Atmospheric constraints on GJ 1214 b from CRIRES+ and prospects for characterisation with ANDES

Abstract

In this study, we aim to constrain the atmospheric composition of GJ 1214 b using all available transits observed with the upgraded CRIRES+ spectrograph at the VLT by searching for the signatures of water vapour, methane, and carbon dioxide. We analysed eight CRIRES+ transit datasets covering the K band (1.90-2.45 microns) at a resolving power of R ~ 100,000. We used the SysRem algorithm to correct for telluric and stellar contributions and employed the cross-correlation technique with templates from petitRADTRANS to search for H2O, CH4, and CO2. Injection-recovery tests across a grid of metallicities (Z) and cloud-deck pressures (pc) were performed to quantify detection limits. We also generated predictions for ANDES observations using end-to-end simulated datasets with EXoPLORE. We detect no significant H2O, CH4, or CO2 signatures. Injection-recovery tests show that such non-detections exclude atmospheres with low-altitude clouds and moderate or low metallicities. CH4 yields the tightest empirical limits, with CO2 unexpectedly ruling out intermediate metallicities (~ 100xsolar) with clouds deeper due to its rapidly rising opacity in compressed, high-Z atmospheres. Our constraints are in line with either a high-Z or a high-altitude aerosol layer, in agreement with recent JWST inferences. The combined analysis of eight CRIRES+ datasets provides the most stringent high-resolution constraints on the atmospheric properties of GJ 1214 b to date. Simulations of a single transit observed with ANDES on the ELT predict modest improvements for H2O, a substantially expanded detectable region for CH4, and the strongest gains for CO2, making the latter a particularly effective tracer for characterising high-metallicity atmospheres in sub-Neptunes.

Figures (12)

• Figure 1: Evolution of the airmass, relative humidity, and mean S/N ratio per spectrum. The pink shaded area marks the in-transit times. Nodding A spectra are represented with empty symbols and Nodding B spectra with filled symbols.
• Figure 2: Steps of the data preparation applied to a representative spectral region of the dataset obtained on the night of March $31$, $2022$. The vertical axis in panel A represents flux in arbitrary units, while in all other panels it represents time, expressed as the planet's orbital phase. Panel A: Original spectrum observed at mid-transit, where telluric absorption lines from H$_2$O can be identified. Panel B: Original spectral matrix extracted using CR2RES, where major S/N differences between spectra (horizontal stripes) and prominent telluric H$_2$O absorption (vertical stripes) can be identified. Panel C: Normalised and masked spectra, where opaque telluric windows are excluded (white stripes). Panel D: Residual spectra after one SysRem pass, where telluric residuals can be observed. Panel E: Residual spectra after nine SysRem passes, where most of the telluric contribution has been removed. Nodding position effects were corrected in all panels by interpolating the pixel-wavelength solution from nodding position B to position A.
• Figure 3: Cross-correlation analysis of potential H$_2$O signals in the transmission spectrum of GJ 1214 b observed with CRIRES$^+$ in the near-infrared. The method is illustrated for the night of March $31$, $2022$. We show the cross-correlation matrix in the Earth's rest frame as a function of the velocity Doppler shifts applied to the template (horizontal axis) and the planet's orbital phase (vertical axis). The results were obtained by using a template with $10\times$ solar metallicity and a $10$ mbar cloud deck. All useful spectral orders were combined. White lines mark the transit start and end times, while the yellow lines indicate the expected exoplanet velocities with respect to Earth.
• Figure 4: Significance analysis of potential H$_2$O signals, illustrated for the night of March $31$, $2022$. Top panel: signal-to-noise ratio map as a function of radial velocity in the exoplanet's rest frame ($v_{\rm rest}$, horizontal axis) and the projected orbital velocity semi-amplitude, $K_{\rm p}$ (vertical axis). Horizontal lines mark the expected $K_{\rm p}$, and vertical lines show the zero rest-frame velocity. Bottom panel: $1$D cross-correlation function at the expected $K_{\rm p}$ ($97.1$ km s$^{-1}$) of GJ 1214 b.
• Figure 5: Cross-correlation maps in S/N units for potential atmospheric signals as a function of $v_{\rm rest}$ and $K_{\rm p}$ for the primary expected absorbing species in the atmosphere of GJ 1214 b. We explored, water vapour (left), methane (middle), and carbon dioxide (right). Horizontal and vertical red lines indicate the expected $K_{\rm p}$ and $v_{\rm rest}$. These example maps were derived using a $10\times$ solar metallicity template with a cloud deck at $10$ mbar. No molecular signals were detected in our cross-correlation analyses.