Abstract
WASP-12b is one of the most well-studied transiting exoplanets, as its highly-inflated radius and its 1.1 day orbit around a G0-type star make it an excellent target for atmospheric categorisation through observation during its secondary eclipse. We present two new secondary eclipse observations of WASP-12b, acquired a year apart with the Wide Field Camera on the Isaac Newton Telescope (INT) and the IO:O instrument on the Liverpool Telescope (LT). These observations were conducted in the $i^\prime$-band, a window expected to be dominated by TiO features if present in appreciable quantities in the upper atmosphere. We measured eclipse depths that disagree with each other by $\sim$3$\sigma$ (0.97 $\pm$ 0.14 mmag on the INT and 0.44 $\pm$ 0.21 mmag on the LT), a result that is mirrored in previous $z^\prime$-band secondary eclipse measurements for WASP-12b. We explore explanations for these disagreements, including systematic errors and variable thermal emission in the dayside atmosphere of WASP-12b caused by temperature changes of a few hundred Kelvin: a possibility we cannot rule out from our analysis. Full-phase curves observed with TESS and CHEOPS have the potential to detect similar atmospheric variability for WASP-12b and other optimal targets, and a strategic, multi-telescope approach to future ground-based secondary eclipse observations is required to discriminate between explanations involving storms and systematics.
Highlights
Secondary eclipse observations are an important tool to characterise the atmospheres of transiting hot Jupiters
Full-phase curves observed with Transiting Exoplanet Survey Satellite (TESS) and CHaracterising ExOPlanets Satellite (CHEOPS) have the potential to detect similar atmospheric variability for WASP-12b and other optimal targets, and a strategic, multi-telescope approach to future ground-based secondary eclipse observations is required to discriminate between explanations involving storms and systematics
We have presented the reduction and analyses of two i band secondary eclipse observations of WASP-12b
Summary
Secondary eclipse observations are an important tool to characterise the atmospheres of transiting hot Jupiters. We selected a small region of the science frames with a good range in fringe amplitudes, and used this region to fit for the proper offset and scaling for the fringe frame using a linear fit between the fringe map and the science frame, taking care to mask any stars in the region This approach was significantly less sensitive to the gradient across the Moon-contaminated science frames and robustly removed the fringes for the entire time-series. We fit and subtracted a second-order 2dimensional polynomial from each of the science frames with the stars masked When stacking the science frames, a small trend was still visible across the detector after the IO:O reduction, despite the flat-fielding For this reason, a second-order polynomial was fit to and subtracted from the entirety of each frame individually with the stars masked. We selected target apertures with radii of 54 pixels, and inner and outer radii of 60 and 79 pixels, respectively
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