A He I upper atmosphere around the warm Neptune GJ 3470 b

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High resolution transit spectroscopy has proven to be a reliable technique for the characterization of the chemical composition of exoplanet atmospheres. Taking advantage of the broad spectral coverage of the CARMENES spectrograph, we initiated a survey aimed at characterizing a broad range of planetary systems. Here, we report our observations of three transits of GJ 3470 b with CARMENES in search of He (23S) absorption. On one of the nights, the He I region was heavily contaminated by OH− telluric emission and, thus, it was not useful for our purposes. The remaining two nights had a very different signal-to-noise ratio (S/N) due to weather. They both indicate the presence of He (2^(3)S) absorption in the transmission spectrum of GJ 3470 b , although a statistically valid detection can only be claimed for the night with higher S/N. For that night, we retrieved a 1.5 ± 0.3% absorption depth, translating into a R_(p)(λ)/R_(p) = 1.15 ± 0.14 at this wavelength. Spectro-photometric light curves for this same night also indicate the presence of extra absorption during the planetary transit with a consistent absorption depth. The He (2^(3)S) absorption is modeled in detail using a radiative transfer code, and the results of our modeling efforts are compared to the observations. We find that the mass-loss rate, Ṁ , is confined to a range of 3 × 10^(10) g s^(−1) for T = 6000 K to 10 × 10^(10) g s^(−1) for T = 9000 K. We discuss the physical mechanisms and implications of the He I detection in GJ 3470 b and put it in context as compared to similar detections and non-detections in other Neptune-size planets. We also present improved stellar and planetary parameter determinations based on our visible and near-infrared observations.
© ESO 2020. Artículo firmado por 27 autores. CARMENES is an instrument for the Centro Astronómico Hispano-Alemán (CAHA) at Calar Alto (Almería, Spain), operated jointly by the Junta de Andalucía and the Instituto de Astrofísica de Andalucía (CSIC). CARMENES was funded by the German Max-Planck-Gesellschaft (MPG), the Spanish Consejo Superior de Investigaciones Científicas (CSIC), the European Union through FEDER/ERF FICTS-2011-02 funds, and the members of the CARMENES Consortium. We acknowledge financial support from the Agencia Estatal de Investigación of the Ministerio de Ciencia, Innovación y Universidades and the European FEDER/ERF funds through projects ESP2016-80435-C2-2-R, ESP2016-76076-R, and BES-2015-074542, and AYA2016-79425-C3-1/2/3-P, the Deutsche Forschungsgemeinschaft through the Research Unit FOR2544 “Blue Planets around Red Stars” and the Priority Program SPP 1992 “Exploring the Diversity of Extrasolar Planets” RE 1664/16-1, the National Natural Science Foundation of China through grants 11503088, 11573073, and 11573075, and the Natural Science Foundation of Jiangsu Province through grant BK20190110. Finally, we wish to thank Dr. Vincent Bourrier and an anonymous referee for discussion and comments that helped to improve the contents of this manuscript.
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