Abstract
ABSTRACT Using high-resolution ground-based transmission spectroscopy to probe exoplanetary atmospheres is difficult due to the inherent telluric contamination from absorption in Earth’s atmosphere. A variety of methods have previously been used to remove telluric features in the optical regime and calculate the planetary transmission spectrum. In this paper we present and compare two such methods, specifically focusing on Na detections using high-resolution optical transmission spectra: (1) calculating the telluric absorption empirically based on the airmass and (2) using a model of the Earth’s transmission spectrum. We test these methods on the transmission spectrum of the hot Jupiter HD 189733 b using archival data obtained with the HARPS spectrograph during three transits. Using models for Centre-to-Limb Variation and the Rossiter–McLaughlin effect, spurious signals which are imprinted within the transmission spectrum are reduced. We find that correcting tellurics with an atmospheric model of the Earth is more robust and produces consistent results when applied to data from different nights with changing atmospheric conditions. We confirm the detection of sodium in the atmosphere of HD 189733 b, with doublet line contrasts of $-0.64 \pm 0.07~{{\ \rm per\ cent}}$ (D2) and $-0.53 \pm 0.07~{{\ \rm per\ cent}}$ (D1). The average line contrast corresponds to an effective photosphere in the Na line located around 1.13 Rp. We also confirm an overall blueshift of the line centroids corresponding to net atmospheric eastward winds with a speed of 1.8 ± 1.2 km s−1. Our study highlights the importance of accurate telluric removal for consistent and reliable characterization of exoplanetary atmospheres using high-resolution transmission spectroscopy.
Highlights
The era of characterising exoplanet atmospheres has accelerated greatly over the last two decades
In this study we approached the challenge of removing telluric lines in high-resolution optical spectra by comparing two popular methods using archival High Accuracy Radial velocity Planet Searcher (HARPS) observations of HD 189733 b
The reference spectrum was used to scale the strength of telluric lines to a constant level as if they had been observed at the same airmass, and are removed during the subsequent transmission spectrum calculation
Summary
The era of characterising exoplanet atmospheres has accelerated greatly over the last two decades. A significant problem arises from the contamination of spectra due to absorption in the Earth’s atmosphere These telluric lines can merge with stellar lines, or appear at a similar or higher strength as features in the planetary transmission spectrum (Redfield et al 2008; Snellen et al 2008). A spectrum of absorption in the Earth’s atmosphere was recovered by using a stellar template to remove the weak stellar lines of the rapidly rotating hot B-star This was employed in subsequent studies (Jensen et al 2011, 2012). We choose to compare two popular methods: one which derives the telluric transmission solely from the data, and one which uses a model of molecular absorption in Earth’s atmosphere Both of these methods account for the variation of telluric line strength over the multiplehour observing window.
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