INFRARED TRANSMISSION SPECTROSCOPY OF THE EXOPLANETS HD 209458b AND XO-1b USING THE WIDE FIELD CAMERA-3 ON THEHUBBLE SPACE TELESCOPE

Abstract

Exoplanetary transmission spectroscopy in the near-infrared using Hubble/NICMOS is currently ambiguous because different observational groups claim different results from the same data, depending on their analysis methodologies. Spatial scanning with Hubble/WFC3 provides an opportunity to resolve this ambiguity. We here report WFC3 spectroscopy of the giant planets HD 209458b and XO-1b in transit, using spatial scanning mode for maximum photon-collecting efficiency. We introduce an analysis technique that derives the exoplanetary transmission spectrum without the necessity of explicitly decorrelating instrumental effects, and achieves nearly photon-limited precision even at the high flux levels collected in spatial scan mode. Our errors are within 6% (XO-1) and 26% (HD 209458b) of the photon-limit at a resolving power of λ/δλ ∼ 70, and are better than 0.01% per spectral channel. Both planets exhibit water absorption of approximately 200 ppm at the water peak near 1.38 µm. Our result for XO-1b contradicts the much larger absorption derived from NICMOS spectroscopy. The weak water absorption we measure for HD 209458b is reminiscent of the weakness of sodium absorption in the first transmission spectroscopy of an exoplanet atmosphere by Charbonneau et al. (2002). Model atmospheres having uniformly-distributed extra opacity of 0.012 cm 2 g −1 account approximately for both our water measurement and the sodium absorption. Our results for HD 209458b support the picture advocated by Pont et al. (2013) in which weak molecular absorptions are superposed on a transmission spectrum that is dominated by continuous opacity due to haze and/or dust. However, the extra opacity needed for HD 209458b is grayer than for HD 189733b, with a weaker Rayleigh component.

Spectroscopy of planetary atmospheres in our Galaxy

“…Knutson et al 2007;Barman 2007;Pont et al 2008;Sing et al 2011b). The hot-Jupiters XO-1b and XO-2b Crouzet et al 2012), WASP-12b ), HD209458b (Deming et al 2013) and HD189733b (Danielski et al 2012) show instead distinctive molecular features in the NIR spectral region (Fig. 6).…”

“…Left and centre: data observed with Spitzer IRAC and MIPS and interpretations (Beaulieu et al 2010;Burrows et al 2010). Right: transit spectrum observed with Hubble WFC3 (Deming et al 2013). Third row: NIR transit spectra recorded with NICMOS for XO1-b (left: and XO2-b (fourth row, Crouzet et al 2012) and interpretations.…”

“…Agol et al 2010;Beaulieu et al 2008Beaulieu et al , 2010Beaulieu et al , 2011Brown 2001;Burke et al 2010;Charbonneau et al 2005Charbonneau et al , 2008Deming et al 2013;Désert et al 2011;Grillmair et al 2008;Knutson et al 2007;Machalek et al 2009;Pont et al 2008;Sing et al 2011a;Stevenson et al 2010;Swain et al 2008Swain et al , 2009b. Parametric models approximate systematic noise via the use of auxiliary information of the instrument, the so-called optical state vectors, which often include the positional drifts of the star on the detector, the focus and the detector temperature changes, the positional angles of the telescope on the sky etc.…”

The EChO science case

“…Phase variations in the IR are important in understanding a planet's atmospheric dynamics and redistribution of absorbed stellar energy from their irradiated day-side to the night-side. Phase variations in the VIS are very useful to infer the cloud distribution [55]. (iv) Exoplanet mapping and meteorological monitoring: The combination of the three prime observational techniques utilized by EChO provides us with information from different parts of the planet atmosphere; from the terminator region via transit spectroscopy, from the day-side hemisphere via eclipse spectroscopy, and from the unilluminated night-side hemisphere using phase variations.…”

“…These were achieved with Spitzer [107]. See also [52,56,121] All three techniques have already been used very successfully from the optical to the near-and mid-infrared, showing molecular, atomic absorption and Rayleigh scattering features in transmission [21,25,36,47,48,55,86,89,101,132,147,148,160,[168][169][170]182] and/or emission spectra [38,74,91,156,159,161,171] of a few of the brightest and hottest transiting gas giants, using the Hubble and Spitzer space telescopes. In addition, infrared phase variations have been measured at several wavelengths using Spitzer, showing only a relatively small temperature difference (300 K) between the planet's day and night-side -implying an efficient redistribution of the absorbed stellar energy [86].…”