Discriminating between hazy and clear hot-Jupiter atmospheres with CARMENES

¹ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA, Leiden, The Netherlands
e-mail: alexsl@strw.leidenuniv.nl
² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁴ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, 38026 La Laguna, Tenerife, Spain
⁶ Department of Physics, Ariel University, Ariel 40700, Israel
⁷ Centro de Astrobiología (CSIC-INTA), ESAC, Camino bajo del castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁸ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁹ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
¹⁰ Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, c/ de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
¹¹ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
¹² Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
¹³ Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Almería, Spain
¹⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁵ Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁶ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain

Received: 10 June 2020
Accepted: 28 August 2020

Abstract

Context. Relatively large radii of some hot Jupiters observed in the ultraviolet and blue-optical are generally interpreted to be due to Rayleigh scattering by high-altitude haze particles. However, the haze composition and its production mechanisms are not fully understood, and observational information is still limited.

Aims. We aim to study the presence of hazes in the atmospheres of HD 209458 b and HD 189733 b with high spectral resolution spectra by analysing the strength of water vapour cross-correlation signals across the red optical and near-infrared wavelength ranges.

Methods. A total of seven transits of the two planets were observed with the CARMENES spectrograph at the 3.5 m Calar Alto telescope. Their Doppler-shifted signals were disentangled from the telluric and stellar contributions using the detrending algorithm SYSREM. The residual spectra were subsequently cross-correlated with water vapour templates at 0.70–0.96 μm to measure the strength of the water vapour absorption bands.

Results. The optical water vapour bands were detected at 5.2σ in HD 209458 b in one transit, whereas no evidence of them was found in four transits of HD 189733 b. Therefore, the relative strength of the optical water bands compared to those in the near-infrared were found to be larger in HD 209458 b than in HD 189733 b.

Conclusions. We interpret the non-detection of optical water bands in the transmission spectra of HD 189733 b, compared to the detection in HD 209458 b, to be due to the presence of high-altitude hazes in the former planet, which are largely absent in the latter. This is consistent with previous measurements with the Hubble Space Telescope. We show that currently available CARMENES observations of hot Jupiters can be used to investigate the presence of haze extinction in their atmospheres.