A Detection of Water in the Transmission Spectrum of the Hot Jupiter WASP-12b and Implications for its Atmospheric Composition

TLDR: In this article, a near-infrared transmission spectrum for WASP-12b based on six transit observations with the Hubble Space Telescope/Wide Field Camera 3 (HST-WFC) was reported.

Abstract: Detailed characterization of exoplanets has begun to yield measurements of their atmospheric properties that constrain the planets’ origins and evolution. For example, past observations of the dayside emission spectrum of the hot Jupiter WASP-12b indicated that its atmosphere has a high carbon-tooxygen ratio (C/O > 1), suggesting it had a dierent formation pathway than is commonly assumed for giant planets. Here we report a precise near-infrared transmission spectrum for WASP-12b based on six transit observations with the Hubble Space Telescope/Wide Field Camera 3. We bin the data in 13 spectrophotometric light curves from 0.84 - 1.67 m and measure the transit depths to a median precision of 51 ppm. We retrieve the atmospheric properties using the transmission spectrum and nd strong evidence for water absorption (7 condence). This detection marks the rst high-condence, spectroscopic identication of a molecule in the atmosphere of WASP-12b. The retrieved 1 water volume mixing ratio is between 10 5 10 2 , which is consistent with C/O > 1 to within 2 . However, we also introduce a new retrieval parameterization that ts for C/O and metallicity under the assumption of chemical equilibrium. With this approach, we constrain C/O to 0:5 +0:2 0:3 at 1 and rule out a carbon-rich atmosphere composition (C/O> 1) at > 3 condence. Further observations and modeling of the planet’s global thermal structure and dynamics would aid in resolving the tension between our inferred C/O and previous constraints. Our ndings highlight the importance of obtaining high-precision data with multiple observing techniques in order to obtain robust constraints on the chemistry and physics of exoplanet atmospheres. Subject headings: planets and satellites: atmospheres | planets and satellites: composition | planets and satellites: individual: WASP-12b

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Table 2 Transit Times

Figure 6. Systematics decorrelation parameters from the light curve fits plotted as a function of normalization c. We show (from top to bottom) the visit-long slope v, ramp rate constant a, ramp amplitude b, and ramp delay d. The normalization constant c is linearly proportional to the per-pixel flux in a spectroscopic channel. Measurements from the G102 and G141 data are shown in light and dark gray, respectively. The error bars indicate 1σ uncertainties from an MCMC fit to the light curves using the analytic model for instrument systematics.

Figure 7. Dilution factor αComp, determined from the ratio of flux from WASP12 BC to WASP-12 A. The median dilution (over the 10 staring mode observations) is shown with black lines (dotted for G102 and dashed for G141). The gray shaded region indicates 1σ uncertainty, determined from the median absolute deviation. To calculate the dilution and uncertainty, we interpolate the spectra from each grism onto a common wavelength scale.

Figure 12. Retrieval results from the C-C parameterization. The left and middle panels show the marginalized distribution for C/O and metallicity. The right panel shows the median (black lines) and 1σ range (shaded regions) of 1000 randomly sampled temperature–pressure profiles for each scenario. Yellow, blue, and red shading correspond to constraints from the fiducial scenario (an uninformative prior on C/O), the oxygen-rich scenario (C/O < 1), and a carbon-rich scenario (C/O > 1). The distribution of C/O values for the carbon-rich model is normalized to have a probability mass of unity over the plotted range; however, the distribution has an extended tail toward higher C/O values that is not shown.

Table 1 Summary of Photometric Observations for WASP-12

Figure 1. Raw HST/WFC3 images taken with the G102 grism. Spatial scan and staring mode data are shown in the top and bottom panels, respectively. The images are cutouts from a 256 × 256 pixel subarray. The spectrum of WASP-12A’s binary companion is visible in the bottom panel near row 10.