分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: JWST is here. The early release observation program (ERO) provides us with the first look at the scientific data and the spectral capabilities. One of the targets from ERO is HAT-P-18b, an inflated Saturn-mass planet with an equilibrium temperature of $\sim$850K. We present the NIRISS/SOSS transmission spectrum of HAT-P-18b from 0.6 to 2.8$\mu m$ and reveal the planet in the infrared beyond 1.6$\mu m$ for the first time. From the spectrum, we see clear water and escaping helium tail features in an otherwise very hazy atmosphere. Our free chemistry retrievals with ATMO show moderate Bayesian evidence (3.79) supporting the presence of methane, but the spectrum does not display any clearly identifiable methane absorption features. The retrieved methane abundance is $\sim$2 orders of magnitude lower than that of solar composition. The methane-depleted atmosphere strongly rejects simple equilibrium chemistry forward models with solar metallicity and C/O ratio and disfavors high metallicity (100 times) and low C/O ratio (0.3). This calls for additional physical processes such as vertical mixing and photochemistry which can remove methane from the atmosphere.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present high-resolution dayside thermal emission observations of the exoplanet WASP-18b using IGRINS on Gemini South. We remove stellar and telluric signatures using standard algorithms, and we extract the planet signal via cross correlation with model spectra. We detect the atmosphere of WASP-18b at a signal-to-noise ratio (SNR) of 5.9 using a full chemistry model, measure H2O (SNR=3.3), CO (SNR=4.0), and OH (SNR=4.8) individually, and confirm previous claims of a thermal inversion layer. The three species are confidently detected (>4$\sigma$) with a Bayesian inference framework, which we also use to retrieve abundance, temperature, and velocity information. For this ultra-hot Jupiter (UHJ), thermal dissociation processes likely play an important role. Retrieving abundances constant with altitude and allowing the temperature-pressure profile to freely adjust results in a moderately super-stellar carbon to oxygen ratio (C/O=0.75^{+0.14}_{-0.17}) and metallicity ([M/H]=1.03^{+0.65}_{-1.01}). Accounting for undetectable oxygen produced by thermal dissociation leads to C/O=0.45^{+0.08}_{-0.10} and [M/H]=1.17^{+0.66}_{-1.01}. A retrieval that assumes radiative-convective-thermochemical-equilibrium and naturally accounts for thermal dissociation constrains C/O<0.34 (2$\sigma$) and [M/H]=0.48^{+0.33}_{-0.29}, in line with the chemistry of the parent star. Looking at the velocity information, we see a tantalising signature of different Doppler shifts at the level of a few km/s for different molecules, which might probe dynamics as a function of altitude and location on the planet disk. Our results demonstrate that ground-based, high-resolution spectroscopy at infrared wavelengths can provide meaningful constraints on the compositions and climate of highly irradiated planets. This work also elucidates potential pitfalls with commonly employed retrieval assumptions when applied to UHJ spectra.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Measuring the abundances of carbon and oxygen in exoplanet atmospheres is considered a crucial avenue for unlocking the formation and evolution of exoplanetary systems. Access to an exoplanet's chemical inventory requires high-precision observations, often inferred from individual molecular detections with low-resolution space-based and high-resolution ground-based facilities. Here we report the medium-resolution (R$\sim$600) transmission spectrum of an exoplanet atmosphere between 3-5 $\mu$m covering multiple absorption features for the Saturn-mass exoplanet WASP-39b, obtained with JWST NIRSpec G395H. Our observations achieve 1.46x photon precision, providing an average transit depth uncertainty of 221 ppm per spectroscopic bin, and present minimal impacts from systematic effects. We detect significant absorption from CO$_2$ (28.5$\sigma$) and H$_2$O (21.5$\sigma$), and identify SO$_2$ as the source of absorption at 4.1 $\mu$m (4.8$\sigma$). Best-fit atmospheric models range between 3 and 10x solar metallicity, with sub-solar to solar C/O ratios. These results, including the detection of SO$_2$, underscore the importance of characterising the chemistry in exoplanet atmospheres, and showcase NIRSpec G395H as an excellent mode for time series observations over this critical wavelength range.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Ultra-hot Jupiters with equilibrium temperature greater than 2000K are uniquely interesting targets as they provide us crucial insights into how atmospheres behave under extreme conditions. This class of giant planets receives intense radiation from their host star and usually has strongly irradiated and highly inflated atmospheres. At such high temperature, cloud formation is expected to be suppressed and thermal dissociation of water vapor could occur. We observed the ultra-hot Jupiter WASP-76b with 7 transits and 5 eclipses using the Hubble Space Telescope (HST) and $Spitzer$ for a comprehensive study of its atmospheric chemical and physical processes. We detect TiO and H$_2$O absorption in the optical and near-infrared transit spectrum. Additional absorption by a number of neutral and ionized heavy metals like Fe, Ni, Ti, and SiO help explain the short wavelength transit spectrum. The secondary eclipse spectrum shows muted water feature but a strong CO emission feature in Spitzer's 4.5 $\mu$m band indicating an inverted temperature pressure profile. We analyzed both the transit and emission spectrum with a combination of self-consistent PHOENIX models and retrieval models (ATMO $\&$ PLATON). Both spectra are well fitted by the self-consistent PHOENIX forward atmosphere model in chemical and radiative equilibrium at solar metallicity, adding to the growing evidence that both TiO/VO and NUV heavy metals opacity are prominent NUV-optical opacity sources in the stratospheres of ultra-hot Jupiters.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Measuring the metallicity and carbon-to-oxygen (C/O) ratio in exoplanet atmospheres is a fundamental step towards constraining the dominant chemical processes at work and, if in equilibrium, revealing planet formation histories. Transmission spectroscopy provides the necessary means by constraining the abundances of oxygen- and carbon-bearing species; however, this requires broad wavelength coverage, moderate spectral resolution, and high precision that, together, are not achievable with previous observatories. Now that JWST has commenced science operations, we are able to observe exoplanets at previously uncharted wavelengths and spectral resolutions. Here we report time-series observations of the transiting exoplanet WASP-39b using JWST's Near InfraRed Camera (NIRCam). The long-wavelength spectroscopic and short-wavelength photometric light curves span 2.0 - 4.0 $\mu$m, exhibit minimal systematics, and reveal well-defined molecular absorption features in the planet's spectrum. Specifically, we detect gaseous H$_2$O in the atmosphere and place an upper limit on the abundance of CH$_4$. The otherwise prominent CO$_2$ feature at 2.8 $\mu$m is largely masked by H$_2$O. The best-fit chemical equilibrium models favour an atmospheric metallicity of 1-100$\times$ solar (i.e., an enrichment of elements heavier than helium relative to the Sun) and a sub-stellar carbon-to-oxygen (C/O) ratio. The inferred high metallicity and low C/O ratio may indicate significant accretion of solid materials during planet formation or disequilibrium processes in the upper atmosphere.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Close-in giant exoplanets with temperatures greater than 2,000 K (''ultra-hot Jupiters'') have been the subject of extensive efforts to determine their atmospheric properties using thermal emission measurements from the Hubble and Spitzer Space Telescopes. However, previous studies have yielded inconsistent results because the small sizes of the spectral features and the limited information content of the data resulted in high sensitivity to the varying assumptions made in the treatment of instrument systematics and the atmospheric retrieval analysis. Here we present a dayside thermal emission spectrum of the ultra-hot Jupiter WASP-18b obtained with the NIRISS instrument on JWST. The data span 0.85 to 2.85 $\mu$m in wavelength at an average resolving power of 400 and exhibit minimal systematics. The spectrum shows three water emission features (at $>$6$\sigma$ confidence) and evidence for optical opacity, possibly due to H$^-$, TiO, and VO (combined significance of 3.8$\sigma$). Models that fit the data require a thermal inversion, molecular dissociation as predicted by chemical equilibrium, a solar heavy element abundance (''metallicity'', M/H = 1.03$_{-0.51}^{+1.11}$ $\times$ solar), and a carbon-to-oxygen (C/O) ratio less than unity. The data also yield a dayside brightness temperature map, which shows a peak in temperature near the sub-stellar point that decreases steeply and symmetrically with longitude toward the terminators.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: JWST has begun its scientific mission, which includes the atmospheric characterization of transiting exoplanets. Some of the first exoplanets to be observed by JWST have equilibrium temperatures below 1000 K, which is a regime where photochemical hazes are expected to form. The optical properties of these hazes, which controls how they interact with light, are critical for interpreting exoplanet observations, but relevant data are not available. Here we measure the optical properties of organic haze analogues generated in water-rich exoplanet atmosphere experiments. We report optical constants (0.4 to 28.6 micron) of organic hazes for current and future observational and modeling efforts covering the entire wavelength range of JWST instrumentation and a large part of Hubble. We use these optical constants to generate hazy model atmospheric spectra. The synthetic spectra show that differences in haze optical constants have a detectable effect on the spectra, impacting our interpretation of exoplanet observations. This study emphasizes the need to investigate the optical properties of hazes formed in different exoplanet atmospheres, and establishes a practical procedure to determine such properties.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We study the emergent spectral fluxes of transiting hot Jupiters, using secondary eclipses from Spitzer. To achieve a large and uniform sample, we have re-analyzed all secondary eclipses for all hot Jupiters observed by Spitzer at 3.6- and/or 4.5 microns. Our sample comprises 457 eclipses of 122 planets, including eclipses of 13 planets not previously published. We use these eclipse depths to calculate the spectral fluxes emergent from the exoplanetary atmospheres, and thereby infer temperature and spectral properties of hot Jupiters. We find that an abrupt rise in brightness temperature, similar to a phase change, occurs on the day side atmospheres of the population at an equilibrium temperature between 1714K and 1818K (99-percent confidence limits). The amplitude of the rise is 291 +/-49 Kelvins, and two viable causes are the onset of magnetic drag that inhibits longitudinal heat redistribution, and/or the rapid dissipation of day side clouds. We also study hot Jupiter spectral properties with respect to metallicity and temperature inversions. Models exhibiting 4.5 micron emission from temperature inversions reproduce our fluxes statistically for the hottest planets, but the transition to emission is gradual, not abrupt. The Spitzer fluxes are sensitive to metallicity for planets cooler than approximately 1200 Kelvins, and most of the hot Jupiter population falls between model tracks having solar to 30X-solar metallicity.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Photochemistry is a fundamental process of planetary atmospheres that is integral to habitability, atmospheric composition and stability, and aerosol formation. However, no unambiguous photochemical products have been detected in exoplanet atmospheres to date. Here we show that photochemically produced sulphur dioxide (SO$_2$) is present in the atmosphere of the hot, giant exoplanet WASP-39b, as constrained by data from the JWST Transiting Exoplanet Early Release Science Program and informed by a suite of photochemical models. We find that SO$_2$ is produced by successive oxidation of sulphur radicals freed when hydrogen sulphide (H$_2$S) is destroyed. The SO$_2$ distribution computed by the photochemical models robustly explains the 4.05 $\mu$m spectral feature seen in JWST transmission spectra [Rustamkulov et al.(submitted), Alderson et al.(submitted)] and leads to observable features at ultraviolet and thermal infrared wavelengths not available from the current observations. The sensitivity of the SO$_2$ feature to the enrichment of heavy elements in the atmosphere ("metallicity") suggests that it can be used as a powerful tracer of atmospheric properties, with our results implying a metallicity of $\sim$10$\times$ solar for WASP-39b. Through providing improved constraints on bulk metallicity and sulphur abundance, the detection of SO$_2$ opens a new avenue for the investigation of giant-planet formation. Our work demonstrates that sulphur photochemistry may be readily observable for exoplanets with super-solar metallicity and equilibrium temperatures $\gtrsim$750 K. The confirmation of photochemistry through the agreement between theoretical predictions and observational data is pivotal for further atmospheric characterisation studies.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Photochemical hazes are important opacity sources in temperate exoplanet atmospheres, hindering current observations from characterizing exoplanet atmospheric compositions. The haziness of an atmosphere is determined by the balance between haze production and removal. However, the material-dependent removal physics of the haze particles is currently unknown under exoplanetary conditions. Here we provide experimentally-measured surface energies for a grid of temperate exoplanet hazes to characterize haze removal in exoplanetary atmospheres. We found large variations of surface energies for hazes produced under different energy sources, atmospheric compositions, and temperatures. The surface energies of the hazes were found to be the lowest around 400 K for the cold plasma samples, leading to the lowest removal rates. We show a suggestive correlation between haze surface energy and atmospheric haziness with planetary equilibrium temperature. We hypothesize that habitable zone exoplanets could be less hazy, as they would possess high-surface-energy hazes which can be removed efficiently.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Transmission spectroscopy provides insight into the atmospheric properties and consequently the formation history, physics, and chemistry of transiting exoplanets. However, obtaining precise inferences of atmospheric properties from transmission spectra requires simultaneously measuring the strength and shape of multiple spectral absorption features from a wide range of chemical species. This has been challenging given the precision and wavelength coverage of previous observatories. Here, we present the transmission spectrum of the Saturn-mass exoplanet WASP-39b obtained using the SOSS mode of the NIRISS instrument on the JWST. This spectrum spans $0.6 - 2.8 \mu$m in wavelength and reveals multiple water absorption bands, the potassium resonance doublet, as well as signatures of clouds. The precision and broad wavelength coverage of NIRISS-SOSS allows us to break model degeneracies between cloud properties and the atmospheric composition of WASP-39b, favoring a heavy element enhancement ("metallicity") of $\sim 10 - 30 \times$ the solar value, a sub-solar carbon-to-oxygen (C/O) ratio, and a solar-to-super-solar potassium-to-oxygen (K/O) ratio. The observations are best explained by wavelength-dependent, non-gray clouds with inhomogeneous coverage of the planet's terminator.