Tighter constraints on the atmosphere of GJ 436 b from combined high-resolution CARMENES and CRIRES$^+$ observations
Authors
A. Peláez-Torres
A. Sánchez-López
L. Nortmann
M. López-Puertas
E. González-Álvarez
H. M. Tabernero
C. Jiang
D. Revilla
G. Morello
J. Orell-Miquel
E. Pallé
P. J. Amado
J. A. Caballero
I. Ribas
A. Reiners
A. Quirrenbach
D. Cont
S. Dreizler
A. Fernández-Martín
A. P. Hatzes
Th. Henning
F. Lesjak
D. Montes
A. Schweizer
T. Trifonov
F. Yan
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.
