Tighter constraints on the atmosphere of GJ 436 b from combined high-resolution CARMENES and CRIRES$^+$ observations

TL;DR

This study combines five CARMENES transits with one CRIRES$^+$ transit to push the limits of high-resolution atmospheric characterization for the warm Neptune GJ 436 b. Using cross-correlation with model transmission spectra, injection-recovery tests, and Bayesian retrievals, the authors find no detectable molecular features in the planet’s atmosphere, consistent with cloud decks at high altitudes or extreme metallicity. The joint data imply either a high-altitude cloud layer around $\sim$1 mbar or metallicities $\gtrsim 900\times$ solar that mute spectral features, and they predict substantial gains with ELT/ANDES, potentially unveiling H$_2$O in cloudy or moderately metal-rich regimes. The results establish the most stringent ground-based constraints to date for GJ 436 b and highlight the remaining observational limits and the promising capabilities of next-generation facilities for sub-Neptune atmospheres.

Abstract

We aim to study the atmospheric properties of the warm Neptune GJ 436 b by combining a set of five transit events observed with the CARMENES spectrograph with one transit from CRIRES$^+$ so as to provide the most constrained results possible at high resolution. We removed telluric and stellar signals from the data using SysRem and potential planetary signals were investigated using the cross-correlation technique. Following standard procedures for undetected species, we performed injection recovery tests and Bayesian retrievals to place constraints on the detectability of the main near-infrared absorbers. In addition, we simulated ELT/ANDES observations by computing end-to-end in silico datasets with EXoPLORE. No molecular signals were detected in the atmosphere of GJ 436 b, which is consistent with previous studies. Combined CARMENES-CRIRES$^+$ injection-recovery and Bayesian retrieval analyses show that the atmosphere is likely covered by high-altitude clouds ($\sim$ $1$ mbar) at low and intermediate metallicities or, alternatively, is very metal-rich ($\gtrsim$ $900\times$ solar), which would suppress spectral features without invoking clouds. Simulations of ELT/ANDES observations suggest a boost by nearly an order of magnitude to the upper limit in the photon-limited regime, reaching $0.1$ mbar at $10$-$300\times$ solar metallicities. The joint analysis of all useful transit observations from CARMENES and CRIRES$^+$ provides the most stringent constraints to date on the atmospheric properties of GJ 436 b. Complementary CCF-based and retrieval approaches consistently indicate that the atmosphere is either cloudy or highly metal enriched. Any weak near-infrared absorption lines, if present, are likely to be below current detection limits. However, according to our simulations, these features may be revealed with ELT/ANDES even in single-transit observations.

Figures (22)

• Figure 1: Spectral coverage of water vapour transmission models for GJ 436 b of the CARMENES NIR channel (grey) and the $\mathrm{CRIRES}^{+}$ H1567 setting (red). Similar figures for CH$_4$ and CO can be found in Appendix \ref{['fig:range_comparison_ch4_co']}.
• Figure 2: Evolution of the airmass, relative humidity, and mean S/N per spectrum over the entire CARMENES NIR range. The orange shaded area marks the in-transit times. Excluded spectra are represented with open symbols.
• Figure 3: Schematic of transmission spectroscopy observations of GJ 436 system used in this work. The orbital phase coverage for the different transits is shown in the lower part.
• Figure 4: Steps of the data preparation applied to a representative spectral region of the dataset obtained on the night of February $04$, $2017$. The vertical axis in panel A represents flux in arbitrary units, while in all other panels it represents time, expressed as 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 CARACAL, where major S/N differences are observed between the spectra (horizontal stripes), and prominent telluric H$_2$O emission can be identified. Panel C: Normalized spectra, where a $3\sigma$ clipping has been applied to filter out outliers and where telluric masks have been applied (white, excluded pixels). All spectra are now normalized to a common continuum. Panel D: Resulting spectra after one SysRem pass, where major residuals can be observed, especially from emission lines. Panel E: Spectra after nine SysRem passes, where most of the telluric contribution has been removed and any exoplanet signal, if present, is buried in the noise.
• Figure 5: Transit depth of H$_2$O for GJ 436 b generated with pRT as a function of cloud deck pressure level (top panel, fixed metallicity of $10\times$ solar) and atmospheric metallicity (bottom panel, clear atmosphere) as multiples of solar. For clarity, an offset of $0.002$ in transit depth was applied to the transmission models in both panels. Clouds at higher altitudes mute more H$_2$O lines.