C/O RATIO AS A DIMENSION FOR CHARACTERIZING EXOPLANETARY ATMOSPHERES

By N. Madhusudhan. 2012

286 citations recorded by the source corpus.

Until recently, infrared observations of exoplanetary atmospheres have typically been interpreted using models that assumed solar elemental abundances. With the chemical composition fixed, attempts have been made to classify hot Jupiter atmospheres on the basis of stellar irradiation. However, recent observations have revealed deviations from predictions based on such classification schemes, and chemical compositions retrieved from some data sets have also indicated non-solar abundances. The data require a two-dimensional (2D) characterization scheme with dependence on both irradiation and chemistry. In this work, we suggest the carbon-to-oxygen (C/O) ratio as an important second dimension for characterizing exoplanetary atmospheres. In hot-hydrogen-dominated atmospheres, the C/O ratio critically influences the relative concentrations of several spectroscopically dominant species. Between a C/O of 0.5 (solar value) and 2, the H2O and CH4 abundances can vary by several orders of magnitude in the observable atmosphere, and new hydrocarbon species such as HCN and C2H2 become prominent for C/O ⩾ 1, while the CO abundance remains almost unchanged. Furthermore, a C/O ⩾ 1 can preclude a strong thermal inversion due to TiO and VO in a hot Jupiter atmosphere, since TiO and VO are naturally underabundant for C/O ⩾ 1. We, therefore, suggest a new 2D classification scheme for hydrogen-dominated exoplanetary atmospheres with irradiation (or temperature) and C/O ratio as the two dimensions. We define four classes in this 2D space (O1, O2, C1, and C2) with distinct chemical, thermal, and spectral properties. Based on the most recent observations, we characterize the thermal structure and C/O ratios of six hot Jupiters (XO-1b, CoRoT-2b, WASP-14b, WASP-19b, WASP-33b, and WASP-12b) in the framework of our proposed 2D classification scheme. While the data for several systems in our sample are consistent with C-rich atmospheres, new observations are required to conclusively constrain their C/O ratios in the day side as well as the terminator regions of their atmospheres. We discuss how observations using existing and forthcoming facilities can constrain C/O ratios in exoplanetary atmospheres.

Subfields: Astronomy and Astrophysics, Instrumentation, Statistical and Nonlinear Physics

View Paper: https://www.semanticscholar.org/paper/09520e222f576529683cd5a7f2c5756caff7631b

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Citations in the corpus

WASP-121 b: a hot Jupiter close to tidal disruption transiting an active F star (2015, background): “F, with the participation of the 4 Whether TiO is seen in emission or absorption in that case depends on the thermal structure of the planetary atmosphere (presence or lack of temperature inversion, see Madhusudhan 2012 for details).”
A POSSIBLE CARBON-RICH INTERIOR IN SUPER-EARTH 55 Cancri e (2012, background): “…support the possibility of a carbon-rich interior in 55 Cancri e. Numerous studies have suggested the possibility of carbon-rich exoplanets, both theoretically (e.g. Kuchner & Seager 2006, Bond et al. 2010, Madhusudhan et al. 2011b) and observationally (Madhusudhan et al. 2011a, Madhusudhan 2012).”
Mass–Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H2O, Na, and K (2019, background): “However, it has been unclear if the low H2O abundances are due to low overall metallicities, and hence low oxygen abundances, or due to high C/O ratios (Madhusudhan et al. 2014b) or some other mechanism altogether.”
QUANTITATIVELY ASSESSING THE ROLE OF CLOUDS IN THE TRANSMISSION SPECTRUM OF GJ 1214b (2013, background): “C/O ratio Recent work has shown that planets may have some range in carbon and oxygen abundances (Madhusudhan et al. 2011a,b; Madhusudhan 2012; Moses et al. 2013).”
Carbon monoxide and water vapor in the atmosphere of the non-transiting exoplanet HD 179949 b (2014, background): “This choice covers the whole range of relative abundances for a hot Jupiter with the above Teq, as a function of its C/O ratio (Madhusudhan 2012).”
SPECTROSCOPIC EVIDENCE FOR A TEMPERATURE INVERSION IN THE DAYSIDE ATMOSPHERE OF HOT JUPITER WASP-33b (2015, methodology): “Madhusudhan et al. (2011) and Madhusudhan (2012) suggested that high C/O ratios could also deplete inversion-causing compounds such as TiO and VO in hot Jupiters, thereby precluding the formation of thermal inversions, and Knutson et al. (2010) proposed that the formation of inversions may instead…”
H2O abundances and cloud properties in ten hot giant exoplanets (2018, methodology): “The atmospheric metallicities for the hot Jupiter sample are calculated assuming that half of the oxygen is in the estimated H2O and the remaining half is in CO, in accordance with expectations for a solar C/O ratio in chemical equilibrium at high temperatures (Madhusudhan 2012).”
TRANSMISSION SPECTROSCOPY OF THE HOT JUPITER WASP-12b FROM 0.7 TO 5 μm (2013, methodology): “Our models include line-by-line molecular absorption due to H2O, CO, CH4, CO2, HCN, C2H2, TiO, and VO, as described in Madhusudhan (2012), and that due to TiH (Burrows et al. 2005; Hill et al. 2012).”
ExoMol line lists - III. An improved hot rotation-vibration line list for HCN and HNC (2013, background): “HCN is likely to feature in carbon rich atmospheres, especially so when disequilibrium effects are taken into account (Madhusudhan 2012; Venot et al. 2012; Moses et al. 2013).”
ROTATION AND WINDS OF EXOPLANET HD 189733 b MEASURED WITH HIGH-DISPERSION TRANSMISSION SPECTROSCOPY (2015, background): “We considered the four main trace gases expected to be spectroscopically active in the observed wavelength range (Madhusudhan 2012): CO, H 2 O, CO 2 , and CH 4 .”
Equilibrium chemistry down to 100 K (2017, background): “Figure 1 in Madhusudhan (2012) actually shows the same effect, although Madhusudhan has used a log-scaling of the C/O-axis where deviations of order 5% are simply not visible.”
Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects (2019, background): “One possibility is a super-solar C/O ratio (&1) in these atmospheres which can cause low H2O abundance and high CO abundance (161, 188, 236).”
A PRECISE WATER ABUNDANCE MEASUREMENT FOR THE HOT JUPITER WASP-43b (2014, background): “We find that a moderately super-solar metallicity composition has equilibrium H2O, CO2, and CO abundances that are within the range of our retrieved values.”
HST hot-Jupiter transmission spectral survey: evidence for aerosols and lack of TiO in the atmosphere of WASP-12b (2013, background): “…and CrH, have been suggested and explored as possible opacity sources in WASP-12b when modelling the red-optical and near-IR data (Swain et al. 2013; Stevenson et al. 2013), with TiH a potentially important Ti-bearing molecule in high C/O ratio atmospheres (Madhusudhan 2012; Stevenson et al. 2013).”
A DETECTION OF WATER IN THE TRANSMISSION SPECTRUM OF THE HOT JUPITER WASP-12b AND IMPLICATIONS FOR ITS ATMOSPHERIC COMPOSITION (2015, background): “…for additional planets (e.g. Brogi et al. 2014; Line et al. 2014), studies of the effect of C/O on atmospheric chemistry (Madhusudhan et al. 2011a; Madhusudhan 2012; Kopparapu et al. 2012; Moses et al. 2013; Venot et al. 2015), inferences of C/O in exoplanet host stars (Teske et al. 2013, 2014),…”
Chemical enrichment of giant planets and discs due to pebble drift (2017, background): “…C/O ratio: if the observed water abundance is far below that expected for the stellar C/O ratio, then one can infer the C/O ratio is close to unity (Madhusudhan 2012), modulo any possible uncertainty due to obscuration of the water feature by clouds or hazes (see Madhusudhan et al. 2016, and…”