CAN TiO EXPLAIN THERMAL INVERSIONS IN THE UPPER ATMOSPHERES OF IRRADIATED GIANT PLANETS?

By D. Spiegel, Katie Silverio, Adam Burrows. 2009

202 citations recorded by the source corpus.

Spitzer Space Telescope infrared observations indicate that several transiting extrasolar giant planets have thermal inversions in their upper atmospheres. Above a relative minimum, the temperature appears to increase with altitude. Such an inversion probably requires a species at high altitude that absorbs a significant amount of incident optical/UV radiation. Some authors have suggested that the strong optical absorbers titanium oxide (TiO) and vanadium oxide (VO) could provide the needed additional opacity, but if regions of the atmosphere are cold enough for Ti and V to be sequestered into solids they might rain out and be severely depleted. With a model of the vertical distribution of a refractory species in gaseous and condensed form, we address the question of whether enough TiO (or VO) could survive aloft in an irradiated planet's atmosphere to produce a thermal inversion. We find that it is unlikely that VO could play a critical role in producing thermal inversions. Furthermore, we find that macroscopic mixing is essential to the TiO hypothesis; without macroscopic mixing, such a heavy species cannot persist in a planet's upper atmosphere. The amount of macroscopic mixing that is required depends on the size of condensed titanium-bearing particles that form in regions of an atmosphere that are too cold for gaseous TiO to exist. We parameterize the macroscopic mixing with the eddy diffusion coefficient Kzz and find, as a function of particle size a, the values that Kzz must assume on the highly irradiated planets HD 209458b, HD 149026b, TrES-4, and OGLE-TR-56b to loft enough titanium to the upper atmosphere for the TiO hypothesis to be correct. On these planets, we find that for TiO to be responsible for thermal inversions Kzz must be at least a few times 107 cm2 s−1, even for a = 0.1 μm, and increases to nearly 1011 cm2 s−1 for a = 10 μm. Such large values may be problematic for the TiO hypothesis, but are not impossible.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/204c15fdb8f9fee914077ca5184655d434e2b229

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In Astronomy and Astrophysics

1. N. Madhusudhan
2. Jon Fortney
3. A. Showman
4. M. Marley
5. M. Line
6. D. Sing
7. V. Parmentier
8. D. Deming

Citations in the corpus

• TRANSMISSION SPECTRA OF THREE-DIMENSIONAL HOT JUPITER MODEL ATMOSPHERES (2009, background): “Spiegel et al. (2009) have recently highlighted the myriad difficulties in modeling the vrtical distribution of TiO in a 1D model.”
• INFLATING HOT JUPITERS WITH OHMIC DISSIPATION (2010, methodology): “In our models, we adopt atmospheric temperature profiles similar to that of Spiegel et al. (2009) for HD209458b and Tres-4b, and the 1D profile of Fortney et al. (2010) for HD189733b.”
• THE IMPRINT OF EXOPLANET FORMATION HISTORY ON OBSERVABLE PRESENT-DAY SPECTRA OF HOT JUPITERS (2016, background): “It is clear that future work should address this process -as also semiconvection in the interior- in more detail building for example on Chamberlain & Hunten (1987); Spiegel et al. (2009).”
• HST hot-Jupiter transmission spectral survey: evidence for aerosols and lack of TiO in the atmosphere of WASP-12b (2013, background): “Further, while most hot Jupiters can experience a vertical cold-trap, which would deplete TiO at depth, WASP-12b is hot enough such that it should not experience a cold-trap anywhere throughout its dayside atmosphere (Spiegel et al. 2009).”
• NEAR-INFRARED SPECTROSCOPY OF THE EXTRASOLAR PLANET HR 8799 b (2010, background): “…providing valuable tests of irradiated planetary atmospheric models and have shown evidence for temperature inversions (e.g., Knutson et al. 2009; Spiegel et al. 2009), global atmospheric dynamics (Showman et al. 2009), and non-equilibrium carbon chemistry (e.g., Cooper & Showman 2006; Swain et…”
• DISEQUILIBRIUM CARBON, OXYGEN, AND NITROGEN CHEMISTRY IN THE ATMOSPHERES OF HD 189733b AND HD 209458b (2011, background): “Hot Jupiters are often divided into two classes, those that appear to have stratospheric thermal inversions, and those that do not (e.g., Fortney et al. 2008a; Hubeny et al. 2003; Harrington et al. 2007; Burrows et al. 2007, 2008; Knutson et al. 2008; Spiegel et al. 2009; Madhusudhan & Seager 2009).”
• An HST optical-to-near-IR transmission spectrum of the hot Jupiter WASP-19b: detection of atmospheric water and likely absence of TiO (2013, background): “Spiegel et al. (2009) find that in the atmosphere of OGLE-TR-56, a planet only ∼100 K cooler than WASP-19b, there could be a TiO cold trap on the planet’s day side.”
• C/O RATIO AS A DIMENSION FOR CHARACTERIZING EXOPLANETARY ATMOSPHERES (2012, background): “While the lack of a thermal inversion in WASP-14b may contradict the prediction of Fortney et al. (2008), it is also possible that condensation and gravitational settling of TiO/VO (Spiegel et al. 2009) might preclude the formation of a thermal inversion.”
• A CORRELATION BETWEEN STELLAR ACTIVITY AND HOT JUPITER EMISSION SPECTRA (2010, background): “Overcoming these effects would require vigorous mixing, but this may not be expected in a stably stratified atmosphere (Spiegel et al. 2009).”
• NO THERMAL INVERSION AND A SOLAR WATER ABUNDANCE FOR THE HOT JUPITER HD 209458B FROM HST/WFC3 SPECTROSCOPY (2016, background): “…in the dayside atmospheres of conventional hot Jupiters because of the strong vertical mixing to keep these heavy molecules aloft at low pressures (Spiegel et al. 2009) and/or the rainout of larger condensate droplets on the cooler nightside (Parmentier et al. 2013) Furthermore, forced inclusion…”
• From thermal dissociation to condensation in the atmospheres of ultra hot Jupiters: WASP-121b in context (2018, background): “…that both CaTiO3 clouds and gaseous TiO are needed to explain the transmission spectrum of WASP-121 points towards the lack of a cold-trap of gaseous TiO by the clouds (Parmentier et al. 2013; Spiegel et al. 2009) and is a hint that the vertical mixing in WASP-121b is strong above the 1 bar level.”
• A TEMPERATURE AND ABUNDANCE RETRIEVAL METHOD FOR EXOPLANET ATMOSPHERES (2009, background): “The intially favored opacity sources, TiO and VO may be unlikely to cause inversions of the observed magnitude (Spiegel et al. 2009).”
• A SYSTEMATIC RETRIEVAL ANALYSIS OF SECONDARY ECLIPSE SPECTRA. II. A UNIFORM ANALYSIS OF NINE PLANETS AND THEIR C TO O RATIOS (2013, background): “However, in a more recent study Spiegel et al. (2009) suggested that vigorous vertical mixing is required to keep both gas phase and condensed phase TiO and VO aloft in upper, inversion forming regions of the atmosphere.”
• 3D mixing in hot Jupiters atmospheres. I. Application to the day/night cold trap in HD 209458b (2013, methodology): “Like in Spiegel et al. (2009), we usePlim = 1 mbar andχlim = 0.5.”
• TRANSITIONS IN THE CLOUD COMPOSITION OF HOT JUPITERS (2016, background): “…cold trap places a combined constraint on the deep vertical mixing rate at the 10-100 bar cold trap, and on the particle size at that cold trap as the mixing rate must be small and/or the particle size must be large for the cold trap to be efficient (Spiegel et al. 2009; Parmentier et al. 2013).”