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/S1743921308027130 Absorption Spectra of the Prototype Hot-Jupiters: determination of atmospheric constituents and structure David K. Sing 1 , A. Lecavelier 1 , J.-M. D´ esert 1 , A. Vidal-Madjar 1 , & G. Ballester 2 1 Institut d’Astrophysique de Paris, CNRS; Universit´ e Pierre et Marie Curie, 98 bis bv Arago, F-75014 Paris, France; sing@iap.fr 2 Lunar and Planetary Laboratory, University of Arizona, Sonett Space Science Building, Tucson, AZ 85721-0063, USA Abstract. The two prototype hot-Jupiter exoplanets HD209458b and HD189733b are currently offering an unprecedented view of their atmospheres. As discussed here, primary transit trans- mission spectra provide the opportunity to identify specific atomic and molecular species, de- termine their abundances, and recover temperature-pressure-altitude information. We present a reanalysis of existing HST/STIS data on HD209458b, providing a complete optical transmission spectrum. Analysis of this spectrum have revealed: (1) the planetary abundance of sodium which is ∼2X solar (2) a depletion of sodium at high altitudes due to condensation or ionization (3) Rayleigh scattering by H 2 (3) a high temperature at pressures of 10’s mbar consistent with the dayside inversion (4) a separate high-altitude hot temperature from the planet’s thermosphere and (5) likely absorption by TiO/VO. While HD209458b and HD189733b are currently the best candidates for these studies, another ∼10 exoplanets are good targets with today’s instruments for future transmission-based atmospheric detections. 1. Which Exoplanets Have Detectable Atmospheres During Transit? Primary transit transmission spectra probes absorption depth, which is a measure of planetary altitude, z, which has a wavelength dependance. Detecting atmospheric signatures become easier with: (1) brighter parent stars, as photometry becomes easier (2) deeper transit depths, which provides larger contrast/signal and (3) a larger atmosphere, resulting from lower surface gravities or higher temperatures providing large scale heights, H = kT/μg. The measured altitude depends upon the atmospheric composition as well as its physical state and can be approximated by, z(λ)= H ln  ξ abs P z =0 σ abs τ eq μg  2πR p H  (1.1) (see Lecavelier et al. 2008a) where ξ abs is the abundance of absorbing species, P z =0 is the zero altitude pressure, R p is the planetary radius, μ is the mean mass of the atmo- spheric particles, σ is the cross section and τ eq is the equivalent optical depth. A transit atmospheric transmission signal is dominated by the absorption directly at the plane- tary terminator over one scale height, as the line of sight column density drops rapidly both horizontally and vertically, mitigating the effects of both vertical and horizontal temperature gradients. With current instruments, only a sub-set of the known transiting planets will have detectable atmospheres, though the list is growing rapidly as detection surveys are be- coming ever more efficient. A plot of the transit signal of an atmospheric scale height vs. 532