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Detection of CO$_2$, CO, and H$_2$O in the atmosphere of the warm sub-Saturn HAT-P-12b

N. Crouzet, B. Edwards, T. Konings, J. Bouwman, M. Min, P. -O. Lagage, L. B. F. M. Waters, J. P. Pye, L. Heinke, M. Guedel, Th. Henning, B. Vandenbussche, O. Absil, I. Argyriou, D. Barrado, A. Boccaletti, C. Cossou, A. Coulais, L. Decin, R. Gastaud, A. Glasse, A. M. Glauser, F. Lahuis, G. Olofsson, P. Patapis, D. Rouan, P. Royer, N. Whiteford, L. Colina, G. Östlin, T. P. Ray, E. F. van Dishoeck

Abstract

The chemical composition of warm gas giant exoplanet atmospheres (with Teq < 1000 K) is not well known due to the lack of observational constraints. HAT-P-12 b is a warm, sub-Saturn-mass transiting exoplanet that is ideal for transmission spectroscopy. One transit of HAT-P-12 b was observed with JWST NIRSpec in the 2.87--5.10 $μ$m range with a resolving power of $\sim$1000. The JWST data are combined with archival observations from HST WFC3 covering the 1.1--1.7 $μ$m range. The data were analysed using two data reduction pipelines and two atmospheric retrieval tools. Atmospheric simulations using chemical forward models were performed. CO2, CO, and H2O are detected at 12.2, 4.1, and 6.0 $σ$ confidence, respectively. Their volume mixing ratios are consistent with an atmosphere of $\sim10\times$ solar metallicity and production of CO2 by photochemistry. CH4 is not detected and seems to be lacking, which could be due to a high intrinsic temperature with strong vertical mixing or other phenomena. SO2 is also not detected and its production seems limited by low upper atmosphere temperatures ($\sim$500 K at $P<10^{-3}$ bar derived from one-dimensional retrievals), insufficient to produce it in detectable quantities ($\gtrsim$ 800 K required according to photochemical models). Retrievals indicate the presence of clouds between 2 and 269 mbar. This study points towards an atmosphere for HAT-P-12 b that could be enriched in carbon and oxygen with respect to its host star. When including the production of CO2 via photochemistry, an atmospheric metallicity that is close to Saturn's can explain the observations. Metallicities inferred for other gas giant exoplanets based on their CO2 mixing ratios may need to account for its photochemical production pathways. This may impact studies on mass-metallicity trends and links between exoplanet atmospheres, interiors, and formation history.

Detection of CO$_2$, CO, and H$_2$O in the atmosphere of the warm sub-Saturn HAT-P-12b

Abstract

The chemical composition of warm gas giant exoplanet atmospheres (with Teq < 1000 K) is not well known due to the lack of observational constraints. HAT-P-12 b is a warm, sub-Saturn-mass transiting exoplanet that is ideal for transmission spectroscopy. One transit of HAT-P-12 b was observed with JWST NIRSpec in the 2.87--5.10 m range with a resolving power of 1000. The JWST data are combined with archival observations from HST WFC3 covering the 1.1--1.7 m range. The data were analysed using two data reduction pipelines and two atmospheric retrieval tools. Atmospheric simulations using chemical forward models were performed. CO2, CO, and H2O are detected at 12.2, 4.1, and 6.0 confidence, respectively. Their volume mixing ratios are consistent with an atmosphere of solar metallicity and production of CO2 by photochemistry. CH4 is not detected and seems to be lacking, which could be due to a high intrinsic temperature with strong vertical mixing or other phenomena. SO2 is also not detected and its production seems limited by low upper atmosphere temperatures (500 K at bar derived from one-dimensional retrievals), insufficient to produce it in detectable quantities ( 800 K required according to photochemical models). Retrievals indicate the presence of clouds between 2 and 269 mbar. This study points towards an atmosphere for HAT-P-12 b that could be enriched in carbon and oxygen with respect to its host star. When including the production of CO2 via photochemistry, an atmospheric metallicity that is close to Saturn's can explain the observations. Metallicities inferred for other gas giant exoplanets based on their CO2 mixing ratios may need to account for its photochemical production pathways. This may impact studies on mass-metallicity trends and links between exoplanet atmospheres, interiors, and formation history.

Paper Structure

This paper contains 36 sections, 1 equation, 16 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Data reduction
  4. JWST NIRSpec reduction
  5. TEATRO reduction
  6. CASCADe reduction
  7. JWST NIRSpec light curve fits
  8. TEATRO light curve fits
  9. CASCADe light curve fits
  10. HST WFC3 data reduction and light curve fits
  11. Atmospheric modelling
  12. Retrieval models
  13. Forward models
  14. Results
  15. JWST NIRSpec light curves
  16. ...and 21 more sections

Figures (16)

  • Figure 1: Transit white light curve of HAT-P-12 b obtained with JWST NIRSpec G395M (black points) and best-fit model (orange line). The residuals are shown in the bottom panel.
  • Figure 2: Sample of ten light curves out of 117 distributed over the $2.85-5.17 \; \mu$m wavelength range. Each light curve covers a $0.02 \; \mu$m wavelength bin. Left: Transit light curves (coloured points) and best-fit models (black lines). Right: Residuals. The central wavelength of each bin is indicated on the right. Systematic trends have not been removed in the left panel and are extremely small. These light curves were obtained with the TEATRO reduction. The light curves are offset for clarity.
  • Figure 3: Transmission spectrum of HAT-P-12 b obtained with JWST NIRSpec G395M from our reductions with TEATRO (black) and CASCADe (purple). No vertical offset has been applied to the spectra.
  • Figure 4: Left panel: Transit depth difference between the CASCADe and TEATRO data reductions expressed in number of standard deviations $\sigma$ as computed from Eq. (\ref{['eq:spectrum difference']}), and best linear fit. Right panel: Histogram of that distribution, and best Gaussian fit.
  • Figure 5: Transmission spectrum of HAT-P-12 b obtained with HST WFC3 G141 using our CASCADe reduction. The top panel shows the spectra for the two observed transits (blue squares) and the combined spectrum binned to a uniform spectral resolution of 0.02 µ m (black dots). The bottom panel shows a comparison of the spectrum derived in this study (blue dots) and the one derived by tsiaras_pop (orange squares). In both panels, the band-averaged transit depth is indicated by the dashed horizontal line, and the shaded area represents its 95% confidence interval. The spectrum from tsiaras_pop has been shifted downwards by 150 ppm to the same mean transit depth as found by CASCADe, for better comparison. The y-axis on the right is in units of planetary atmospheric scale height assuming a hydrogen-dominated atmosphere and an equilibrium temperature of $\sim$500 K as derived from the ARCiS retrieval.
  • ...and 11 more figures