Study of Hot-Jupiter atmospheres using infrared photometry

The study of upper atmospheres of Hot-Jupiters has burgeoned in the past two years. This thesis aims to detect thermal radiation from 3 unique Hot-Jupiters, discovered by the XO team, using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope in order to obtain broadband spectra in the infrared to constrain the upper atmospheric temperature structure of Hot-Jupiters. The infrared spectral energy distributions of 3 Hot-Jupiters XO-1b, XO-2b and XO-3b have revealed a presence of a thermally inverted stratosphere in all 3 of them. Since the 3 XO Hot-Jupiters occupy a wide range of substellar point fluxes (Fp ∼ 0.49--4.2 x 109 erg cm-2 s-1) it allows us to set a threshold bolometric substellar point flux on the planet above which the intense optical/UV light from the star drives a thermal inversion---a hot stratosphere.;Both XO-1b and XO-2b have substellar point fluxes below F p ∼ 1.0 x 109 erg cm-2 s-1 and yet their spectral energy distribution points to a thermally inverted hot stratosphere. XO-3b has a high substellar point flux and as predicted, possesses a thermal inversion as well. Puzzlingly the Hot-Jupiter HD-189733b, which is not in our study, has a similar substellar point flux like XO-1b and yet does not have a hot stratosphere. We thus suggest that other factors can determine the presence of hot stratospheres in Hot-Jupiters: the substellar point UV flux would be a more appropriate measure of planetary insolation than bolometric flux as upper atmospheric absorbers (TiO, VO and sulfur compounds H2 and HS) absorb in the optical/UV spectrum and furthermore dynamic effects in the atmosphere like equatorial jets and shocks as suggested by 3D global circulation models can induce stratospheres without extra upper atmospheric absorbers.;Obtaining day-side spectra of many more Hot-Jupiters with a wide range of substellar point flux either with existing Cold Spitzer observations in all 4 IRAC channels or during the Warm Spitzer mission could shed more light on why some Hot-Jupiters have hot stratospheres and how exactly they arise.;The timing centroids of the secondary eclipses of the 3 Hot-Jupiters have been used to refine their orbital eccentricity. The 3-sigma upper limit for XO-1b is excos(o) < 0.0036, where e is eccentricity and o is the argument of the periastron. Similarly we refined the 3-sigma upper limit on excos(o) < 0.012 for XO-2b. We refined the eccentricity of XO-3b using the weighted average of the timing centroids of our secondary eclipse measurements to e = 0.277 +/- 0.004. Radius-age trajectories suggest that if the age of XO-3b is t = 2.82+0.58-0.82 GYr and assuming solar metallicity, the increased tidal heating rate would require a lowered tidal dissipation parameter Qp ≲ 106. Even in the absence of an accurate parallax measurement the radius-age trajectory of XO-3b seems to imply that at least some amount tidal heating must be responsible for the inflated radius Rp = 1.22+0.07-0.07 RJup of XO-3b.