Transiting Planets Proceedings IAU Symposium No. 253, 2008 Fr´ ed´ eric Pont, Dimitar Sasselov & Matthews Holman, eds. c  2009 International Astronomical Union doi:10.1017/S1743921308027105 Ground-based detections of sodium in HD 209458b’s atmosphere in two data sets S. Albrecht and I. Snellen and E. de Mooij and R. Le Poole Leiden Observatory, University of Leiden, P.O. Box 9513, NL-2300 RA Leiden, The Netherlands email: albrecht@strw.leidenuniv.nl Abstract. We present two separate ground-based detections of sodium in the transmission spectrum of HD 209458b. First we reanalyzed an archival data set from the HDS spectrograph on Subaru, which shows sodium at a > 5σ level. Secondly, our preliminary results of a UVES/VLT data set indicate sodium absorption at a similar level, although the data cover the eclipse only partially. Both results are fully consistent with the HST results of Charbonneau et al. (2002). The Na D absorption feature seems to be resolved in the narrowest passband. 1. Introduction Planetary atmospheres are expected to absorb in a wavelength-dependent manner. A stellar spectrum observed through the atmosphere of a transiting planet carries the spectral imprint of the atomic and molecular species in that atmosphere. Comparing in-transit with out-of-transit spectra of the host star can therefore reveal information about the composition and structure of the planet’s atmosphere (e.g. Seager & Sasselov 2000). For hot jupiters two prominent absorption features are expected in the visible wavelength band (Brown 2001); sodium absorption around the Na D lines (5889 ˚ A and 5895 ˚ A), and absorption by potassium (7665 ˚ A and 7699 ˚ A). Charbonneau et al. (2002) were the first to detect the sodium feature in the transmission spectrum of an extra solar planet, HD 209458b. Recently, Redfield et al. (2008) detected sodium absorption by the atmosphere of HD 189733b via transmission spectroscopy from the ground. In this paper we present two independent data sets which provide the first ground-based detection of sodium in the transmission spectrum of HD 209458b. First we will summarize the reanalysis and results of an archival data set from HDS/Subaru. The results have been recently published by Snellen et al. (2008). Subsequently, we will present our preliminary analysis and results of our UVES/VLT data set. 2. HDS Subaru results HD 209458 was observed on the night of October 24, 2002, using HDS on the Subaru telescope. The observations have been described in Winn et al. (2004) and Narita et al. (2005). We concentrated on the 30 spectra from the red CCD (5500 ˚ A <λ< 6800 ˚ A), which were made with an exposure time of 500 seconds, a resolution of R ≈ 45 000, a SN ratio between 300 - 400 in the continuum, and of which nine where obtained before ingress and three after egress†. After initial reduction and correction for the change in the blaze function we selected 59 strong stellar lines and the Na D doublet to perform our analysis. First, for each line, the line center was determined through Gaussian fitting. † For a more in depth description of the data set see Winn et al. (2004), Narita et al. (2005), and Snellen et al. (2008). 520 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921308027105 Downloaded from https://www.cambridge.org/core. IP address: 3.235.21.12, on 21 May 2020 at 15:21:25, subject to the Cambridge Core terms of use, available at