Theoretical Spectral Models of the Planet HD 209458b with a Thermal Inversion and Water Emission Bands

Abstract

We find that a theoretical fit to all the HD 209458b data at secondary eclipse requires that the day-side atmosphere of HD 209458b have a thermal inversion and a stratosphere. This inversion is caused by the capture of optical stellar flux by an absorber of uncertain origin that resides at altitude. One consequence of stratospheric heating and temperature inversion is the flipping of water absorption features into emission features from the near-to the mid-infrared, and we see evidence of such a water emission feature in the recent HD 209458b IRAC data of Knutson et al. In addition, an upper-atmosphere optical absorber may help explain both the weaker-thanexpected Na D feature seen in transit and the fact that the transit radius at 24 mm is smaller than the corresponding radius in the optical. Moreover, it may be a factor in why HD 209458b's optical transit radius is as large as it is. We speculate on the nature of this absorber and the planets whose atmospheres may, or may not, be affected by its presence.

Spectroscopy of planetary atmospheres in our Galaxy

“…On a large scale, the transmission spectra of hot-Jupiters seem to be dominated by the signature of water vapour (Barman 2007(Barman , 2008Beaulieu et al 2010;Burrows et al 2007Burrows et al , 2008Burrows et al , 2010Charbonneau et al 2008;Grillmair et al 2008;Knutson et al 2008;Madhusudhan and Seager (2009);Tinetti et al 2007aTinetti et al , 2007b, whereas warm Neptunes, such as GJ 436b and GJ 3470b, are expected to be methane-rich (Beaulieu et al 2011;Fukui et al 2013). The analysis of GJ 436b cannot be considered conclusive, though, given the activity of the star (Knutson et al 2011) and the lack of spectroscopic data: only photometric data, often recorded at different times, are available for this target.…”

Characterizing the Atmospheres of Hot Jupiters: From Spectra to Multi-Color Maps

“…Similarly, it has been suggested , Burrows et al 2007, Fortney et al 2008) that the presence of gas-phase TiO or VO in the atmosphere might also provide the necessary opacity. These compounds are predicted to have condensed out of the relatively cool atmospheres of HD 189733b and TrES-1, whereas they might still remain in gas phase in the hottest regions on the day side of HD 209458b.…”

“…The black line corresponds to the default model (no temperature inversion) with a redistribution parameter Pn = 0.1, which describes the case where 10% of the incident energy is redistributed to the night side. The grey line correspond to a model with Pn = 0.1 and an additional optical absorber at high altitudes, which produces a thermal inversion around pressures of 0.001 bar (Burrows et al 2007(Burrows et al , 2008. Squares show the values for these models after integrating over the Spitzer bandpasses.…”

“…2) from Knutson et al (2008c). These models employ the same formalism described 258 H. A. Knutson in Burrows et al (2007Burrows et al ( , 2008, which uses a dimensionless redistribution parameter P n that, in approximate fashion, accounts for the cooling of the dayside and the warming of the nightside by zonal winds near an optical depth of order unity. The larger P n is, the cooler is the dayside atmosphere and the lower the emergent planetary flux at superior conjunction.…”

Transiting Exoplanets with JWST

“…Other notable exoplanet atmosphere discoveries by Spitzer come from spectrophotometry and include discovery of a temperature inversion on the day side of HD 209458b [32,33] and a tentative detection of water vapor in transmission spectra during primary transit [34,35].…”