical observations designed to search for indications of this diffuse absorbing gas.
Observations
We observed WASP-12 with WHT on three nights in January 2012. Nights 2 & 3 were cloudy so we discuss the night starting on the 9th January 2012 only. Images were taken simultaneously in NaI, g’ and u' bands using the ultrafast, triple-beam CCD camera, ULTRACAM (Fig. 2), Figure 2: ULTRACAM’s optical system uses dichroic with time-resolution 3 sec (for g’ & u’) and 15 sec (for NaI). filters to enable simultaneous multiband observations
ULTRACorrect
Our images were contaminated by a Moon reflection (Fig. 3a), which changed in shape and intensity throughout the night. This introduced systematic effects of magnitude ~0.1% into the pipeline-produced light curves; a significant issue at the precision of this work. We therefore performed a bespoke reduction, building a new tool, ULTRACorrect, to remove the Moon's reflected light. The algorithm iteratively median smooths the background using
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Figure 3: (a) raw ultracam image (note the bright bacground feature), (b) ULTRACorrect fit, (c) final corrected image
a large kernel, separating the moon reflection (Fig. 3b) from heavily defocussed star PSFs (Fig. 3c). ULTRACorrect makes no assumptions about the origin of background features and can be used on large data sets, correcting ~100,000 pixels per second.
Results & Discussion
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Our WASP-12 lightcurves (Fig. 4) show asymmetric transits in NaI, g’ and possibly u' bands, consistent with the unevenly-distributed extended gas cloud enshrouding the system indicated by the near-UV data (Fossati et al 2010; Haswell et al 2012).
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Figure 4: Transit lightcurves of WASP-12 b in Na, g’ & u’ bands overlaid with TAP model fits. All data had the Moon contamination removed before modelling.
Our high cadence data puts stringent constraints on the timings of WASP-12 b transits. All three lightcurves have been modelled using TAP (Gazak et al 2012) resulting in consistent transit timing estimates for NaI, g’ and u’, which agree with transit times derived from ephemerides by Haswell et al (2012) and Maciejewski et al (2013) (Fig. 5) weakening the argument for the presence of transit timing variations with amplitude of 0.0007 days in this system. Haswell Maciejewski Bochinski: u’ Bochinski: g’ Bochinski: NaI 2455936.5545
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Figure 5: Comparison between mid-transit timings as calculated from ephemerides by Haswell et al (2012) & Maciejewski et al (2013) and derived from TAP model fits in u’, g’ & NaI bands (respectively 2455936.5557(9) BJDTDB, 2455936.5560(2) BJDTDB and 2455936.5559(2) BJDTDB)
Contact:
Name: Jakub J Bochinski E-mail:
[email protected] Twitter: @JakubBochinski
References:
Fossati et al 2010, ‘Metals in the Exosphere of the Highly Irradiated Planet WASP-12b’,714, L222 Gazak et al 2012, ‘Transit Analysis Package: An IDL Graphical User Interface for Exoplanet Transit Photometry’, AA, 2012, 697967 Haswell et al 2012, ‘Near-UV Absorption, Chromospheric Activity, and Star-Planet Interactions in the WASP-12 system’, ApJ, 760, 79
Lecavelier des Etangs et al 2012, ‘Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b’, A&A, 543, L4 Maciejewski et al 2013, ‘Multi-site campaign for transit timing variations of WASP-12 b’, A&A, 551, A108 Vidal-Madjar et al 2003, ‘An extended upper atmosphere around the extrasolar planet HD209458b’, Nature, 422, 6928
