Cloudy with a chance of metals: Indications of CO_2 in the atmosphere of GJ1214b from high-resolution K-band spectroscopy

Sub-Neptune extra-solar planets are abundant in the Milky Way, yet their atmospheric properties remain poorly understood. They frequently exhibit muted transmission spectra, with GJ,1214,b being the most prominent example. Following years of intense observing campaigns yielding featureless planetary spectra, more recent observations with JWST have revealed the first possible atmospheric signatures of H_2O, CH_4, and CO_2. We present high-resolution transmission spectroscopy of GJ,1214,b based on eight transits obtained with the CRIRES+$ spectrograph in the K band. We used to remove telluric and stellar signals from the data and searched for signatures of H_2O, CO, CH_4, H_2S, NH_3, and CO$_2 using the cross-correlation technique. SYSREM We obtained non-detections for the first five molecules and used injection recovery tests to derive upper limits on the atmosphere. For CO_2 we measure a cross-correlation signal at S/N ∼ 3.6, with a detailed investigation of the signal showing no obvious indication that it is caused by correlated noise. A Welch t-test confirmed that the in-trail, in-transit distribution is significantly different from the out-of-trail distribution at a 3.4 σ confidence. Interpreting the data using a Bayesian retrieval framework, with multiple molecular species and free chemistry, resulted in a retrieved planet temperature of T_ =398^ iso +283 _ -197 K, consistent with a value intermediate between the day- and night-side temperatures from JWST-derived temperature-pressure profiles at high altitudes, as expected for the planetary terminator. In addition, a metallicity of M/H $ = 0.48^ +0.89 _ -1.70 $ was derived from the abundances of the retrieved molecules, along with an opacity deck pressure of log_ 10 (P_ ) = -3.04^ c +2.52 _ -1.53 . A simpler equilibrium chemistry retrieval assuming CO_2 as the sole opacity source returned a compatible temperature, with smaller formal uncertainties (T_ =509^ iso +102 _ -59 K), slightly higher metallicity ( M/H $ =1.51^ +0.68 _ -0.75 $), and higher opacity deck pressure (log_ 10 (P_c) = -0.88^ +1.95 _ -2.48 ). While these sets of values correspond to relatively large signal amplitudes predicted for CO_2 features in the mid-infrared, they are compatible with JWST NIRSpec observations within the models' 1.5σ uncertainties. Further modelling and additional data are required to confirm the atmospheric signatures and obtain a comprehensive interpretation of low- and high-resolution data. Overall, our results support previous findings that CO_2 is likely to be a significant component of the atmosphere of GJ,1214,b.