Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems. VIII. Molecular Mapping Performance with JWST/MIRI MRS: VHS 1256 b as a case study

Mathilde Mâlin, Anthony Boccaletti, Benjamin Charnay, Laurent Pueyo, Alexis Bidot, Polychronis Patapis, Sasha Hinkley, Simon Petrus, Niall Whiteford, Marshall Perrin, Beth A. Biller, Gabriele Cugno, Thayne Currie, Camilla Danielski, Thomas Henning, Kielan K. W. Hoch, Markus Janson, Jens Kammerer, Elisabeth C Matthews, Evert Nasedkin, Paulina Palma-Bifani, Isabel Rebollido, Matthias Samland, Andrew Skemer, Jordan M. Stone, Genaro Suárez, Ben J. Sutlieff, Motohide Tamura, Christopher A. Theissen, Johanna M. Vos, Zhoujian Zhang, Alice Zurlo

Abstract

VHS 1256 b was the first planetary-mass companion to be observed with the James Webb Space Telescope's Mid-Infrared Instrument (JWST/MIRI) using the Medium-Resolution Spectrometer (MRS). The MRS provides high-quality integral-field spectral data in the mid-infrared (IR) wavelengths from 4.9 to 18 um. This dataset serves as a testbed for applying cross-correlation techniques to characterize exoplanet atmospheres. We implement the so-called molecular mapping approach, which consists of performing a spectral cross-correlation between each spectral pixel and atmospheric model templates. We compare these results with those obtained from cross-correlation of the extracted spectrum. Using a self-consistent Exo-REM atmospheric model grid, we constrain the temperature, surface gravity, C/O ratio, and metallicity, finding values consistent with those obtained from other analysis methods. We detect CO (S/N $\sim$ 25) and H2O (S/N $\sim$ 76), with tentative detections of NH3 and CH4 (S/N$\sim$ 3). We test cross-correlation to measure trace-species abundances and isotopic ratios. We measure a volume mixing ratio of [NH3] =-5.73^{+0.15}_{-0.14} and an isotopic ratio $^{12}\mathrm{C}/^{13}\mathrm{C}=77.8^{+13}_{-10}$, both consistent with free-chemistry retrievals. The derived NH3 volume mixing ratio, combined with the measured temperature and radius, is consistent with VHS 1256 b having a mass above the deuterium-burning limit. These results demonstrate the diagnostic power of mid-IR spectroscopy and highlight cross-correlation as a robust method for characterizing directly imaged exoplanets, even in future higher-contrast regimes where spectral extraction becomes challenging. Future MIRI MRS observations across a wider range of temperatures and masses will further expand our understanding of planetary atmospheric chemistry.

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems. VIII. Molecular Mapping Performance with JWST/MIRI MRS: VHS 1256 b as a case study

Abstract

VHS 1256 b was the first planetary-mass companion to be observed with the James Webb Space Telescope's Mid-Infrared Instrument (JWST/MIRI) using the Medium-Resolution Spectrometer (MRS). The MRS provides high-quality integral-field spectral data in the mid-infrared (IR) wavelengths from 4.9 to 18 um. This dataset serves as a testbed for applying cross-correlation techniques to characterize exoplanet atmospheres. We implement the so-called molecular mapping approach, which consists of performing a spectral cross-correlation between each spectral pixel and atmospheric model templates. We compare these results with those obtained from cross-correlation of the extracted spectrum. Using a self-consistent Exo-REM atmospheric model grid, we constrain the temperature, surface gravity, C/O ratio, and metallicity, finding values consistent with those obtained from other analysis methods. We detect CO (S/N 25) and H2O (S/N 76), with tentative detections of NH3 and CH4 (S/N 3). We test cross-correlation to measure trace-species abundances and isotopic ratios. We measure a volume mixing ratio of [NH3] =-5.73^{+0.15}_{-0.14} and an isotopic ratio , both consistent with free-chemistry retrievals. The derived NH3 volume mixing ratio, combined with the measured temperature and radius, is consistent with VHS 1256 b having a mass above the deuterium-burning limit. These results demonstrate the diagnostic power of mid-IR spectroscopy and highlight cross-correlation as a robust method for characterizing directly imaged exoplanets, even in future higher-contrast regimes where spectral extraction becomes challenging. Future MIRI MRS observations across a wider range of temperatures and masses will further expand our understanding of planetary atmospheric chemistry.
Paper Structure (42 sections, 1 equation, 17 figures, 1 table)

This paper contains 42 sections, 1 equation, 17 figures, 1 table.

Table of Contents

  1. Introduction
  2. VHS 1256: a Rosetta Stone for giant exoplanet atmospheric characterization
  3. Molecular Mapping: a powerful tool to characterize exoplanets' atmospheres
  4. Objectives of this study
  5. Data and methods
  6. Observations and data reduction
  7. Spectral extraction
  8. Exo-REM
  9. Cross-correlation processing and detection metrics
  10. Signal-to-noise of detections
  11. Molecular mapping
  12. 1D cross-correlation
  13. Statistical significance between two models
  14. Molecular mapping applied to MIRI MRS data
  15. Correlation maps with the atmospheric template spectrum
  16. ...and 27 more sections

Figures (17)

  • Figure 1: Median cubes in the band A of each MRS channel. VHS 1256 b is identified by the arrow. The color scale is the same for each band.
  • Figure 2: Updated spectra for VHS 1256 b in comparison to previously published spectra miles_jwst_2023 in grey. The $S/N$ per wavelength is indicated in the bottom subplot.
  • Figure 3: Correlation maps in each MRS band. The red cross indicated the position of the centroid of the PSF, previously identified from the median cubes.
  • Figure 4: Correlation maps with molecular templates presented in the band where the detection is the highest. The position of VHS 1256 b measured in the cubes is indicated by the black cross.
  • Figure 5: Left: Continuum-subtracted spectrum of VHS 1256 b (black), shown with zooms on wavelength ranges corresponding to the features of the targeted molecules. The corresponding molecular templates are overplotted for comparison. Right: Cross-correlation functions with different molecular templates: H$_2$O, CO, NH$_3$, and CH$_4$, presented in the bands for which the $S/N$ is the highest. The autocorrelation functions (ACF) are shown with dashed lines.
  • ...and 12 more figures