Transmission Spectra as Diagnostics of Extrasolar Giant Planet Atmospheres

By T. Brown. 2001

354 citations recorded by the source corpus.

Atmospheres of transiting extrasolar giant planets (EGPs) such as HD 209458b must impose features on the spectra of their parent stars during transits; these features contain information about the physical conditions and chemical composition of the atmospheres. The most convenient observational index showing these features is the "spectrum ratio" ℜ(λ), defined as the wavelength-dependent ratio of spectra taken in and out of transit. The principal source of structure in ℜ is the variation with wavelength of the height at which the EGP atmosphere first becomes opaque to tangential rays—one may think of the planet as having different radii, and hence different transit depths, at each wavelength. The characteristic depth of absorption lines in ℜ scales with the atmospheric scale height and with the logarithm of the opacity ratio between continuum and strong lines. For close-in EGPs, line depths of 10-3 relative to the stellar continuum can occur. The atmospheres of EGPs probably consist mostly of molecular species, including H2, CO, H2O, and CH4, while the illuminating flux is characteristic of a Sun-like star. Thus, the most useful diagnostics are likely to be the near-infrared bands of these molecules, and the visible/near-IR resonance lines of the alkali metals. I describe a model that estimates ℜ(λ) for EGPs with prescribed radius, mass, temperature structure, chemical composition, and cloud properties. This model assumes hydrostatic and chemical equilibrium in an atmosphere with chemistry involving only H, C, N, and O. Other elements (He, Na, K, Si) are included as nonreacting minor constituents. Opacity sources include Rayleigh scattering, the strongest lines of Na and K, collision-induced absorption by H2, scattering by cloud particles, and molecular lines of CO, H2O, and CH4. The model simulates Doppler shifts from height-dependent winds and from planetary rotation, and deals in a schematic way with photoionization of Na and K by the stellar UV flux. Using this model, I investigated the diagnostic potential of various spectral features for planets similar to HD 209458b. Clouds are the most important determinants of the depth of features in ℜ; they decrease the strength of all features as they reach higher in the atmosphere. The relative strengths of molecular lines provide diagnostics for the heavy-element abundance, temperature, and the vertical temperature structure, although diagnostics for different physical properties tend to be somewhat degenerate. Planetary rotation with likely periods leaves a clear signature on the line profiles, as do winds with speeds comparable to that of rotation. Successful use of these diagnostics will require spectral observations with signal-to-noise ratio (S/N) of 103 or better and resolving power R = λ/δλ ranging from 103 to 106, depending on the application. Because of these stringent demands, it will be important to evolve analysis methods that combine information from many lines into a few definitive diagnostic indices.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/c7d2a80f303f356f1f7dbad924e37357d2a75e7d

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In Astronomy and Astrophysics

1. N. Madhusudhan
2. Jon Fortney
3. A. Showman
4. M. Marley
5. D. Sing
6. Sara Seager
7. D. Deming
8. D. Charbonneau

In Atomic and Molecular Physics, and Optics

1. Jon Fortney
2. D. Sing
3. M. Marley
4. Sara Seager
5. Jonathan Tennyson
6. J. D'esert
7. J. Winn
8. R. Freedman

In Instrumentation

1. Jon Fortney
2. N. Madhusudhan
3. A. Showman
4. M. Marley
5. D. Sing
6. Sara Seager
7. H. Knutson
8. V. Parmentier

Citations in the corpus

• The Transit Light Source Effect: False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets (2017, background): “In the transiting exoplanet case, the incident light is measured by observing the disk-integrated stellar spectrum before the transit (e.g., Brown 2001), the assumption being that the disk-integrated spectrum is identical to the light incident on the planetary atmosphere.”
• Transits and Occultations (2010, methodology): “For more details on differential aperture photometry using an ensemble of comparison stars, see Gilliland & Brown (1988), Kjeldsen & Frandsen (1992), and Everett & Howell (2001).”
• TRANSMISSION SPECTRA OF THREE-DIMENSIONAL HOT JUPITER MODEL ATMOSPHERES (2009, background): “…after the detection of the transits of planet HD 209458b (Charbonneau et al. 2000; Henry et al. 2000), atmosphere modelers realized that transits could provide a probe of the atmospheric composition of transiting planet atmospheres (Seager & Sasselov 2000; Brown 2001; Hubbard et al. 2001).”
• The effect of condensates on the characterization of transiting planet atmospheres with transmission spectroscopy (2005, methodology): “The physics behind this process were laid out theoretically and modeled by Seager & Sasselov (2000), Brown (2001), and Hubbard et al. (2001).”
• Hubble Space Telescope STIS Optical Transit Transmission Spectra of the Hot Jupiter HD 209458b (2008, methodology): “This strong Na absorption is much closer to the original cloudless predictions (Seager & Sasselov 2000; Brown 2001; Hubbard et al. 2001), albeit with a more complicated Na line absorption profile (Sing et al. 2008).”
• A Gaussian process framework for modelling instrumental systematics: application to transmission spectroscopy (2011, background): “…planet is wavelength dependent, as the effective size of a planet’s occulting disk depends on the height in the atmosphere at which the planet becomes opaque to starlight, which in turn depends on the atomic and molecular species present in the atmosphere (e.g. Seager & Sasselov 2000; Brown 2001).”
• HST hot-Jupiter transmission spectral survey: evidence for aerosols and lack of TiO in the atmosphere of WASP-12b (2013, methodology): “This observing technique has proven to produce high signal-to-noise (S/N) spectra which are photometrically accurate near the Poisson limit during a transit event (e.g Brown 2001; Sing et al. 2011; Huitson et al. 2012).”
• Atmospheric circulation and tides of ``51 Pegasus b-like'' planets (2002, background): “Finally, transit observations of Pegasi planets using high resolution spectroscopy should in the near future yield constraints on the atmospheric temperature, cloud/haze abundance, and winds (Brown 2001; see also Seager & Sasselov 2000; Hubbard et al. 2001).”
• Detection of an Extrasolar Planet Atmosphere (2001, methodology): “Our current models (Brown 2001) account for the ionization of sodium by the large stellar flux incident upon the planetary atmosphere.”
• Hubble Space Telescope transmission spectroscopy of the exoplanet HD 189733b: high‐altitude atmospheric haze in the optical and near‐ultraviolet with STIS (2011, background): “The resulting photometric light curves exhibit all the expected systematic instrumental effects taken during similar high S/N transit observations before HST servicing mission four, as originally noted in Brown (2001).”
• QUANTITATIVELY ASSESSING THE ROLE OF CLOUDS IN THE TRANSMISSION SPECTRUM OF GJ 1214b (2013, background): “It has long been recognized that clouds could impact the transmission spectrum of transiting exoplanets as well (Seager & Sasselov 2000; Brown 2001; Hubbard et al. 2001).”
• THE FLAT TRANSMISSION SPECTRUM OF THE SUPER-EARTH GJ1214b FROM WIDE FIELD CAMERA 3 ON THE HUBBLE SPACE TELESCOPE (2011, background): “…in the transmission spectrum scale as nH×2HRp/R2?, where nH is set by the opacities involved and can be 1-10 for strong absorption features, H is the atmospheric scale height, Rp is the planetary radius, and R? is the stellar radius (e.g. Seager & Sasselov 2000; Brown 2001; Hubbard et al. 2001).”
• HOW TO DISTINGUISH BETWEEN CLOUDY MINI-NEPTUNES AND WATER/VOLATILE-DOMINATED SUPER-EARTHS (2013, background): “This is as a result of the grazing geometry in which the transmission spectrum is formed (Brown 2001), combined with the exponential decrease in gas density with altitude.”
• A Unified Theory for the Atmospheres of the Hot and Very Hot Jupiters: Two Classes of Irradiated Atmospheres (2007, background): “If the opacity on the limb is everywhere larger in the optical this would lead to weaker, narrower Na absorption peak than expected by the first-generation models (Seager & Sasselov 2000; Brown 2001; Hubbard et al. 2001).”
• Atmosphere, Interior, and Evolution of the Metal-rich Transiting Planet HD 149026b (2005, background): “However, stars HD 209458, OGLE-TR10, and TrES-1are all consistent with solar abundances (Brown 2001; Konacki et al. 2005; Sozzetti et al. 2004), although the planet radii differ by ∼0.3 RJ at similar plan- 7 Bodenheimer et al. (2003) and Laughlin et al. (2005) have computed evolution models with 40…”
• Infrared Transmission Spectra for Extrasolar Giant Planets (2006, background): “Temperature influences the transmission spectrum by way of its influence on the atmospheric scale height, as discussed by Brown (2001), and on the absorption coefficients.”
• Ground-based detection of sodium in the transmission spectrum of exoplanet HD209458b (2008, background): “If this velocity shift is real, it is rather puzzling how it could be produced, since it is about an order of magnitude higher thant e expected sound speed in the upper layer of the planet’s atmosphere (Brown 2001).”