分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present a new Keck/NIRSPEC observation of metastable helium absorption from the upper atmosphere of HD 189733b, a hot Jupiter orbiting a nearby moderately active star. We measure an average helium transit depth of $0.420 \pm 0.013$% integrated over the [-20, 20] km/s velocity range. Comparing this measurement to eight previously published transit observations with different instruments, we find that our depth is 32% (9$\sigma$) lower than the average of the three CARMENES transits, but only 16% (4.4$\sigma$) lower than the average of the five GIANO transits. We perform 1D hydrodynamical simulations of the outflow, and find that XUV variability on the order of 33%--common for this star--can change the helium absorption depth by 60%. We conclude that changes in stellar XUV flux can explain the observational variability in helium absorption. 3D models are necessary to explore other sources of variability, such as shear instability and changing stellar wind conditions.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The GRAVITY instrument has been revolutionary for near-infrared interferometry by pushing sensitivity and precision to previously unknown limits. With the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) in GRAVITY+, these limits will be pushed even further, with vastly improved sky coverage, as well as faint-science and high-contrast capabilities. This upgrade includes the implementation of wide-field off-axis fringe-tracking, new adaptive optics systems on all Unit Telescopes, and laser guide stars in an upgraded facility. GRAVITY+ will open up the sky to the measurement of black hole masses across cosmic time in hundreds of active galactic nuclei, use the faint stars in the Galactic centre to probe General Relativity, and enable the characterisation of dozens of young exoplanets to study their formation, bearing the promise of another scientific revolution to come at the VLTI.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We observed a full-orbit phase curve of the hot Jupiter WASP-43b with MIRI/LRS as part of the Transiting Exoplanet Community Early Release Science Program. Here we report preliminary findings for the instrument performance from the team's MIRI Working Group. Overall we find that MIRI's performance for phase curve observations is excellent, with a few minor caveats. The key takeaways for Cycle 2 planning with MIRI/LRS are: (1) long-duration observations (> 24 hours) have now been successfully executed; (2) for phase curves, we recommend including a one-hour burn-in period prior to taking science data to mitigate the effects of the ramp systematic; and (3) we do not yet recommend partial phase curve observations. In addition, we also find that: the position of the spectrum on the detector is stable to within 0.03 pixels over the full 26.5-hour observation; the light curves typically show a systematic downward ramp that is strongest for the first 30 minutes, but continues to decay for hours; from 10.6-11.8 microns, the ramp effect has remarkably different behavior, possibly due to a different illumination history for the affected region of the detector; after trimming the integrations most affected by the initial ramps and correcting the remaining systematics with analytic models, we obtain residuals to the light-curve fits that are typically within 25% of the photon noise limit for 0.5-micron spectroscopic bins; non-linearity correction is not a significant source of additional noise for WASP-43, though it may be an issue for brighter targets; the gain value of 5.5 electrons/DN currently on CRDS and JDox is known to be incorrect, and the current best estimate for the gain is approximately 3.1 electrons/DN; new reference files for the JWST calibration pipeline reflecting these findings are under development at STScI.
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: 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.
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: We validate the presence of a two-planet system orbiting the 0.15--1.4 Gyr K4 dwarf TOI 560 (HD 73583). The system consists of an inner moderately eccentric transiting mini-Neptune (TOI 560 b, $P = 6.3980661^{+0.0000095}_{-0.0000097}$ days, $e=0.294^{+0.13}_{-0.062}$, $M= 0.94^{+0.31}_{-0.23}M_{Nep}$) initially discovered in the Sector 8 \tess\ mission observations, and a transiting mini-Neptune (TOI 560 c, $P = 18.8805^{+0.0024}_{-0.0011}$ days, $M= 1.32^{+0.29}_{-0.32}M_{Nep}$) discovered in the Sector 34 observations, in a rare near-1:3 orbital resonance. We utilize photometric data from \tess\, \textit{Spitzer}, and ground-based follow-up observations to confirm the ephemerides and period of the transiting planets, vet false positive scenarios, and detect the photo-eccentric effect for TOI 560 b. We obtain follow-up spectroscopy and corresponding precise radial velocities (RVs) with the iSHELL spectrograph at the NASA Infrared Telescope Facility and the HIRES Spectrograph at Keck Observatory to validate the planetary nature of these signals, which we combine with published PFS RVs from Magellan Observatory. We detect the masses of both planets at $> 3-\sigma$ significance. We apply a Gaussian process (GP) model to the \tess\ light curves to place priors on a chromatic radial velocity GP model to constrain the stellar activity of the TOI 560 host star, and confirm a strong wavelength dependence for the stellar activity demonstrating the ability of NIR RVs in mitigating stellar activity for young K dwarfs. TOI 560 is a nearby moderately young multi-planet system with two planets suitable for atmospheric characterization with James Webb Space Telescope (JWST) and other upcoming missions. In particular, it will undergo six transit pairs separated by $<$6 hours before June 2027.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Photochemistry is a fundamental process of planetary atmospheres that is integral to habitability, atmospheric composition and stability, and aerosol formation. However, no unambiguous photochemical products have been detected in exoplanet atmospheres to date. Here we show that photochemically produced sulphur dioxide (SO$_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$750 K. The confirmation of photochemistry through the agreement between theoretical predictions and observational data is pivotal for further atmospheric characterisation studies.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Transiting exoplanets orbiting young nearby stars are ideal laboratories for testing theories of planet formation and evolution. However, to date only a handful of stars with age <1 Gyr have been found to host transiting exoplanets. Here we present the discovery and validation of a sub-Neptune around HD 18599, a young (300 Myr), nearby (d=40 pc) K star. We validate the transiting planet candidate as a bona fide planet using data from the TESS, Spitzer, and Gaia missions, ground-based photometry from IRSF, LCO, PEST, and NGTS, speckle imaging from Gemini, and spectroscopy from CHIRON, NRES, FEROS, and Minerva-Australis. The planet has an orbital period of 4.13 d, and a radius of 2.7Rearth. The RV data yields a 3-sigma mass upper limit of 30.5Mearth which is explained by either a massive companion or the large observed jitter typical for a young star. The brightness of the host star (V~9 mag) makes it conducive to detailed characterization via Doppler mass measurement which will provide a rare view into the interior structure of young planets.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report the discovery and characterization of a transiting warm sub-Neptune planet around the nearby bright ($V=8.75$ mag, $K=7.15$ mag) solar twin HD 183579, delivered by the Transiting Exoplanet Survey Satellite (TESS). The host star is located $56.8\pm0.1$ pc away with a radius of $R_{\ast}=0.97\pm0.02R_{\odot}$ and a mass of $M_{\ast}=1.03\pm0.05\ M_{\odot}$. We confirm the planetary nature by combining space and ground-based photometry, spectroscopy, and imaging. We find that HD 183579b (TOI-1055b) has a radius of $R_{p}=3.53\pm0.13\ R_{\oplus}$ on a $17.47$ day orbit with a mass of $M_{p}=11.2\pm5.4\ M_{\oplus}$ ($3\sigma$ mass upper limit of $27.4M_{\oplus}$). HD 183579b is the fifth brightest known sub-Neptune planet system in the sky, making it an excellent target for future studies of the interior structure and atmospheric properties. By performing a line-by-line differential analysis using the high resolution and signal-to-noise ratio HARPS spectra, we find that HD 183579 joins the typical solar twin sample, without a statistically significant refractory element depletion.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: With the increasing number of planets discovered by TESS, the atmospheric characterization of small exoplanets is accelerating. L98-59 is a M-dwarf hosting a multi-planet system, and so far, four small planets have been confirmed. The innermost planet b is $\sim15\%$ smaller and $\sim60\%$ lighter than Earth, and should thus have a predominantly rocky composition. The Hubble Space Telescope observed five primary transits of L98-59b in $1.1-1.7\ \mu$m, and here we report the data analysis and the resulting transmission spectrum of the planet. We measure the transit depths for each of the five transits and, by combination, we obtain a transmission spectrum with an overall precision of $\sim20$ ppm in for each of the 18 spectrophotometric channels. With this level of precision, the transmission spectrum does not show significant modulation, and is thus consistent with a planet without any atmosphere or a planet having an atmosphere and high-altitude clouds or haze. The scenarios involving an aerosol-free, H$_2$-dominated atmosphere with H$_2$O or CH$_4$ are inconsistent with the data. The transmission spectrum also disfavors, but does not rules out, an H$_2$O-dominated atmosphere without clouds. A spectral retrieval process suggests that an H$_2$-dominated atmosphere with HCN and clouds or haze may be the preferred solution, but this indication is non-conclusive. Future James Webb Space Telescope observations may find out the nature of the planet among the remaining viable scenarios.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present observations of the 1.35+/-0.07 Earth-radius planet L 98-59 c using Wide Field Camera~3 on the Hubble Space Telescope. L 98-59 is a nearby (10.6 pc), bright (H=7.4 mag), M3V star that harbors three small, transiting planets. As one of the closest known transiting multi-planet systems, L 98-59 offers one of the best opportunities to probe and compare the atmospheres of rocky planets that formed in the same stellar environment. We measured the transmission spectrum of L 98-59 c during a single transit, with the extracted spectrum showing marginal evidence for wavelength-dependent transit depth variations which would indicate the presence of an atmosphere. Forward modeling was used to constrain possible atmospheric compositions of the planet based on the shape of the transmission spectrum. Although L 98-59 is a fairly quiet star, we have seen evidence for stellar activity, and therefore we cannot rule out a scenario where the source of the signal originates with inhomogeneities on the host-star surface. While intriguing, our results are inconclusive and additional data is needed to verify any atmospheric signal. Fortunately, additional data will soon be collected from both HST and JWST. Should this result be confirmed with additional data, L 98-59 c would be the first planet smaller than 2 Earth-radii with a detected atmosphere, and among the first small planets with a known atmosphere to be studied in detail by the JWST.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: WD 1856+534 b is a Jupiter-sized, cool giant planet candidate transiting the white dwarf WD 1856+534. Here, we report an optical transmission spectrum of WD 1856+534 b obtained from ten transits using the Gemini Multi-Object Spectrograph. This system is challenging to observe due to the faintness of the host star and the short transit duration. Nevertheless, our phase-folded white light curve reached a precision of 0.12 %. WD 1856+534 b provides a unique transit configuration compared to other known exoplanets: the planet is $8\times$ larger than its star and occults over half of the stellar disc during mid-transit. Consequently, many standard modeling assumptions do not hold. We introduce the concept of a `limb darkening corrected, time-averaged transmission spectrum' and propose that this is more suitable than $(R_{\mathrm{p}, \lambda} / R_{\mathrm{s}})^2$ for comparisons to atmospheric models for planets with grazing transits. We also present a modified radiative transfer prescription. Though the transmission spectrum shows no prominent absorption features, it is sufficiently precise to constrain the mass of WD 1856+534 b to be > 0.84 M$_\mathrm{J}$ (to $2 \, \sigma$ confidence), assuming a clear atmosphere and a Jovian composition. High-altitude cloud decks can allow lower masses. WD 1856+534 b could have formed either as a result of common envelope evolution or migration under the Kozai-Lidov mechanism. Further studies of WD 1856+534 b, alongside new dedicated searches for substellar objects around white dwarfs, will shed further light on the mysteries of post-main sequence planetary systems.