tokens: smaller than the weaker CO lines in the planet transmission spectrum. Furthermore, in Doppler space the nature of this spurious signal is substantially different from the planet absorption, and largely canceled prior to cross-correlation. Winn et al. ( 2007 ) demonstrated that HD 189733 is a slow rotator with a rotational period close to 13 days. Thus any configuration of spots can be considered static during one single transit observation (less than 2 hr). Compared to the observer, the star moves in radial velocity by only 0.2 km s −1 during the transit (mostly due to the change in barycentric velocity), which is only 1/15 of the full width half maximum of the CRIRES IP. This means that any distortion of the stellar lines caused by unocculted spots is static in both radial velocity and time. In this case our data analysis designed to remove telluric lines (Section 3.4 ) will also re-normalize residuals from uncorrected star spots, further dampening the already small signal. However, we know from past transit observations (Pont et al. 2007 , 2008 ) that HD 189733 b usually crosses some spots during transit. In this case, the distortion of the stellar line suddenly disappears (when the spot is occulted) and reappears, generating a pulse radial velocity signal in the data, uncorrected by our analysis. The duration of this pulse is dependent on the size of the crossed spot (or region of spots). Although probably unrealistic, we discuss the worst-case scenario in which all spots are clustered in a region of radius 0.1 R S (based on the 1% coverage and spots significantly darker than the photosphere). For an impact parameter of b = 0.66, the length of the chord crossed by the planet is 1.5 stellar radii. With 39 frames observed in transit, the planet crosses 0.038 stellar radii per frame. This unrealistically big group of spots would therefore affect 2.6 frames of our data. For a spot signal 1/10 of the planet signal, occulted star spots can only contaminate 8% of the detected CO signal, or based on the detection presented in Section 5.1 . We therefore conclude that—at least in the case of HD 189733 b—the contamination due to uncorrected star spots is negligible. 3.4. Removal of Telluric Lines Telluric lines change in depth during the observations, but not in position. Therefore, the telluric spectrum is most effectively removed by modeling the flux of each data column (i.e., each wavelength channel) in time. The main cause of telluric variation is the change in geometric airmass. Changes in the IP (due to, e.g., in-slit guiding, seeing, and performances of the adaptive optics), as well as changes in the water vapor content in the Earth’s atmosphere could also affect the flux in the core of telluric lines. We initially normalize each spectrum by its median continuum level. We then model the time-variation of the flux in each spectral channel via the simple functional form:where A is the geometric airmass, and we divide through the fit. The
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