Water vapor detection in the transmission spectra of HD 209458 b with the CARMENES NIR channel

Abstract

Aims. We aim at detecting water vapor in the atmosphere of the hot Jupiter HD 209458 b and perform a multi-band study in the near infrared with CARMENES. Methods. The water vapor absorption lines from the atmosphere of the planet 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 shift to remove the telluric and stellar lines using Sysrem, which performs a principal component analysis including proper error propagation. The residual spectra contain the signal from thousands of planetary molecular lines well below the noise level. We retrieve the information from those lines by cross-correlating the residual spectra with models of the atmospheric absorption of the planet. Results. We find a cross-correlation signal with a signal-to-noise ratio (S/N) of 6.4, revealing H 2 O in HD 209458 b. We obtain a net blueshift of the signal of -5.2 +2.6 −1.3 km s −1 that, despite the large error bars, is a firm indication of day-to night-side winds at the terminator of this hot Jupiter. Additionally, we performed a multi-band study for the detection of H 2 O individually from the three near infrared bands covered by CARMENES. We detect H 2 O from its 0.96-1.06 µm band with a S/N of 5.8, and also find hints of a detection from the 1.06-1.26 µm band, with a low S/N of 2.8. No clear planetary signal is found from the 1.26-1.62 µm band. Conclusions. Our significant H 2 O signal at 0.96-1.06 µm in HD 209458 b represents the first detection of H 2 O from this band individually, 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. H 2 O is detected from the 0.96-1.06 µm band in HD 209458 b, but hardly in HD 189733 b, which supports a stronger aerosol extinction in the latter, in line with previous studies. Future data gathered at more stable conditions and with larger S/N at both optical and near-infrared wavelengths could help to characterize the presence of aerosols in HD 209458 b and other planets.

Searching for thermal inversion agents in the transmission spectrum of KELT-20b/MASCARA-2b: detection of neutral iron and ionised calcium H&K lines

“…The optimisation could be performed by injecting weak artificial planetary signals at the expected velocity and choosing the number of iterations that give the highest S/N for each order (e.g. Brogi et al 2013;Birkby et al 2013Birkby et al , 2017Nugroho et al 2017;Sánchez-López et al 2019). However, this could potentially bias the optimisation to recover the injected signal only at a specific velocity and/or for a specific model, as Cabot et al (2019) have shown.…”

“…This has become one of the most robust approaches in the attempt to characterize the atmospheres of exoplanets (e.g. Redfield et al 2008;Snellen et al 2008Snellen et al , 2010Jensen et al 2011;Brogi et al 2012Brogi et al , 2013Birkby et al 2013;Brogi et al 2014;Lockwood et al 2014;Snellen et al 2014;Hoeijmakers et al 2015;Schwarz et al 2015;Wyttenbach et al 2015;Birkby et al 2017;Brogi et al 2018;Hawker et al 2018;Pino et al 2018;Wang et al 2018;Alonso-Floriano et al 2019;Brogi & Line 2019;Hoeijmakers et al 2019;Sánchez-López et al 2019;Gibson et al 2020;Merritt et al 2020). Using this technique, Nugroho et al (2017) directly detected a high-resolution emission signature of TiO on the dayside of WASP-33b using the Subaru telescope, providing simultaneous evidence of the existence of both this molecule and a hot stratosphere in the atmosphere of the planet.…”

Detection of Fe I and Fe II in the atmosphere of MASCARA-2b using a cross-correlation method

“…As described inBirkby et al (2017),Brogi et al (2018),Alonso-Floriano et al (2019) andSánchez-López et al (2019) we checked the significance of detected signals by calculating its SNR. To do this, we divided the co-added cross-correlation values by the standard deviation away from the expected signal (0 km s −1 ) in the region from -50 km s −1 to -150 km s −1 and from 50 km s −1 to 150 km s −1 .…”

Is there Na I in the atmosphere of HD 209458b?

“…In this study we only use the optical observations. The observations were carried out under programs H16-3.5-24, H16-3.5-22, and H18-3.5-22, and the infrared data were studied by Sánchez-López et al (2019) and Alonso-Floriano et al (2019). The observing strategy was the same as for the HARPS-N observations.…”