The Peculiar Atmospheric Chemistry of KELT-9b

The atmospheric temperatures of the ultra-hot Jupiter KELT-9b straddle the transition between gas giants and stars, and therefore between two traditionally distinct regimes of atmospheric chemistry. Previous theoretical studies assume the atmosphere of KELT-9b to be in chemical equilibrium. Despite the high ultraviolet flux from KELT-9, we show using photochemical kinetics calculations that the observable atmosphere of KELT-9b is predicted to be close to chemical equilibrium, which greatly simplifies any theoretical interpretation of its spectra. It also makes the atmosphere of KELT-9b, which is expected to be cloudfree, a tightly constrained chemical system that lends itself to a clean set of theoretical predictions. Due to the lower pressures probed in transmission (compared to emission) spectroscopy, we predict the abundance of water to vary by several orders of magnitude across the atmospheric limb depending on temperature, which makes water a sensitive thermometer. Carbon monoxide is predicted to be the dominant molecule under a wide range of scenarios, rendering it a robust diagnostic of the metallicity when analyzed in tandem with water. All of the other usual suspects (acetylene, ammonia, carbon dioxide, hydrogen cyanide, methane) are predicted to be subdominant at solar metallicity, while atomic oxygen, iron and magnesium are predicted to have relative abundances as high as 1 part in 10,000. Neutral atomic iron is predicted to be seen through a forest of optical and near-infrared lines, which makes KELT-9b suitable for high-resolution ground-based spectroscopy with HARPS-N or CARMENES. We summarize future observational prospects of characterizing the atmosphere of KELT-9b.


INTRODUCTION
The highly-inflated gas-giant exoplanet KELT-9b is a standout/oddball: it is the first hot Jupiter to be found orbiting a A/B star, has an equilibrium temperature in excess of 4000 K, and a z -band brightness temperature of about 4600 K (Gaudi et al. 2017). Based on existing empirical trends (Heng 2016;Stevenson 2016), we expect its atmosphere to be cloudfree. Bell & Cowan (2018) have previously suggested that the transition between the dayside and nightside of KELT-9b may coincide with a transition between the dominance of atomic and molecular hydrogen, respectively. This implies that the latent heat associated with transforming hydrogen from its atomic to molecular forms (and vice versa) may alter the efficiency of day-night heat redistribution. (See also Komacek & Tan 2018.) In the current Letter, we study the atomic and molecular chemistry of KELT-9b, which we believe to be peculiar as KELT-9b exhibits chemical traits of both a gas giant and a star within the same atmosphere.
Despite being irradiated in the ultraviolet about 700 times stronger than for WASP-33b (Gaudi et al. 2017), our expectation is that the abnormally high temperatures allow the observable atmosphere of KELT-9b to remain close to chemical equilibrium (Section 2). An atmosphere in chemical equilibrium is considerably easier to interpret, because the atomic and molecular abundances are determined only by the temperature, pressure and elemental abundances (e.g., Chapter 7 of Heng 2017). Chemistry becomes a local, rather than global, problem. From a qualitative standpoint, we may begin to understand the atmospheric chemistry of KELT-9b from the following net reactions (e.g., Moses et al. 2011;Heng & Tsai 2016), where M is a third body. Lower and higher pressures favor atomic and molecular hydrogen, respectively. This implies that the transition from H 2 to H should occur at lower temperatures in transmission versus emission spectra, since the pressure probed in transmission is a factor H/R ∼ 10 lower (Fortney 2005;Heng & Kitzmann 2017), with H being the pressure scale height and R being the transit radius. Since carbon monoxide (CO) needs molecular hydrogen to make water (H 2 O) and methane (CH 4 ), this conversion may be inhibited depending on the unknown temperature of the transit chord of KELT-9b. If the temperature probed in transmission is close to the equilibrium temperature of about 4000 K, then we expect atomic hydrogen to reign and for water to be scarce. It is abundant if the temperature is somewhat lower. One of our goals is to establish quantitatively how sensitive this dependence on temperature is. In this way, water may be used as a thermometer for the atmospheric limb temperature of KELT-9b. Even if water is undetected, an upper limit set on its relative abundance is a lower limit on temperature (Section 3). Carbon monoxide is the dominant molecule and we expect its abundance to serve as a direct diagnostic for the elemental abundances of carbon and oxygen.
At the same time our study was released, several other studies explored the atmospheric chemistry of ultra-hot Jupiters as well. Parmentier et al. (2018) combined a three-dimensional general circulation model (GCM) with the NASA CEA code, which assumes chemical equilibrium, and focused on WASP-121b. Lothringer et al. (2018) used one-dimensional PHOENIX models coupled to the Astrophysical Chemical Equilibrium Solver to study KELT-9b. Mansfield et al. (2018) confronted Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) of HAT-P-7b with both three-dimensional GCMs and one-dimensional radiative transfer models that assume chemical equilibrium. Kreidberg et al. (2018) also confronted these chemical-equilibrium models with HST-WFC3 data, but for WASP-103b. Arcangeli et al. (2018) used one-dimensional chemical-equilibrium models to confront HST-WFC3 data of WASP-18b. The key point is that none of these studies actually examine if chemical equilibrium is a reasonable assumption, but instead assume it. In the current study, we use the ARGO photochemical kinetics code (Rimmer & Helling 2016), together with a validated stellar spectral energy distribution of KELT-9, to study this issue.
From the study of stellar atmospheres, we expect the dayside atmosphere of KELT-9b to resemble that of a K star with its spectrum being dominated by atoms (Gaudi et al. 2017). What is less obvious is that we expect to see a forest of optical and near-infrared lines associated with both neutral and singly-ionized iron (e.g., Nussbaumer & Storey 1988;Solanki et al. 1992;Corrégé & Hibbert 2006;Aldenius & Johansson 2007;Peterson et al. 2017). In Section 4, we show simulated ground-based, high-resolution spectra associated with them. The overarching motivation of this study is to theoretically establish KELT-9b as a unique laboratory for atmospheric chemistry and thereby position it as a target for future observations across all wavelengths.

IS THE ATMOSPHERE OF KELT-9B IN CHEMICAL EQUILIBRIUM?
Based on our earlier study of WASP-12b (Oreshenko et al. 2017;Tsai et al. 2017), we had some expectation that the atmosphere of KELT-9b would be close to chemical equilibrium because of the higher temperatures involved. To check our expectation, we use the ARGO photochemical kinetics code (Rimmer & Helling 2016) to compute model atmospheres of KELT-9b. A zenith angle of 48 • is assumed, which corresponds to its cosine being 2/3. Unless otherwise stated, we assume the solar metallicity values of Asplund et al. (2009) We assume two temperature-pressure profiles using the analytical formula of Guillot (2010), which only requires three parameters: the equilibrium temperature (a known quantity: 4050 K), a mean infrared opacity and a mean optical opacity. (See also Chapter 4 of Heng 2017.) Since the hydrogen anion (H − ) is expected to be the dominant source of spectral continuum (Arcangeli et al. 2018), we choose a mean infrared opacity of 0.01 cm 2 g −1 such that the pressure probed by the transit chord is ∼ 10 mbar, consistent with a first-principles calculation using the H − cross section for a hydrogen-dominated atmosphere (John 1988). We then choose the mean optical opacity to be half and four times of the mean infrared opacity, such that profiles without and with a temperature inversion are generated, respectively. One of these profiles is motivated by the expectation that very hot Jupiters are expected to have temperature inversions (e.g., Arcangeli et al. 2018).
While the use of fixed temperature-pressure profiles implies that our photochemical kinetics and radiative transfer calculations are not self-consistent, they are justified, as a first approach, for three reasons. First, our primary aim is to examine the role of photochemistry. A predominance of the currently available photochemical cross sections do not depend on temperature, even if in principle they should (Venot et al. 2013). Second, this non-self-consistent approach is prevalent in the published literature (e.g., Line et al. 2011;Moses et al. 2011Moses et al. , 2013Visscher 2012;Zahnle & Marley 2014;Venot et al. 2015;Tsai et al. 2017). Third, selfconsistency between chemical kinetics and radiative transfer is expected to drive the system closer to chemical equilibrium (Drummond et al. 2016), implying that our approach is conservative.
We paid close attention to the ultraviolet fluxes of KELT-9 reported by Gaudi et al. (2017). We use a PHOENIX stellar model (Husser et al. 2013) interpolated to the stellar parameters of KELT-9 reported in Gaudi et al. (2017). We then binned the model ultraviolet fluxes and compared them to the measured values ( Figure 1). We see that our PHOENIX model ultraviolet fluxes are ∼ 10% higher than the KELT-9 measurements, implying that the ultraviolet spectrum of 10 16 10 14 10 12 10 10 10 8 10 6 10 4 10 2 10 0 mixing ratio  10 16 10 14 10 12 10 10 10 8 10 6 10 4 10 2 10 0 mixing ratio   The dashed curves assume chemical equilibrium, while the solid curves include both vertical mixing and photochemistry. The dotted curves include vertical mixing only. The top and middle panels assume temperature-pressure profiles with and without temperature inversions, respectively. A vertical mixing strength of Kzz = 10 10 cm 2 s −1 is assumed. Bottom panel: Comparing the measured ultraviolet fluxes from the stellar surface of KELT-9/HD 195689 versus those from the PHOENIX stellar model. The model values are binned in the same way as the measured ones for comparison, but the entire PHOENIX curve is used in the photochemistry calculations. KELT-9 we are using for our photochemical kinetics calculations is conservative. Being an A/B star, KELT-9 is expected to be fully radiative and not possess a magnetic dynamo that drives X-ray emission (Schröder & Schmitt 2007). As such, we do not need to concern ourselves with X-ray emission from KELT-9 impinging upon the atmosphere of KELT-9b and influencing the photochemistry. Cooper & Showman (2006) used a three-dimensional GCM coupled to a chemical relaxation method (dealing only with methane-to-carbon-monoxide interconversion), which assumes that chemical abundances relax towards their chemical-equilibrium values on a prescribed timescale, and found that vertical mixing is more important than horizontal mixing for disequilibrium chemistry. A Newtonian relaxation scheme was used in lieu of radiative transfer. Agúndez et al. (2014) found horizontal mixing to be more important, but their conclusion is based on a simplified two-dimensional dynamical model that assumes the zonal wind to follow solid-cylinder rotation, although they do perform full chemical kinetics calculations. Drummond et al. (2018) coupled a three-dimensional GCM to the chemical relaxation method and two-stream radiative transfer and reach a more subdued conclusion than Agúndez et al. (2014) regarding the importance of horizontal mixing. The studies of Cooper & Showman (2006) and Drummond et al. (2018) predate that of Tsai et al. (2018), who tested the accuracy of the chemical relaxation method for the first time. Tsai et al. (2018) showed that, contrary to the implementation of Cooper & Showman (2006) and Drummond et al. (2018), a single chemical timescale corresponding to a single rate-limiting reaction cannot be assumed, and that order-of-magnitude differences in the chemical timescale exist between the approximation of Cooper & Showman (2006) and more accurate calculations using full chemical kinetics. Given this evolving literature, our use of one-dimensional, plane-parallel models of chemical kinetics with photochemistry and vertical mixing only, to study whether the atmosphere of KELT-9b is in chemical equilibrium, is not unreasonable as a first approach.
In steady state, the conservation of mass states that the horizontal and vertical velocities are related by v h /R ∼ v z /H. The same argument implies that the dynamical timescales associated with vertical and horizontal mixing are comparable at the order-of-magnitude level.
This implies that the eddy diffusion coefficient associated with vertical mixing is K zz ∼ 0.1Hv z ∼ 10 10 cm 2 s −1 , where we have approximated the characteristic length scale as 0.1H (Smith 1998). Figure 1 shows the chemical abundances for three atmospheric models of KELT-9b: with photochemistry and vertical mixing, with vertical mixing only and in chemical equilibrium. Regardless of whether the temperature-pressure profile has an inversion or not, we see that vertical mixing is relatively unimportant (i.e., the dotted and dashed curves mostly coincide) compared to photochemistry. Photochemistry drives some of the atomic and molecular abundances away from chemical equilibrium at 1 mbar, but the model atmosphere remains close to chemical equilibrium at pressures of 1 mbar, which are the pressures conceivably probed by infrared and optical spectroscopy at low spectral resolution. When a temperature inversion is present, the higher temperatures at photospheric pressures negate the disequilibrium effects of photochemistry. If the temperatures are above 4000 K, then the molecules start to thermally dissociate into their constituent atoms and ions.
Although various species are in chemical disequilibrium at low pressures or high altitudes, they reside in the optically thin part of the atmosphere, implying that the assumption of chemical equilibrium remains reasonable for regions of the atmosphere that are observed in the optical and infrared range of wavelengths-at least, at low spectral resolution.

THE PECULIAR ATMOSPHERIC CHEMISTRY OF KELT-9B
To further explore the atmospheric chemistry (and for reasons of computational efficiency), we switch to using FastChem, which is an open-source chemical-equilibrium code 1 that treats gas-phase chemistry associated with the elements Al, Ar, Ca, Cl, Co, Cr, Cu, F, Fe, H, He, K, Mg, Mn, Na, Ne, P, S, Si, Ti, V and Zn, including ions (Stock et al. 2018). Figures 2, 3 and 4 show the volume mixing ratios (relative abundances by number) of molecules, atoms and ions. For simplicity and ease of interpretation, we approximate the pressures probed in transmission and emission to be 10 mbar and 0.1 bar, respectively. We then explore trends associated with temperature and carbon-to-oxygen ratio (C/O). Some general trends are worth pointing out. First, molecules that are usually prominent in hot Jupiters are subdominant, including acetylene (C 2 H 2 ), ammonia (NH 3 ), carbon dioxide (CO 2 ), hydrogen cyanide (HCN) and methane (CH 4 ), which have mixing ratios 10 −6 . We have not explicitly shown the mixing ratios for methane and acetylene in Figure 2, because their abundances mostly fall below the lower bound- ary of the plot (10 −15 ) at these high temperatures and solar metallicity. Second, atomic oxygen (O), magnesium (Mg) and iron (Fe) may have mixing ratios as high as ∼ 10 −4 , which is bested only by CO and sometimes water (H 2 O). Even nickel (Ni) may attain mixing ratios ∼ 10 −6 . Intriguingly, titanium (Ti) may be as abundant as ∼ 10 −7 .

Carbon monoxide as a metallicity diagnostic
In Figures 2 and 4, we see that the abundance of CO remains remarkably constant, across a broad range of temperatures (2000-4500 K), C/O values and for pressures of 0.01-0.1 bar. It is not only the dominant carbon carrier by several orders of magnitude in mixing ratio (its nearest competitor is CO 2 ), but it is the dominant molecule alongside water at low temperatures and C/O values.
In Figure 3, we further investigate the role of CO as a metallicity diagnostic. At temperatures of 2000-3000 K, almost all of the available carbon or oxygen is locked up in CO and the ratio of the CO number density to the total amount of either atomic C or O is very close to unity, depending on the value of C/O. This correspondence starts to break down at 4000 K and hotter, as CO dissociates into its constituents. Measuring CO alone would not constrain C/H or O/H directly, because C/O is a priori unknown. Additional molecules such as water are needed to set constraints on C/O being smaller or larger than unity and therefore to fully utilize CO as a metallicity diagnostic. In Figure 3, we also keep the ratios of the elemental abundances fixed, but in-crease the elemental abundances themselves by the same factor. For temperatures of 2000-3000 K and up to about 100× the solar metallicity, CO is a good tracer of the metallicity. By contrast, CO 2 is absent at these temperatures unless the metallicity approaches 1000-10,000 times solar and the temperature is 2000 K or lower. We consider these conditions to be unlikely for the atmosphere of KELT-9b. We predict CO, rather than CO 2 (e.g., Moses et al. 2013), to play the role of the metallicity diagnostic.

Water as a thermometer
In emission, the abundance of water holds steady until about 3000 K or higher, beyond which it starts to dissociate ( Figure 2). A less intuitive outcome is that, because of the lower pressure probed in transmission, the drop in the water abundance is much steeper across temperature. In fact, at solar metallicity, it drops by more than 6 orders of magnitude between 2000-4500 K. Such a sensitivity to temperature implies that water may be used as a thermometer in the transmission spectrum of KELT-9b. Used in tandem with carbon monoxide, the elemental abundances of carbon and oxygen may be robustly determined. The atmosphere of KELT-9b becomes a tightly constrained chemical system.
A caveat is that water may also become scarce when C/O approaches unity (Kuchner & Seager 2005;Madhusudhan 2012), as shown in Figure 4. However, when C/O 1, we expect acetylene, methane and hydrogen cyanide to become important. These species are spectroscopically active in the infrared range of wavelengths and should be detectable under these conditions. At these high temperatures, graphite formation (Moses et al. 2013) is unimportant, which would in principle allow C/O > 1 in the gas phase.

Neutral atoms and heavy-metal content
A prediction of having a very hot atmosphere is that the neutral species of atomic metals become prominent. At solar metallicity, magnesium and iron have mixing ratios ∼ 10 −4 in both emission and transmission from 2000-3500 K, dropping to ∼ 10 −5 in transmission (with a slower drop in emission). Above 3000 K in transmission, atomic oxygen attains mixing ratios ∼ 10 −4 . Aluminum, nickel and sodium have mixing ratios as high as ∼ 10 −6 . All of these properties imply that it is possible to directly constrain the heavy-metal content of the atmosphere of KELT-9b. We will make predictions for detecting iron via ground-based, high-resolution spectroscopy in Section 4.

Thomson scattering by electrons versus extinction by hydrogen anions
Another prediction is that free electrons have mixing ratios as high as ∼ 10 −4 (Figure 2). For temperatures from 2000-4500 K, the mixing ratio of free electrons is comparable to or greater than that of hydrogen anions (H − ). However, Thomson scattering has a somewhat small, wavelengthindependent cross section of σ T ≈ 6.65 × 10 −25 cm 2 . The cross section associated with hydrogen anions (σ H − ) has a bound-free component up to 1.6419 µm, the photodetachment threshold and a free-free component that extends into the mid-infrared range of wavelengths (John 1988). Figure 1 of Arcangeli et al. (2018) illustrates this graphically, where we see that σ H − ∼ 10 −17 -10 −16 cm 2 . We conclude that Thomson scattering is a negligible effect compared to the extinction of light by H − .

Ground-based high-resolution spectroscopy of neutral iron lines
To investigate the predicted prominence of spectral lines associated with neutral and singly-ionized iron, we now simulate ground-based, resolution ∼ 100, 000 observations of them. This technique has previously been established and consists of cross-correlating the observed system (star plus exoplanet) spectrum with a template of the atom or molecule being sought (Snellen et al. 2010;Brogi et al. 2012). To construct this template, we use the spectroscopic line list of Fe and Fe + from the Vienna Atomic Line Database (Ryabchikova et al. 2015). We compute opacities from this line list using standard methods (e.g., Grimm & Heng 2015;Heng 2017). To generate transmission spectra, we use the analytical formula of Heng & Kitzmann (2017). For illustration, we assume a constant opacity of 0.01 cm 2 g −1 to mimic a spectral continuum, a reference pressure of 0.1 bar at the white-light radius measured by Gaudi et al. (2017), a transit chord (terminator) temperature of 3000 K and volume mixing ratios of of 5 × 10 −5 and 10 −7 for Fe and Fe + , respectively. Figure 5 shows our theoretical transmission spectra containing Fe and Fe + versus Fe + only in the visible range of wavelengths. At the expected temperatures of KELT-9b, the energy levels of Fe are preferentially populated compared to Fe + . It is therefore unsurprising that Fe is dominant over Fe + as an opacity source. We expect the star to be deficient in Fe lines, because calculations assuming collisional ionization equilibrium have shown that the abundance of Fe drops rapidly as the temperature exceeds 10,000 K; the abundance of Fe + becomes dominant only at these temperatures (Kaastra et al. 2008).
In Figure 5, we also show a simulated transit observation of KELT-9b with the HARPS-N spectrograph located at the 3.6-meter TNG telescope at La Palma, which has a spectral resolution of about 110,000. Due to the brightness of KELT-9, HARPS-N is expected to provide a signal-to-noise ratio (SNR) of at least 100 in the stellar continuum at 500 nm after a single exposure of 200 s. The transit duration is 3 hours, implying that 54 observations may be obtained in-transit, resulting in a combined SNR of 735 at 500 nm.
Here, our PHOENIX model for KELT-9 (Husser et al. 2013) is convolved with the line-spread function to match the rotational velocity broadening (v sin i = 111.4 km s −1 ), because we are interested in resolving individual spectral lines. We also multiplied it with a SkyCalc model from ESO (Noll et al. 2012;Jones et al. 2013) of the telluric transmission spectrum across the wavelength range accessible by HARPS-N (387-691 nm). Our prediction is that iron can be observed at high signal-to-noise after observing a single transit with a high-resolution spectrograph on a 4-meter-class telescope, e.g., with HARPS-N or CARMENES.

Hubble Space Telescope (HST)
Ultraviolet transit spectroscopy using HST is now an established technique (e.g., Ehrenreich et al. 2015). It has been used to detect both atomic hydrogen and metals (e.g., Vidal-Madjar et al. 2004). Besides the semi-forbidden lines, there is a zoo of near-ultraviolet resonant lines associated with neutral and singly-ionized metal atoms, e.g., see Tables 9.3 and 9.4 of Draine (2011). These include the Fe I doublet at 0.37 and 0.39 µm, a set of Ni I lines from 0.34-0.35 µm, a set of Ti I lines from 0.33-0.52 µm, the C I triplet at 0.17 µm, the O I triplet at 0.13 µm and a Mg I line at 0.29 µm.

James Webb Space Telescope (JWST)
Our predictions may be decisively tested by JWST via the measurement of the transmission spectrum, dayside emission spectrum and nightside emission spectrum. Such a program will conceivably fit within a small JWST proposal (≤ 25 hours). Detecting CO and H 2 O jointly will set constraints

Characterizing Exoplanet Satellite (CHEOPS)
The CHEOPS bandpass covers 0.33-1.1 µm (Broeg et al. 2013). For hot Jupiters with equilibrium temperatures 1300 K, a CHEOPS secondary eclipse translates directly into a measurement of the geometric albedo (Seager 2010). For somewhat hotter temperatures, the secondary eclipse measurement samples a combination of reflected light and thermal emission that depends on the dayside-nightside heat redistribution efficiency (Heng & Demory 2013). The dayside of KELT-9b is so hot that its blackbody peak wavelength is about 0.65 µm, which is in the middle of the CHEOPS bandpass ( Figure 6). Similar to HST, CHEOPS will reside in low-Earth orbit, meaning Earth crossings of the South Atlantic Anomaly will prevent continuous monitoring in any direction. KELT-9 is visible to CHEOPS for ≈ 2.5 months per year and observations can be obtained at an observing efficiency between 50-70% (with the upper limit being the maximum obtainable), yielding about 35 occultations annually.
The stellar effective temperature is 10,150 K, while for the planetary equilibrium temperature we assume no redistribution (f = 2/3 in equation [3.9] of Seager 2010) and a Bond albedo of 0.1 (geometric albedo of 0.067 assuming isotropic scattering). Both the star and exoplanet are assumed to emit as blackbodies, and we integrate their fluxes in the CHEOPS bandpass, which is shown in Figure 6. Using equations (3.40) and (3.49) in Seager (2010), we compute the secondary eclipse depths due to thermal emission and reflected light, respectively. For KELT-9b, these are 427.9 parts per million (ppm) and 45.6 ppm, which gives 473.5 ppm in total. The thermal emission dominates reflected light by a factor of more than 9, implying that CHEOPS secondary eclipses are unambiguously sampling the thermal emission from KELT-9b.
To compute the precision attained with a single secondary eclipse of KELT-9b, we use the CHEOPS Exposure Time Calculator (ETC). 2 As input, we use the properties of the KELT-9 star (Gaudi et al. 2017). The exposure time was set to 1 second. Stellar granulation noise was excluded from the calculation as the star does not have a convective envelope. With these inputs, we estimate a precision of about 23 ppm for a single secondary eclipse ( Figure 6). This translates into a signal-to-noise ratio of ≈ 21, equivalent to better than 5% precision in the thermal emission flux. The ability to obtain multiple secondary eclipses without data stacking means that we can quantify the climate variability of KELT-9b, for which only an upper limit was previously set on HD 189733b using Spitzer data (Agol et al. 2010).
One may use the secondary eclipse measurements to perform eclipse mapping (de Wit et al. 2012;Majeau et al. 2012). The eclipse mapping technique is sensitive to the brightness temperature contrast on the planet dayside. Assuming a hotter region on the planet photosphere with a brightness temperature 1500 K larger than the continuum would yield a 30-ppm amplitude signal in both occultation ingress and egress in the CHEOPS bandpass. We estimate that a photometric precision of 15 ppm over 2 minutes is required to achieve a 4-σ detection of the ingress/egress features. We expect CHEOPS to achieve a precision of about 100 ppm over 2 minutes, which would require about 45 occultations to retrieve an optical eclipse map of that planet. These observations bear two caveats. First, the impact parameter of b = 0.17 (Gaudi et al. 2017) will negatively impact our ability to constrain the latitudinal location of the hot spot. Second, the level of stellar activity on short timescales (∼minutes) probed at visible wavelengths will be critical to assess the feasibility of these observations.

Summary
In the current study, we have explored the atmospheric chemistry of ultra-hot Jupiters for which KELT-9b is an archetype. In contrast to other studies (Arcangeli et al. 2018;Kreidberg et al. 2018;Lothringer et al. 2018;Mansfield et al. 2018;Parmentier et al. 2018), we do not assume chemical equilibrium in our calculations, but instead perform photochemical kinetics calculations, which explicitly quantify the effects of atmospheric mixing and photochemistry. We then further explore the atmospheric chemistry using chemicalequilibrium calculations that include a set of metals. Our findings include: • Photochemistry, rather than atmospheric mixing, is the main driver for disequilibrium chemistry at low pressures. Nevertheless, at pressures probed by optical and infrared spectroscopy at low resolution, chemical equilibrium is a reasonable approximation.
• When analyzed in tandem, the abundances of carbon monoxide and water allows one to completely solve for C/H, O/H and temperature, provided that chemical equilibrium is assumed.
• Metals in their atomic form are predicted to be observable. In particular, atomic iron should be seen via a forest of optical and near-infrared lines using high-resolution ground-based spectrographs such as HARPS-N and CARMENES.

Future work
While we have argued that horizontal mixing does not significantly introduce chemical disequilibrium in ultra-hot Jupiters, a more rigorous investigation would involve running three-dimensional GCMs of KELT-9b-like exoplanets that include a treatment of disequilibrium chemistry (rather than assuming chemical equilibrium) and a varying specific heat capacity (due to the co-existence of atomic and molecular hydrogen in the same atmosphere). Existing work in the published literature already hint that results from hot-Jupiter GCMs (executed at lower temperatures than for KELT-9b) may not carry over straightforwardly. For example, Showman et al. (2013) studied the transition between moderate and intense stellar insolation using GCMs. To lowest order, the zonal wind speeds are driven by the dayside-nightside temperature contrast, which is unknown for KELT-9b. A higher dayside-nightside temperature contrast should lead to faster winds, but this cannot increase without bound (Fromang et al. 2016). Under moderate stellar insolation, the usual flow pattern of a zonal jet appears (see Heng & Showman 2015 for a review). Under intense stellar insolation, a combination of the damping of planetary-scale waves and frictional drag inhibit zonal jet formation and dayside-to-nightside flow.
It remains an open question if the atmospheric flow of ultrahot-Jupiters occupy yet another regime and if these flows are capable of significantly driving the atmosphere away from chemical equilibrium. Future work needs to elucidate the flow behavior and its influence on the chemistry of KELT-9b-like exoplanets in three dimensions.