arXiv subjects

David K. Sing

Publications and source records attributed to David K. Sing.

At least 55 records · Page 3Linked to original sources

arXiv2025

Statistical trends in JWST transiting exoplanet atmospheres

Our brains are hardwired for pattern recognition as correlations are useful for predicting and understanding nature. As more exoplanet atmospheres are being characterized with JWST, we are starting to unveil their properties on a population level. Here we present a framework for comparing exoplanet transmission spectroscopy from 3 to 5$μ$m with four bands: L (2.9 - 3.7$μ$m), SO$_2$ (3.95 - 4.1$μ$m), CO$_2$ (4.25 - 4.4$μ$m) and CO (4.5 - 4.9$μ$m). Together, the four bands cover the major carbon, oxygen, nitrogen, and sulfur-bearing molecules including H$_2$O, CH$_4$, NH$_3$, H$_2$S, SO$_2$, CO$_2$, and CO. Among the eight high-precision gas giant exoplanet planet spectra we collected, we found strong correlations between the SO$_2$-L index and planet mass (r=-0.41$\pm$0.09) and temperature (r=-0.64$\pm$0.08), indicating SO$_2$ preferably exists (SO$_2$-L$>$-0.5) among low mass ($\sim<$0.3M$_J$) and cooler ($\sim<$1200K) targets. We also observe strong temperature dependency for both CO$_2$-L and CO-L indices. Under equilibrium chemistry and isothermal thermal structure assumptions, we find that the planet sample favors super-solar metallicity and low C/O ratio ($<$0.7). In addition, the presence of a mass-metallicity correlation is favored over uniform metallicity with the eight planets. We further introduce the SO$_2$-L versus CO$_2$-L diagram alike the color-magnitude diagram for stars and brown dwarfs. All reported trends here will be testable and be further quantified with existing and future JWST observations within the next few years.

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arXiv2024

An HST Transmission Spectrum of the Closest M-Dwarf Transiting Rocky Planet LTT 1445Ab

Which rocky exoplanets have atmospheres? This presumably simply question is the first that must be answered to understand the prevalence of nearby habitable planets. A mere 6.9 pc from Earth, LTT 1445A is the closest transiting M-dwarf system, and its largest known planet, at $\rm 1.31\; R_{\oplus}$ and 424 K, is one of the most promising targets in which to search for an atmosphere. We use HST/WFC3 transmission spectroscopy with the G280 and G141 grisms to study the spectrum of LTT 1445Ab between $\rm 0.2-1.65\;μm$. In doing so, we uncover a UV flare on the neighboring star LTT 1445C that is completely invisible at optical wavelengths; we report one of the first simultaneous near-UV/optical spectra of an M~dwarf flare. The planet spectrum is consistent with a flat line (with median transit depth uncertainties of 128 and 52 ppm for the G280 and G141 observations, respectively), though the infrared portion displays potential features that could be explained by known opacity sources such as HCN. Some atmospheric retrievals weakly favor ($\sim2σ$) an atmosphere, but it remains challenging to discern between stellar contamination, an atmosphere, and a featureless spectrum at this time. We do, however, confidently rule out $\leq100\times$ solar metallicity atmospheres. Although stellar contamination retrievals cannot fit the infrared features well, the overall spectrum is consistent with stellar contamination from hot or cold spots. Based on the UV/optical data, we place limits on the extent of stellar variability expected in the near-infrared ($30-40$ ppm), which will be critical for future JWST observations.

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arXiv2024

Effects of Planetary Parameters on Disequilibrium Chemistry in Irradiated Planetary Atmospheres: From Gas Giants to Sub-Neptunes

A primary goal of characterizing exoplanet atmospheres is to constrain planetary bulk properties, such as their metallicity, C/O ratio, and intrinsic heat. However, there are significant uncertainties in many aspects of atmospheric physics, such as the strength of vertical mixing. Here we use PICASO and the photochem model to explore how atmospheric chemistry is influenced by planetary properties like metallicity, C/O ratio, $T_{\rm int}$, $T_{\rm eq}$, and $K_{\rm zz}$ in hydrogen-dominated atmospheres. We vary the $T_{\rm eq}$ of the planets between 400 K- 1600 K, across ``cold",``warm," and `hot" objects. We also explore an extensive range of $T_{\rm int}$ values between 30-500 K, representing sub-Neptunes to massive gas giants. We find that gases like CO and CO$_2$ show a drastically different dependence on $K_{\rm zz}$ and C/O for planets with cold interiors (e.g., sub-Neptunes) compared to planets with hotter interiors (e.g., Jupiter mass planets), for the same $T_{\rm eq}$. We also find that gases like CS and CS$_2$ can carry a significant portion of the S- inventory in the upper atmosphere near $T_{\rm eq}$ $\le$ 600 K, below which SO$_2$ ceases to be abundant. For solar C/O, we show that the CO/CH$_4$ ratio in the upper atmospheres of planets can become $\le$1 for planets with low $T_{\rm eq}$, but only if their interiors are cold ($T_{\rm int}$$\le$100 K). We find that photochemical haze precursor molecules in the upper atmosphere show very complex dependence on C/O, $K_{\rm zz}$, $T_{\rm eq}$, and $T_{\rm int}$ for planets with cold interiors (e.g., sub-Neptunes). We also briefly explore fully coupling PICASO and photochem to generate self-consistent radiative-convective-photochemical-equilibrium models.

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arXiv2024

TESS Giants Transiting Giants. VII. A Hot Saturn Orbiting an Oscillating Red Giant Star

We present the discovery of TOI-7041 b (TIC 201175570 b), a hot Saturn transiting a red giant star with measurable stellar oscillations. We observe solar-like oscillations in TOI-7041 with a frequency of maximum power of $ν_{\rm max} = 218.50\pm2.23$ $μ$Hz and a large frequency separation of $Δν= 16.5282\pm0.0186$ $μ$Hz. Our asteroseismic analysis indicates that TOI-7041 has a radius of $4.10 \pm 0.06$(stat) $\pm$ 0.05(sys) $R_\odot$, making it one of the largest stars around which a transiting planet has been discovered with the Transiting Exoplanet Survey Satellite (TESS), and the mission's first oscillating red giant with a transiting planet. TOI-7041 b has an orbital period of $9.691 \pm 0.006$ days and a low eccentricity of $e = 0.04 \pm 0.04$. We measure a planet radius of $1.02 \pm 0.03$ $R_J$ with photometry from TESS, and a planet mass of $0.36 \pm 0.16$ $M_J$ ($114 \pm 51$ $M_\oplus$) with ground-based radial velocity measurements. TOI-7041 b appears less inflated than similar systems receiving equivalent incident flux, and its circular orbit indicates that it is not undergoing tidal heating due to circularization. The asteroseismic analysis of the host star provides some of the tightest constraints on stellar properties for a TESS planet host and enables precise characterization of the hot Saturn. This system joins a small number of TESS-discovered exoplanets orbiting stars that exhibit clear stellar oscillations and indicates that extended TESS observations of evolved stars will similarly provide a path to improved exoplanet characterization.

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arXiv2024

Quartz Clouds in the Dayside Atmosphere of the Quintessential Hot Jupiter HD 189733 b

Recent mid-infrared observations with JWST/MIRI have resulted in the first direct detections of absorption features from silicate clouds in the transmission spectra of two transiting exoplanets, WASP-17 b and WASP-107 b. In this paper, we measure the mid-infrared ($5-12$ $μ$m) dayside emission spectrum of the benchmark hot Jupiter HD 189733 b with MIRI LRS by combining data from two secondary eclipse observations. We confirm the previous detection of H$_2$O absorption at 6.5 $μ$m from Spitzer/IRS and additionally detect H$_2$S as well as an absorption feature at 8.7 $μ$m in both secondary eclipse observations. The excess absorption at 8.7 $μ$m can be explained by the presence of small ($\sim$0.01 $μ$m) grains of SiO$_2$[s] in the uppermost layers of HD 189733 b's dayside atmosphere. This is the first direct detection of silicate clouds in HD 189733 b's atmosphere, and the first detection of a distinct absorption feature from silicate clouds on the day side of any hot Jupiter. We find that models including SiO$_2$[s] are preferred by $6-7σ$ over clear models and those with other potential cloud species. The high altitude location of these silicate particles is best explained by formation in the hottest regions of HD 189733 b's dayside atmosphere near the substellar point. We additionally find that HD 189733 b's emission spectrum longward of 9 $μ$m displays residual features not well captured by our current atmospheric models. When combined with other JWST observations of HD 189733 b's transmission and emission spectrum at shorter wavelengths, these observations will provide us with the most detailed picture to date of the atmospheric composition and cloud properties of this benchmark hot Jupiter.

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arXiv2024

Atmospheric retrievals suggest the presence of a secondary atmosphere and possible sulfur species on L 98-59 d from JWST NIRSpec G395H transmission spectroscopy

L 98-59 d is a Super-Earth planet orbiting an M-type star. We performed retrievals on the transmission spectrum of L 98-59 d obtained using NIRSpec G395H during a single transit, from JWST Cycle 1 GTO 1224. The wavelength range of this spectrum allows us to detect the presence of several atmospheric species. We found that the spectrum is consistent with a high mean molecular weight atmosphere. The atmospheric spectrum indicates the possible presence of the sulfur-bearing species H$_2$S and SO$_2$, which could hint at active volcanism on this planet if verified by future observations. We also tested for signs of stellar contamination in the spectrum, and found signs of unocculted faculae on the star. The tentative signs of an atmosphere on L 98-59 d presented in this work from just one transit bodes well for possible molecular detections in the future, particularly as it is one of the best targets among small exoplanets for atmospheric characterization using JWST.

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arXiv2024

Hints of a sulfur-rich atmosphere around the 1.6 R$_{\oplus}$ Super-Earth L98-59 d from JWST NIRSpec G395H transmission spectroscopy

Detecting atmospheres around planets with a radius below 1.6 R$_{\oplus}$, commonly referred to as rocky planets (Rogers_2015, Rogers_2021), has proven to be challenging. However, rocky planets orbiting M-dwarfs are ideal candidates due to their favorable planet-to-star radius ratio. Here, we present one transit observation of the Super-Earth L98-59d (1.58 R$_{\oplus}$, 2.31 M$_{\oplus}$), at the limit of rocky/gas-rich, using the JWST NIRSpec G395H mode covering the 2.8 to 5.1 microns wavelength range. The extracted transit spectrum from a single transit observation deviates from a flat line by 2.6 to 5.6$σ$, depending on the data reduction and retrieval setup. The hints of an atmospheric detection are driven by a large absorption feature between 3.3 to 4.8 microns. A stellar contamination retrieval analysis rejected the source of this feature as being due to stellar inhomogeneities, making the best fit an atmospheric model including sulfur-bearing species, suggesting that the atmosphere of L98-59d may not be at equilibrium. This result will need to be confirmed by the analysis of the second NIRSpec G395H visit in addition to the NIRISS SOSS transit observation.

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arXiv2024

Sulphur dioxide in the mid-infrared transmission spectrum of WASP-39b

The recent inference of sulphur dioxide (SO$_2$) in the atmosphere of the hot ($\sim$1100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST observations suggests that photochemistry is a key process in high temperature exoplanet atmospheres. This is due to the low ($<$1 ppb) abundance of SO$_2$ under thermochemical equilibrium, compared to that produced from the photochemistry of H$_2$O and H$_2$S (1-10 ppm). However, the SO$_2$ inference was made from a single, small molecular feature in the transmission spectrum of WASP-39b at 4.05 $μ$m, and therefore the detection of other SO$_2$ absorption bands at different wavelengths is needed to better constrain the SO$_2$ abundance. Here we report the detection of SO$_2$ spectral features at 7.7 and 8.5 $μ$m in the 5-12 $μ$m transmission spectrum of WASP-39b measured by the JWST Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS). Our observations suggest an abundance of SO$_2$ of 0.5-25 ppm (1$σ$ range), consistent with previous findings. In addition to SO$_2$, we find broad water vapour absorption features, as well as an unexplained decrease in the transit depth at wavelengths longer than 10 $μ$m. Fitting the spectrum with a grid of atmospheric forward models, we derive an atmospheric heavy element content (metallicity) for WASP-39b of $\sim$7.1-8.0 $\times$ solar and demonstrate that photochemistry shapes the spectra of WASP-39b across a broad wavelength range.

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arXiv2024

Hydrogen sulfide and metal-enriched atmosphere for a Jupiter-mass exoplanet

As the closest transiting hot Jupiter to Earth, HD 189733b has been the benchmark planet for atmospheric characterization. It has also been the anchor point for much of our theoretical understanding of exoplanet atmospheres from composition, chemistry, aerosols to atmospheric dynamics, escape, and modeling techniques. Prior studies of HD 189733b have detected carbon and oxygen-bearing molecules H2O and CO in the atmosphere. The presence of CO2 and CH4 has been claimed but later disputed. The inferred metallicity based on these measurements, a key parameter in tracing planet formation locations, varies from depletion to enhancement, hindered by limited wavelength coverage and precision of the observations. Here we report detections of H2O (13.4 sigma), CO2 (11.2 sigma), CO (5 sigma), and H2S (4.5 sigma) in the transmission spectrum (2.4-5 micron) of HD 189733b. With an equilibrium temperature of ~1200K, H2O, CO, and H2S are the main reservoirs for oxygen, carbon, and sulfur. Based on the measured abundances of these three major volatile elements, we infer an atmospheric metallicity of 3-5 times stellar. The upper limit on the methane abundance at 5 sigma is 0.1 ppm which indicates a low carbon-to-oxygen ratio (<0.2), suggesting formation through the accretion of water-rich icy planetesimals. The low oxygen-to-sulfur and carbon-to-sulfur ratios also support the planetesimal accretion formation pathway.

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arXiv2024

Latitudinal Asymmetry in the Dayside Atmosphere of WASP-43b

We present two-dimensional near-infrared temperature maps of the canonical hot Jupiter WASP-43b using a phase-curve observation with JWST NIRSpec/G395H. From the white-light planetary transit, we improve constraints on the planet's orbital parameters and measure a planet-to-star radius ratio of $0.15883^{+0.00056}_{-0.00053}$. Using the white-light phase curve, we measure a longitude of maximum brightness of $6.9^{+0^\circ.5}_{-0^\circ.5}$ east of the substellar point and a phase-curve offset of $10.0^{+0^\circ.8}_{-0^\circ.8}$. We also find an $\approx4σ$ detection of a latitudinal hotspot offset of $-13.4^{+3^\circ.2}_{-1^\circ.7}$, the first significant detection of a non-equatorial hotspot in an exoplanet atmosphere. We show that this detection is robust to variations within planetary parameter uncertainties, but only if the transit is used to improve constraints, showing the importance of transit observations to eclipse mapping. Maps retrieved from the NRS1 and NRS2 detectors are similar, with hotspot locations consistent between the two detectors at the $1σ$ level. Our JWST data show brighter (hotter) nightsides and a dimmer (colder) dayside at the shorter wavelengths relative to fits to \textit{Spitzer} 3.6 and 4.5 \microns\ phase curves. Through comparison between our phase curves and a set of general circulation models, we find evidence for clouds on the nightside and atmospheric drag or high metallicity reducing the eastward hotspot offset.

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arXiv2024

HST SHEL: Enabling Comparative Exoplanetology with HST/STIS

The Hubble Space Telescope (HST) has been our most prolific tool to study exoplanet atmospheres. As the age of JWST begins, there is a wealth of HST archival data that is useful to strengthen our inferences from JWST. Notably, HST/STIS and its 0.3-1 $μ$m wavelength coverage extends past JWST's 0.6 $μ$m wavelength cutoff and holds an abundance of potential information: alkali (Na, K) and molecular (TiO, VO) species opacities, aerosol information, and the presence of stellar contamination. However, time series observations with HST suffer from significant instrumental systematics and can be highly dependent on choices made during the transit fitting process. This makes comparing transmission spectra of planets with different data reduction methodologies challenging, as it is difficult to discern if an observed trend is caused by differences in data reduction or underlying physical processes. Here, we present the Sculpting Hubble's Exoplanet Legacy (SHEL) program, which aims to build a consistent data reduction and light curve analysis methodology and associated database of transmission spectra from archival HST observations. In this paper, we present the SHEL analysis framework for HST/STIS and its low-resolution spectroscopy modes, G430L and G750L. We apply our methodology to four notable hot Jupiters: WASP-39 b, WASP-121 b, WASP-69 b, and WASP-17 b, and use these examples to discuss nuances behind analysis with HST/STIS. Our results for WASP-39 b, WASP-121 b, and WASP-17 b are consistent with past publications, but our analysis of WASP-69 b differs and shows evidence of either a strong scattering slope or stellar contamination. The data reduction pipeline and tutorials are available on Github.

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arXiv2024

A warm Neptune's methane reveals core mass and vigorous atmospheric mixing

Observations of transiting gas giant exoplanets have revealed a pervasive depletion of methane, which has only recently been identified atmospherically. The depletion is thought to be maintained by disequilibrium processes such as photochemistry or mixing from a hotter interior. However, the interiors are largely unconstrained along with the vertical mixing strength and only upper limits on the CH$_4$ depletion have been available. The warm Neptune WASP-107 b stands out among exoplanets with an unusually low density, reported low core mass, and temperatures amenable to CH$_4$ though previous observations have yet to find the molecule. Here we present a JWST NIRSpec transmission spectrum of WASP-107 b which shows features from both SO$_2$ and CH$_4$ along with H$_2$O, CO$_2$, and CO. We detect methane with 4.2$σ$ significance at an abundance of 1.0$\pm$0.5 ppm, which is depleted by 3 orders of magnitude relative to equilibrium expectations. Our results are highly constraining for the atmosphere and interior, which indicate the envelope has a super-solar metallicity of 43$\pm$8$\times$ solar, a hot interior with an intrinsic temperature of T$_{\rm int}$=460$\pm$40 K, and vigorous vertical mixing which depletes CH4 with a diffusion coefficient of Kzz = 10$^{11.6\pm0.1}$ cm$^2$/s. Photochemistry has a negligible effect on the CH$_4$ abundance, but is needed to account for the SO$_2$. We infer a core mass of 11.5$_{-3.6}^{+3.0}$ M$_{\odot}$, which is much higher than previous upper limits, releasing a tension with core-accretion models.

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arXiv2024

Nightside clouds and disequilibrium chemistry on the hot Jupiter WASP-43b

Hot Jupiters are among the best-studied exoplanets, but it is still poorly understood how their chemical composition and cloud properties vary with longitude. Theoretical models predict that clouds may condense on the nightside and that molecular abundances can be driven out of equilibrium by zonal winds. Here we report a phase-resolved emission spectrum of the hot Jupiter WASP-43b measured from 5-12 $μ$m with JWST's Mid-Infrared Instrument (MIRI). The spectra reveal a large day-night temperature contrast (with average brightness temperatures of 1524$\pm$35 and 863$\pm$23 Kelvin, respectively) and evidence for water absorption at all orbital phases. Comparisons with three-dimensional atmospheric models show that both the phase curve shape and emission spectra strongly suggest the presence of nightside clouds which become optically thick to thermal emission at pressures greater than ~100 mbar. The dayside is consistent with a cloudless atmosphere above the mid-infrared photosphere. Contrary to expectations from equilibrium chemistry but consistent with disequilibrium kinetics models, methane is not detected on the nightside (2$σ$ upper limit of 1-6 parts per million, depending on model assumptions).

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arXiv2024

JWST/NIRCam Transmission Spectroscopy of the Nearby Sub-Earth GJ 341b

We present a JWST/NIRCam transmission spectrum from $3.9-5.0$ $μ$m of the recently-validated sub-Earth GJ 341b ($\mathrm{R_P} = 0.92$ $\mathrm{R_{\oplus}}$, $\mathrm{T_{eq}} = 540$ K) orbiting a nearby bright M1 star ($\mathrm{d} = 10.4$ pc, $\mathrm{K_{mag}}=5.6$). We use three independent pipelines to reduce the data from the three JWST visits and perform several tests to check for the significance of an atmosphere. Overall, our analysis does not uncover evidence of an atmosphere. Our null hypothesis tests find that none of our pipelines' transmission spectra can rule out a flat line, although there is weak evidence for a Gaussian feature in two spectra from different pipelines (at 2.3 and $2.9σ$). However, the candidate features are seen at different wavelengths (4.3 $μ$m vs 4.7 $μ$m), and our retrieval analysis finds that different gas species can explain these features in the two reductions (CO$_2$ at $3.1σ$ compared to O$_3$ at $2.9σ$), suggesting that they are not real astrophysical signals. Our forward model analysis rules out a low mean molecular weight atmosphere ($< 350\times$ solar metallicity) to at least $3σ$, and disfavors CH$_4$-dominated atmospheres at $1-3σ$, depending on the reduction. Instead, the forward models find our transmission spectra are consistent with no atmosphere, a hazy atmosphere, or an atmosphere containing a species that does not have prominent molecular bands across the NIRCam/F444W bandpass, such as a water-dominated atmosphere. Our results demonstrate the unequivocal need for two or more transit observations analyzed with multiple reduction pipelines, alongside rigorous statistical tests, to determine the robustness of molecular detections for small exoplanet atmospheres.

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arXiv2023

Insight into the Formation of $β$ Pic b through the Composition of Its Parent Protoplanetary Disk as Revealed by the $β$ Pic Moving Group Member HD 181327

It has been suggested that $β$ Pic b has a supersolar metallicity and subsolar C/O ratio. Assuming solar carbon and oxygen abundances for the star $β$ Pic and therefore the planet's parent protoplanetary disk, $β$ Pic b's C/O ratio suggests that it formed via core accretion between its parent protoplanteary disk's H$_{2}$O and CO$_{2}$ ice lines. $β$ Pic b's high metallicity is difficult to reconcile with its mass $M_{\text{p}}~=~11.7~M_{\text{Jup}}$ though. Massive stars can present peculiar photospheric abundances that are unlikely to record the abundances of their former protoplanetary disks. This issue can be overcome for early-type stars in moving groups by infering the elemental abundances of the FGK stars in the same moving group that formed in the same molecular cloud and presumably share the same composition. We infer the photospheric abundances of the F dwarf HD 181327, a $β$ Pic moving group member that is the best available proxy for the composition of $β$ Pic b's parent protoplanetary disk. In parallel, we infer updated atmospheric abundances for $β$ Pic b. As expected for a planet of its mass formed via core-accretion beyond its parent protoplanetary disk's H$_{2}$O ice line, we find that $β$ Pic b's atmosphere is consistent with stellar metallicity and confirm that is has superstellar carbon and oxygen abundances with a substellar C/O ratio. We propose that the elemental abundances of FGK dwarfs in moving groups can be used as proxies for the otherwise difficult-to-infer elemental abundances of early-type and late-type members of the same moving groups.

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arXiv2023

Double Trouble: Two Transits of the Super-Earth GJ 1132 b Observed with JWST NIRSpec G395H

The search for rocky planet atmospheres with JWST has focused on planets transiting M dwarfs. Such planets have favorable planet-to-star size ratios, enhancing the amplitude of atmospheric features. Since the expected signal strength of atmospheric features is similar to the single-transit performance of JWST, multiple observations are required to confirm any detection. Here, we present two transit observations of the rocky planet GJ 1132 b with JWST NIRSpec G395H, covering 2.8-5.2 $μ$m. Previous HST WFC3 observations of GJ 1132 b were inconclusive, with evidence reported for either an atmosphere or a featureless spectrum based on analyses of the same dataset. Our JWST data exhibit substantial differences between the two visits. One transit is consistent with either a H$_2$O-dominated atmosphere containing ~1% CH$_4$ and trace N$_2$O ($χ^{2}_ν$ = 1.13) or stellar contamination from unocculted starspots ($χ^{2}_ν$ = 1.36). However, the second transit is consistent with a featureless spectrum. Neither visit is consistent with a previous report of HCN. Atmospheric variability is unlikely to explain the scale of the observed differences between the visits. Similarly, our out-of-transit stellar spectra show no evidence of changing stellar inhomogeneity between the two visits - observed 8 days apart, only 6.5% of the stellar rotation rate. We further find no evidence of differing instrumental systematic effects between visits. The most plausible explanation is an unlucky random noise draw leading to two significantly discrepant transmission spectra. Our results highlight the importance of multi-visit repeatability with JWST prior to claiming atmospheric detections for these small, enigmatic planets.

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arXiv2023

Hydrodynamic atmospheric escape in HD 189733 b: Signatures of carbon and hydrogen measured with the Hubble Space Telescope

One of the most well-studied exoplanets to date, HD 189733 b, stands out as an archetypal hot Jupiter with many observations and theoretical models aimed at characterizing its atmosphere, interior, host star, and environment. We report here on the results of an extensive campaign to observe atmospheric escape signatures in HD 189733 b using the Hubble Space Telescope and its unique ultraviolet capabilities. We have found a tentative, but repeatable in-transit absorption of singly-ionized carbon (C II, $5.2\% \pm 1.4\%$) in the epoch of June-July/2017, as well as a neutral hydrogen (H I) absorption consistent with previous observations. We model the hydrodynamic outflow of HD 189733 b using an isothermal Parker wind formulation to interpret the observations of escaping C and O nuclei at the altitudes probed by our observations. Our forward models indicate that the outflow of HD 189733 b is mostly neutral within an altitude of $\sim 2$ R$_\mathrm{p}$ and singly ionized beyond that point. The measured in-transit absorption of C II at 133.57 nm is consistent with an escape rate of $\sim 1.1 \times 10^{11}$ g$\,$s$^{-1}$, assuming solar C abundance and outflow temperature of $12\,100$ K. Although we find a marginal neutral oxygen (O I) in-transit absorption, our models predict an in-transit depth that is only comparable to the size of measurement uncertainties. A comparison between the observed Lyman-$α$ transit depths and hydrodynamics models suggests that the exosphere of this planet interacts with a stellar wind at least one order of magnitude stronger than solar.

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arXiv2023

Photochemically-produced SO$_2$ in the atmosphere of WASP-39b

Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability. However, no unambiguous photochemical products have been detected in exoplanet atmospheres to date. Recent observations from the JWST Transiting Exoplanet Early Release Science Program found a spectral absorption feature at 4.05 $μ$m arising from SO$_2$ in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 M$_J$) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of $\sim$1100 K. The most plausible way of generating SO$_2$ in such an atmosphere is through photochemical processes. Here we show that the SO$_2$ distribution computed by a suite of photochemical models robustly explains the 4.05 $μ$m spectral feature identified by JWST transmission observations with NIRSpec PRISM (2.7$σ$) and G395H (4.5$σ$). SO$_2$ is produced by successive oxidation of sulphur radicals freed when hydrogen sulphide (H$_2$S) is destroyed. The sensitivity of the SO$_2$ feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of $\sim$10$\times$ solar. We further point out that SO$_2$ also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations.

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