Water vapor detection in the transmission spectra of HD 209458 b with the CARMENES NIR channel
Abstract
Aims: We aim at detecting H2O in the atmosphere of the hot Jupiter HD 209458 b and perform a multi-band study in the near infrared with CARMENES. Methods: The H2O absorption lines from the planet's atmosphere are Doppler-shifted due to the large change in its radial velocity during transit. This shift is of the order of tens of km s-1, whilst the Earth's telluric and the stellar lines can be considered quasi-static. We took advantage of this to remove the telluric and stellar lines using SYSREM, a principal component analysis algorithm. The residual spectra contain the signal from thousands of planetary molecular lines well below the noise level. We retrieve this information by cross-correlating the spectra with models of the atmospheric absorption. Results: We find evidence of H2O in HD 209458 b with a signal-to-noise ratio (S/N) of 6.4. The signal is blueshifted by --5.2 +2.6-1.3 km s-1, which, despite the error bars, is a firm indication of day-to-night winds at the terminator of this hot Jupiter. Additionally, we performed a multi-band study for the detection of H2O individually from the three NIR bands covered by CARMENES. We detect H2O from its 1.0 μm band with a S/N of 5.8, and also find hints from the 1.15 μm band, with a low S/N of 2.8. No clear planetary signal is found from the 1.4 μm band. Conclusions: Our significant signal from the 1.0 μm band in HD 209458 b represents the first detection of H2O from this band, the bluest one to date. The unfavorable observational conditions might be the reason for the inconclusive detection from the stronger 1.15 and 1.4 μm bands. H2O is detected from the 1.0 μm band in HD 209458 b, but hardly in HD 189733 b, which supports a stronger aerosol extinction in the latter.