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Abstract:

Sulfur chemistry has emerged as an important probe of exoplanet atmospheres in the JWST era, although observational constraints have thus far been largely limited to SO2 and H2S in warm and hot exoplanets. Recent JWST observations have revealed CS2 in several cooler gas giant exoplanets, yielding a new tracer of sulfur chemistry. However, the detailed chemical pathways responsible for the formation of CS2 remain poorly understood. Here, we use TOI-6894 b, a temperate gas giant with evidence for CS2, as a test case for one-dimensional photochemical kinetic-transport modeling and sensitivity analyses of CS2 chemistry. We show that CS2 is produced through coupled thermochemical and photochemical processes involving CH4 and H2S as the primary carbon and sulfur reservoirs, with S2 photolysis driving disequilibrium sulfur chemistry. Our models provide a physically consistent explanation for the observed CS2 feature in TOI-6894 b. Extending our analysis to gas giant exoplanets spanning a wide range of Teq, we find that CS2 abundance peaks in temperate to warm atmospheres (Teq ∼ 500–700 K), and declines toward both lower and higher temperatures. This temperature dependence provides a unified framework for interpreting current CS2 observations, accounting for reported detections in temperate to warm planets and the lack of detections in colder and hotter giant exoplanets. Our results establish CS2 as a complementary probe of sulfur inventories and atmospheric metallicity in cool gas giant exoplanets.

Abstract:

A key science goal for the Planetary Camera and Spectrograph (ELT-PCS) instrument is to image rocky exoplanets in reflected light around nearby M- and K-dwarfs. These observations will require a contrast of 10−9 at 100 mas angular separation, and 10−8 at 10 mas. To achieve such an extreme performance, every instrument design choice must be carefully evaluated. For ELT-PCS’s science instruments, perhaps the most critical design choice is which integral field unit (IFU) technology to use. To this end, we have developed a modular, lab-based integral field spectrograph test bench with which to compare the performance of two popular IFU technologies: image slicers and lenslet arrays. The test bench implements a Lyot coronagraph and a spatial light modulator to simulate a high-contrast imaging system, which feeds each IFU with a diffraction-limited PSF image. Each IFU is coupled to the spectrograph, which achieves R≈3000 with the image slicer and R≈100 with the lenslet array. The calibration and data reduction for each IFU design are described and the initial results of a contrast comparison are presented. The lenslet array achieves a deeper contrast at angular separations of less than 6λ/D. The image slicer, however, gains more at small separations from applying spectral deconvolution, with a 3.4× greater gain at 4λ/D. Spectral deconvolution further yields a contrast curve close to the detector noise floor. Further work will focus on carrying out a fully-controlled characterisation of the lenslet array IFU and a more comprehensive study into the effects of adaptive optics residuals on the contrast.

Abstract:

Ultrahot Jupiters (UHJs; Teq ≳ 2000 K) enable simultaneous detection of volatile (ice-forming) and refractory (rock-forming) elements in planetary atmospheres, providing a powerful diagnostic of planet formation and atmospheric processing. We present a comprehensive high-resolution cross-correlation spectroscopy analysis of the UHJ MASCARA-1 b (Teq ≈ 2600 K) using the IGRINS and IGRINS-2 spectrographs. We detect robust (signal-to-noise ratio > 4) signals from H2O, CO, OH, Fe i, Mg i, Ca i, and Ti i, marking the most complete atmospheric inventory of MASCARA-1 b to date. Using a chemically consistent atmospheric inference framework, we constrain elemental abundances to a typical precision of ≈0.2 dex, retrieving a solar atmospheric metallicity ([M/H]⊙ (Formula presented) =0.07−0.13+0.17 ≈1.2 × solar), a C/O ratio (C/O (Formula presented) =0.65−0.08+0.08 ) consistent with solar value (C/O = 0.59), an enhanced refractory abundance ([ (Formula presented) R /H]⊙= (Formula presented) 0.40−0.17+0.23≈2.5× solar; ≈3.8 × stellar), and a moderately supersolar refractory-to-volatile ratio ([ (Formula presented) R/V ]⊙ =  (Formula presented) 0.36−0.09+0.11 ≈ 2.3× solar). Comparison with formation models suggests that MASCARA-1 b most likely accreted material between the soot–H2O or H2O–CO snowlines (at 68% confidence). We additionally find stellar values for atmospheric Ti/Fe and Ca/Fe ratios (at 68% confidence). The Mg/Fe is also found to be consistent with stellar value at 95% confidence. Therefore, we do not find strong indication of nightside cold trapping in MASCARA-1 b. As homogeneous refractory-to-volatile measurements expand across the UHJ population, particularly with upcoming Extremely Large Telescopes, these diagnostics will enable statistically robust tests of emerging trends in giant planet formation and atmospheric evolution.