arXiv subjects

S. -M. Tsai

Publications and source records attributed to S. -M. Tsai.

4 recordsLinked to original sources

arXiv2026

JWST-SUPER I: New insights into irradiated warm Neptunes atmospheres from MIRI observations of HD 106315 c

Sulphur-bearing molecules have recently emerged as powerful tracers of atmospheric photochemistry in exoplanets observed with JWST. In several warm giant planets, SO$_2$ has been detected as a product of UV-driven chemical processing, suggesting a close connection between stellar irradiation, atmospheric metallicity, and sulphur chemistry. Whether these trends extend to Neptune-mass planets orbiting hotter stars remains largely unexplored. We investigate the atmospheric composition of the warm Neptune HD 106315 c, a Neptune-mass planet orbiting an F-type host star and subjected to a strong ultraviolet (UV) irradiation environment. Here, we report the low-resolution transmission spectrum between 5 and 12 $μ$m of HD 106315 c obtained with the MIRI Low-Resolution Spectrometer on-board JWST. Our work also includes re-analysis of archival data from HST WFC3 G141 mode of HD 106315 c, as well as contemporaneous XMM and Swift monitoring of the star in the UV and X-ray wavelength ranges. Together with the archival HST WFC3 data, we detect H$_2$O and find tentative evidence for NH$_3$, retrieving abundances of $\log_{10}$(H$_2$O)$=-1.40^{+0.40}_{-0.74}$ and $\log_{10}$(NH$_3$)$=-2.47^{+0.57}_{-0.89}$, together with an isothermal terminator temperature of $747^{+150}_{-155}$ K. We place stringent upper limits on CH$_4$, SO$_2$, and CS$_2$, finding no robust evidence for any sulphur-bearing species despite the intense irradiation received by the planet. The inferred water abundance implies a strongly metal-enriched atmosphere. Elevated intrinsic temperatures can reconcile the non-detections of CH$_4$ and CS$_2$ through carbon--sulphur coupling, while the absence of SO$_2$ points toward a reduced atmospheric sulphur inventory or a near-solar to mildly enhanced C/O ratio, and places HD 106315 c near the transition between sulphur-rich and sulphur-poor chemical regimes.

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arXiv2026

Benchmarking Two Chemical Networks used in General Circulation Models of Hot Jupiters

Chemical kinetics is becoming an increasingly vital component of hot Jupiter general circulation models (GCMs). Here we simulate the hot Jupiter WASP-96b using two chemical networks, a reduced chemical network frequently used in the GCM literature (which we refer to as V19) and a more recent effective network making use of tables of net reactions (MiniCHEM), coupled to the same GCM in order to provide a robust benchmark. We find a numerical escape criterion used by the Unified Model chemical kinetics solver to stop integration for the duration of the chemical timestep, independent of the chemical network, results in artificial quenching, overestimating of HCN, CH$_4$, and NH$_3$ abundances by factors of 1.5 to 3. With this criterion disabled, agreement between the two networks is improved, except for HCN and NH$_3$, where different reaction rates and included species results in lower abundances in the V19 network. While many rates differ between the networks, the lower quenched NH$_3$ abundances in the V19 simulations are, in particular, due to the choice of NH$_2$ + NH$_3$ $\rightarrow$ N$_2$H$_3$ + H$_2$ reaction rate, which is poorly constrained in the literature. This reaction also impacts the quenching of HCN, which is additionally affected by the lack of CH$_2$NH$_2$ in the V19 network. While there are reasons to favour the MiniCHEM HCN and NH$_3$ abundances, ultimately, improved experimental and theoretical determination of reaction rates are needed to address the uncertainties and better characterize the quenching behaviour.

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arXiv2025

Osiris revisited: Confirming a solar metallicity and low C/O in HD 209458b

HD 209458b is the prototypical hot Jupiter and one of the best targets available for precise atmosphere characterisation. Now that spectra from both Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) are available, we can reveal the atmospheric properties in unprecedented detail. In this study, we perform a new data reduction and analysis of the original HST/WFC3 spectrum, accounting for the wavelength dependence of the instrument systematics that was not considered in previous analyses. This allows us to precisely and robustly measure the much-debated H$_2$O abundance in HD 209458b's atmosphere. We combine the newly reduced spectrum with archival JWST/NIRCam data and run free chemistry atmospheric retrievals over the 1.0 - 5.1 $μ$m wavelength range, covering possible features of multiple absorbing species, including CO$_2$, CO, CH$_4$, NH$_3$, HCN, Na, SO$_2$, and H$_2$S. We detect H$_2$O and CO$_2$ robustly at above 7 $σ$ significance, and find a 3.6 $σ$ preference for cloudy models compared to a clear atmosphere. For all other absorbers we tested, only upper limits of abundance can be measured. We use Bayesian model averaging to account for a range of different assumptions about the cloud properties, resulting in a water volume mixing ratio of $0.95^{+0.35}_{-0.17} \:\times$ solar and a carbon dioxide abundance of $0.94^{+0.16}_{-0.09} \:\times$ solar. Both results are consistent with solar values and comparable to predictions from the VULCAN 1D photochemistry model. Combining these values with a prior on the CO abundance from ground-based measurements, we derive an overall atmospheric composition comparable to solar metallicity of $\mathrm{[M/H]} = 0.10^{+0.41}_{-0.40}$ and very low C/O of $0.054^{+0.080}_{-0.034}$ with a 3 $σ$ upper limit of 0.454. This indicates a strong enrichment in oxygen and depletion in carbon during HD 209458b's formation.

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arXiv2021

A new approach to spectroscopic phase curves: The emission spectrum of WASP-12b observed in quadrature with HST/WFC3

We analyse emission spectra of WASP-12b from a partial phase curve observed over three epochs with the Hubble Space Telescope, covering eclipse, quadrature, and transit, respectively. As the 1.1-day period phase curve was only partially covered over three epochs, traditional methods to extract the planet flux and instrument systematic errors cannot recover the thermal emission away from the secondary eclipse. To analyse this partial phase curve, we introduce a new method, which corrects for the wavelength-independent component of the systematic errors. Our new method removes the achromatic instrument and stellar variability, and uses the measured stellar spectrum in eclipse to then retrieve a relative planetary spectrum in wavelength at each phase. We are able to extract the emission spectrum of an exoplanet at quadrature outside of a phase curve for the first time; we recover the quadrature spectrum of WASP-12b up to an additive constant. The dayside emission spectrum is extracted in a similar manner, and in both cases we are able to estimate the brightness temperature, albeit at a greatly reduced precision. We estimate the brightness temperature from the dayside (Tday=3186+-677 K) and from the quadrature spectrum (Tquad=2124+-417 K) and combine them to constrain the energy budget of the planet. We compare our extracted relative spectra to global circulation models of this planet, which are generally found to be a good match. However, we do see tentative evidence of a steeper spectral slope in the measured dayside spectrum compared to our models. We find that we cannot match this increased slope by increasing optical opacities in our models. We also find that this spectral slope is unlikely to be explained by a non-equilibrium water abundance, as water advected from the nightside is quickly dissociated on the dayside.

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