New Avenues in Atmospheric Modelling of Exoplanets

In this thesis I explore various aspects of atmospheric characterisation of exoplanets with the primary goal of understanding their chemical compositions and physical processes. My research led to the development of new self-consistent models of exoplanetary atmospheres, a new paradigm for atmospheric retrievals of thermal emission spectra, as well as chemical detections using both high-resolution Doppler spectroscopy as well as low-resolution transit spectroscopy. I firstly computed the molecular and atomic cross sections of various species prevalent in the atmospheres of such exoplanets in order to compute their spectra. The absorption cross sections were calculated through the broadening of spectral lines obtained from high resolution line lists. These cross sections and subsequent spectral models have led to the detections of numerous chemical species (HCN, TiO, Li, Na, K, CO, and H$_2$O) in the atmospheres of several exoplanets. Recent advances in observations have heralded the need for accurate models of exoplanetary atmospheres. I have built a new self-consistent atmospheric model, GENESIS, custom built for exoplanets and demonstrated for irradiated and non-irradiated atmospheres over a wide range of atmospheric parameter space. The model treats line-by-line radiative transfer through the Feautrier method and radiative-convective equilibrium through the Rybicki Complete Linearisation method in a plane parallel atmosphere. This model allows for a detailed exploration of radiative processes and chemical compositions and their effects on observed emission spectra. I compared this model against several others in the literature and found good agreement between the atmospheric properties and emission spectra. Thermal inversions have been seen on the dayside atmospheres of some hot Jupiters and have been predicted to be caused by TiO or VO due to their visible opacity. I used the GENESIS model to investigate the effect of visible opacity and deduced that many new species (AlO, CaO, NaH and MgH), hitherto unexplored, are also capable of causing thermal inversions on hot Jupiters. I have explored the effect of these species as a function of their overall atmospheric abundance as well as determining the required abundance for each of these species to form an inversion. Secondly, I show that a low infrared opacity caused by a low H$_2$O abundance can also lead to strong thermal inversions even with sub-solar abundances of these visible absorbers due to the change in infrared opacity. As a demonstration of this work I have shown that the thermal inversion on WASP-121b can be explained by all the visible absorbers listed above. These thermal inversions are of great importance as the species responsible may be observed with current observational capabilities, thus providing testable observations for these species. I have also developed a new hybrid retrieval method for exoplanetary emission spectra, HyDRA. This uses the latest atmospheric modelling tools to fit the observed spectra of exoplanet atmospheres. We explore a wide range of parameter space and determine the temperature profile and abundances of various species present in the dayside atmosphere through the emission spectra. These retrieved abundances are then used to explore disequilibrium processes which may be present through integration into the GENESIS self-consistent model. Such a framework allows constraints on departures of the temperature structures from radiative-convective equilibrium as well as chemical compositions from thermochemical equilibrium. I explored HST and Spitzer observations of WASP-43b and confirmed the data were in agreement with radiative-convective equilibrium in the dayside atmosphere. The HyDRA retrieval framework has also been extended to model the atmospheres of ultra-hot Jupiters with temperatures in excess of 2500~K. Such high temperatures can cause molecular species such as H$_2$O to thermally dissociate and for ionic species such as H- to form. Such effects have been used to explain the largely featureless WFC3 spectra seen for many ultra-hot Jupiters. I have included both of these effects into the HyDRA retrieval model to retrieve the atmosphere of the planet WASP-18b. I find that the retrieved abundances for H$_2$O and CO and the thermal inversion in the atmosphere do not change significantly compared to previous retrievals of WASP-18b which did not include thermal dissociation or H- opacity. I also see no significant evidence for H- or thermal dissociation in the atmosphere. With future instrumentation we may be more likely to constrain such effects in the emission spectra. I have also used the HyDRA retrieval framework to perform a set of homogeneous retrievals for eight well known hot Jupiters with high precision HST WFC3 spectra. These planets all also have Spitzer observations which I also use to explore the atmospheric temperature profile and chemical composition, in particular explore the H$_2$O abundance. The eight explored planets span a wide range of equilibrium temperatures, including four which fall into the category of ultra-hot Jupiters. We find that the coolest planets in the study generally have better constrained H$_2$O abundances near solar composition due to strong H$_2$O absorption features. On the other hand, three of the hottest exoplanets exhibit thermal inversions and indicate very poorly constrained or sub-solar H$_2$O abundances. This study shows that even currently explored exoplanets exhibit a wide range of atmospheric properties and that we will be able to explore this diversity further with more exoplanetary spectra coming up in the next few years. Finally, I have used the GENESIS model to enable chemical detections of molecular species using high resolution Doppler spectroscopy of hot Jupiters. I generated high resolution emission spectra of the hot Jupiters HD189733b and HD209458b for cross correlation with the data obtained with the VLT CRIRES spectrograph. This helped us find evidence for H$_2$O, CO and HCN in the atmosphere of both planets. In the future this method has great potential for new chemical detections due to its sensitivity to trace species and shows great promise in the detection of biosignatures on smaller rocky planets

A weak spectral signature of water vapour in the atmosphere of HD 179949 b at high spectral resolution in the L band

High-resolution spectroscopy (R≤20,000) is currently the only known method to constrain the orbital solution and atmospheric properties of non-transiting hot Jupiters. It does so by resolving the spectral features of the planet into a forest of spectral lines and directly observing its Doppler shift while orbiting the host star. In this study, we analyse VLT/CRIRES (R=100,000) L-band observations of the non-transiting giant planet HD 179949 b centred around 3.5μm. We observe a weak (3.0σ, or S/N=4.8) spectral signature of H2O in absorption contained within the radial velocity of the planet at superior-conjunction, with a mild dependence on the choice of line list used for the modelling. Combining this data with previous observations in the K band, we measure a detection significance of 8.4σ for an atmosphere that is most consistent with a shallow lapse-rate, solar C/O ratio, and with CO and H2O being the only major sources of opacity in this wavelength range. As the two sets of data were taken 3 yr apart, this points to the absence of strong radial-velocity anomalies due, e.g. to variability in atmospheric circulation. We measure a projected orbital velocity for the planet of K_P=(145.2±2.0) km/s (1σ ) and improve the error bars on this parameter by ∼70 per cent. However, we only marginally tighten constraints on orbital inclination (66.2+3.7−3.1 deg) and planet mass (0.963+0.036−0.031 Jupiter masses), due to the dominant uncertainties of stellar mass and semi-major axis. Follow ups of radial-velocity planets are thus crucial to fully enable their accurate characterization via high-resolution spectroscopy

β\beta Pictoris b through the eyes of the upgraded CRIRES+

Context: High-resolution spectrographs fed by adaptive optics (AO) provide a unique opportunity to characterize directly imaged exoplanets. Observations with such instruments allow us to probe the atmospheric composition, spin rotation, and radial velocity of the planet, thereby helping to reveal information on its formation and migration history. The recent upgrade of the Cryogenic High-Resolution Infrared Echelle Spectrograph (CRIRES+) at the VLT makes it a highly suitable instrument for characterizing directly imaged exoplanets. Aims: In this work, we report on observations of $\beta$ Pictoris b with CRIRES+ and use them to constrain the planets atmospheric properties and update the estimation of its spin rotation. Methods: The data were reduced using the open-source \textit{pycrires} package. We subsequently forward-modeled the stellar, planetary, and systematic contribution to the data to detect molecules in the planet's atmosphere. We also used atmospheric retrievals to provide new constraints on its atmosphere. Results: We confidently detected water and carbon monoxide in the atmosphere of $\beta$ Pictoris b and retrieved a slightly sub-solar carbon-to-oxygen ratio, which is in agreement with previous results. The interpretation is hampered by our limited knowledge of the C/O ratio of the host star. We also obtained a much improved constraint on its spin rotation of $19.9 \pm 1.0$ km/s, which gives a rotation period of $8.7 \pm 0.8$ hours, assuming no obliquity. We find that there is a degeneracy between the metallicity and clouds, but this has minimal impact on the retrieved C/O, $v\sin{i}$, and radial velocity. Our results show that CRIRES+ is performing well and stands as a highly useful instrument for characterizing directly imaged planets.Comment: Accepted for publication in A&

Retrieval survey of metals in six ultra-hot Jupiters: Trends in chemistry, rain-out, ionisation and atmospheric dynamics

Ground-based high-resolution spectroscopy (HRS) has detected numerous chemical species and atmospheric dynamics in exoplanets, most notably ultra-hot Jupiters (UHJs). However, quantitative estimates on abundances have been challenging but are essential for accurate comparative characterisation and to determine formation scenarios. In this work we retrieve the atmospheres of six UHJs (WASP-76~b, MASCARA-4~b, MASCARA-2~b, WASP-121~b, HAT-P-70~b and WASP-189~b) with ESPRESSO and HARPS-N/HARPS observations, exploring trends in eleven neutral species and dynamics. While Fe abundances agree well with stellar values, Mg, Ni, Cr, Mn and V show more variation, highlighting the difficulty in using a single species as a proxy for metallicity. We find that Ca, Na, Ti and TiO are under-abundant, potentially due to ionisation and/or night-side rain-out. Our retrievals also show that relative abundances between species are more robust, consistent with previous works. We perform spatially- and phase-resolved retrievals for WASP-76~b and WASP-121~b given their high signal-to-noise observations, and find the chemical abundances in each of the terminator regions are broadly consistent. We additionally constrain dynamics for our sample through Doppler shifts and broadening of the planetary signals during the primary eclipse, with median blue shifts between $\sim$0.9-9.0~km/s due to day-night winds. Furthermore, we constrain spectroscopic masses for MASCARA-2~b and HAT-P-70~b consistent with their known upper limits, but we note that these may be biased due to degeneracies. This work highlights the importance of future HRS studies to further probe differences and trends between exoplanets.Comment: 26 pages, 11 figures, 5 tables, published in A

JWST measurements of 13 C, 18 O, and 17 O in the atmosphere of Super-Jupiter VHS 1256 b

Isotope ratios have recently been measured in the atmospheres of directly imaged and transiting exoplanets from ground-based observations. The arrival of JWST allows us to characterize exoplanetary atmospheres in further detail and opens up wavelengths inaccessible from the ground. In this work we constrain the carbon and oxygen isotopes 13C, 18O, and 17O from CO in the atmosphere of the directly imaged companion VHS 1256 b through retrievals of the ∼4.1–5.3 μm NIRSpec G395H/F290LP observations from the early-release science program (ERS 1386). We detect and constrain 13C16O, 12C18O, and 12C17O at 32σ, 16σ, and 10σ confidence respectively, thanks to the very high signal-to-noise observations. We find the ratio of abundances are more precisely constrained than their absolute values, with 12C/13C=62−2+2 , in between previous measurements for companions (∼30) and isolated brown dwarfs (∼100). The oxygen isotope ratios are 16O/18O=425−28+33 and 16O/17O=1010−100+120 . All of the ratios are lower than the local interstellar medium and solar system, suggesting that abundances of the more minor isotopes are enhanced compared to the primary. This could be driven by isotope fractionation in protoplanetary disks, which can potentially alter the carbon and oxygen ratios through isotope selective photodissociation, gas/ice partitioning, and isotopic exchange reactions. In addition to CO, we constrain 1H 216O and 12C16O2 (the primary isotopologues of both species), but find only upper limits on 12C1H4 and 14N1H3. This work highlights the power of JWST to constrain isotopes in exoplanet atmospheres, with great promise in determining formation histories in the future