The widest broadband transmission spectrum (0.38–1.71 μm) of HD 189733b from ground-based chromatic Rossiter–McLaughlin observations

¹ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
e-mail: moshagh@iac.es
² Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
³ Institut für Astrophysik, Georg-August Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁵ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, c/ de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
⁶ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
⁷ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁹ University of Bern, Center for Space and Habitability, Gesellschaftsstrasse 6, CH-3012, Bern, Switzerland
¹⁰ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
¹¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹² Centro de Astrobiología (CSIC-INTA), ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹³ Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Almería, Spain
¹⁴ 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
¹⁵ Centro de Astrobiología (CSIC-INTA), Crta. Ajalvir km 4, Torrejón de Ardoz, Madrid, Spain

Received: 19 August 2020
Accepted: 17 September 2020

Abstract

Multiband photometric transit observations (spectro-photometric) have been used mostly so far to retrieve broadband transmission spectra of transiting exoplanets in order to study their atmospheres. An alternative method was proposed, and has only been used once, to recover broadband transmission spectra using chromatic Rossiter–McLaughlin observations. We use the chromatic Rossiter–McLaughlin technique on archival and new observational data obtained with the HARPS and CARMENES instruments to retrieve transmission spectra of HD 189733b. The combined results cover the widest retrieved broadband transmission spectrum of an exoplanet obtained from ground-based observation. Our retrieved spectrum in the visible wavelength range shows the signature of a hazy atmosphere, and also includes an indication for the presence of sodium and potassium. These findings all agree with previous studies. The combined visible and near-infrared transmission spectrum exhibits a strong steep slope that may have several origins, such as a super-Rayleigh slope in the atmosphere of HD 189733b, an unknown systematic instrumental offset between the visible and near-infrared, or a strong stellar activity contamination. The host star is indeed known to be very active and might easily generate spurious features in the retrieved transmission spectra. Using our CARMENES observations, we assessed this scenario and place an informative constraint on some properties of the active regions of HD 189733. We demonstrate that the presence of starspots on HD 189733 can easily explain our observed strong slope in the broadband transmission spectrum.