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An Optical Transmission Spectrum for the Ultra-hot Jupiter WASP-121b Measured with the <i>Hubble Space Telescope</i>
摘要： We present an atmospheric transmission spectrum for the ultra-hot Jupiter WASP-121b, measured using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Across the 0.47–1 mm wavelength range, the data imply an atmospheric opacity comparable to—and in some spectroscopic channels exceeding—that previously measured at near-infrared wavelengths (1.15–1.65 mm). Wavelength-dependent variations in the opacity rule out a gray cloud deck at a confidence level of 3.7σ and may instead be explained by VO spectral bands. We find a cloud-free model assuming chemical equilibrium for a temperature of 1500 K and a metal enrichment of 10–30× solar matches these data well. Using a free-chemistry retrieval analysis, we estimate a VO abundance of -6.6 +0.3 -0.2 dex. We find no evidence for TiO and place a 3σ upper limit of ?7.9 dex on its abundance, suggesting TiO may have condensed from the gas phase at the day–night limb. The opacity rises steeply at the shortest wavelengths, increasing by approximately five pressure scale heights from 0.47 to 0.3 mm in wavelength. If this feature is caused by Rayleigh scattering due to uniformly distributed aerosols, it would imply an unphysically high temperature of 6810 ± 1530 K. One alternative explanation for the short-wavelength rise is absorption due to SH (mercapto radical), which has been predicted as an important product of non-equilibrium chemistry in hot Jupiter atmospheres. Irrespective of the identity of the NUV absorber, it likely captures a significant amount of incident stellar radiation at low pressures, thus playing a significant role in the overall energy budget, thermal structure, and circulation of the atmosphere.
关键词： planets and satellites: gaseous planets,methods: observational,planets and satellites: atmospheres
更新于2025-09-23 15:23:52
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Atmospheric Characterization of Directly Imaged Exoplanets with <i>JWST</i> /MIRI
摘要： The Mid-Infrared instrument (MIRI) on board the James Webb Space Telescope will perform the first ever characterization of young giant exoplanets observed by direct imaging in the 5–28 μm spectral range. This wavelength range is key for both determining the bolometric luminosity of the cool known exoplanets and for accessing the strongest ammonia bands. In conjunction with shorter wavelength observations, MIRI will enable a more accurate characterization of the exoplanetary atmospheric properties. Here we consider a subsample of the currently known exoplanets detected by direct imaging, and we discuss their detectability with MIRI, either using the coronagraphic or the spectroscopic modes. By using the Exo-REM atmosphere model, we calculate the mid-infrared emission spectra of 14 exoplanets, and we simulate MIRI coronagraphic or spectroscopic observations. Specifically, we analyze four coronagraphic observational setups, which depend on (i) the target-star and reference-star offset (0, 3, 14 mas), (ii) the wavefront-error (130, 204 nm root mean square), and (iii) the telescope jitter amplitude (1.6, 7 mas). We then determine the signal-to-noise and integration time values for the coronagraphic targets whose planet-to-star contrasts range from 3.9 to 10.1 mag. We conclude that all the MIRI targets should be observable with different degrees of difficulty, which depends on the final in-flight instrument performances. Furthermore, we test for detection of ammonia in the atmosphere of the coolest targets. Finally, we present the case of HR 8799 b to discuss what MIRI observations can bring to the knowledge of a planetary atmosphere, either alone or in combination with shorter wavelength observations.
关键词： planets and satellites: fundamental parameters,planets and satellites: atmospheres,telescopes,planets and satellites: gaseous planets,instrumentation: spectrographs,instrumentation: high angular resolution
更新于2025-09-23 15:21:21
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Circumbinary exoplanets and brown dwarfs with the Laser Interferometer Space Antenna
摘要： Aims. We explore the prospects for the detection of giant circumbinary exoplanets and brown dwarfs (BDs) orbiting Galactic double white dwarfs (DWDs) binaries with the Laser Interferometer Space Antenna (LISA). Methods. By assuming an occurrence rate of 50%, motivated by white dwarf pollution observations, we built a Galactic synthetic population of P-type giant exoplanets and BDs orbiting DWDs. We carried this out by injecting different sub-stellar populations, with various mass and orbital separation characteristics, into the DWD population used in the LISA mission proposal. We then performed a Fisher matrix analysis to measure how many of these three-body systems show a periodic Doppler-shifted gravitational wave perturbation detectable by LISA. Results. We report the number of circumbinary planets (CBPs) and BDs that can be detected by LISA for various combinations of mass and semi-major axis distributions. We identify pessimistic and optimistic scenarios corresponding, respectively, to 3 and 83 (14 and 2218) detections of CBPs (BDs), observed during the length of the nominal LISA mission. These detections are distributed all over the Galaxy following the underlying DWD distribution, and they are biased towards DWDs with higher LISA signal-to-noise ratio and shorter orbital period. Finally, we show that if LISA were to be extended for four more years, the number of systems detected will be more than doubled in both the optimistic and pessimistic scenarios. Conclusions. Our results present promising prospects for the detection of post-main sequence exoplanets and BDs, showing that gravitational waves can prove the existence of these populations over the totality of the Milky Way. Detections by LISA will deepen our knowledge on the life of exoplanets subsequent to the most extreme evolution phases of their hosts, clarifying whether new phases of planetary formation take place later in the life of the stars. Such a method is strongly complementary to electromagnetic studies within the solar region and opens a window into the investigation of planets and BDs everywhere in the entire Galaxy, and possibly even in nearby galaxies in the Local Group.
关键词： gravitational waves,planets and satellites: detection,white dwarfs,planets and satellites: gaseous planets,brown dwarfs
更新于2025-09-23 15:19:57
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MOPSS. I. Flat Optical Spectra for the Hot Jupiters WASP-4 b and WASP-52b
摘要： We present the first results from MOPSS, The Michigan Optical Planetary Spectra Survey, aimed at creating a database of optical planetary transmission spectra all observed, reduced, and analyzed with a uniform method for the benefit of enabling comparative exoplanet studies. We discuss our methods, and present results for our first two targets observed with the Magellan Baade 6.5 m telescope, one transit of the Hot Jupiter WASP-4b and two transits of the Hot Saturn WASP-52b. Both targets present flat, featureless spectra, corresponding to the presence of aerosols. We find that the cloud decks must begin no lower than 10?4 bar for both planets. For WASP-52b, we also consider the effects of star spots on the transmission spectrum, including unocculted spots and spots on the stellar limb influencing the light curve limb-darkening parameters. We discuss the usefulness of this program in the coming James Webb Space Telescope era.
关键词： planets and satellites: individual (WASP-52b, WASP-4b),planets and satellites: gaseous planets,planets and satellites: atmospheres,starspots,planets and satellites: composition
更新于2025-09-19 17:15:36
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Spatially resolved spectroscopy across stellar surfaces
摘要： Context. High-precision stellar analyses require hydrodynamic modeling to interpret chemical abundances or oscillation modes. Exoplanet atmosphere studies require stellar background spectra to be known along the transit path while detection of Earth analogs require stellar microvariability to be understood. Hydrodynamic 3D models can be computed for widely di?erent stars but have been tested in detail only for the Sun with its resolved surface features. Model predictions include spectral line shapes, asymmetries, and wavelength shifts, and their center-to-limb changes across stellar disks. Aims. We observe high-resolution spectral line pro?les across spatially highly resolved stellar surfaces, which are free from the e?ects of spatial smearing and rotational broadening present in full-disk spectra, enabling comparisons to synthetic pro?les from 3D models. Methods. During exoplanet transits, successive stellar surface portions become hidden and di?erential spectroscopy between various transit phases provides spectra of small surface segments temporarily hidden behind the planet. Planets cover no more than ～1% of any main-sequence star, enabling high spatial resolution but demanding very precise observations. Realistically measurable quantities are identi?ed through simulated observations of synthetic spectral lines. Results. In normal stars, line pro?le ratios between various transit phases may vary by ～0.5%, requiring S /N (cid:38) 5000 for meaningful spectral reconstruction. While not yet realistic for individual spectral lines, this is achievable for cool stars by averaging over numerous lines with similar parameters. Conclusions. For bright host stars of large transiting planets, spatially resolved spectroscopy is currently practical. More observable targets are likely to be found in the near future by ongoing photometric searches.
关键词： line: pro?les,hydrodynamics,stars: atmospheres,planets and satellites: gaseous planets,techniques: spectroscopic,stars: solar-type
更新于2025-09-10 09:29:36
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Methane in Analogs of Young Directly Imaged Exoplanets
摘要： We present detections of methane in R ～ 1300, L-band spectra of VHS 1256 b and PSO 318.5, two low-gravity, red, late L dwarfs that share the same colors as the HR 8799 planets. These spectra reveal shallow methane features, which indicate VHS 1256 b and PSO 318.5 have photospheres that are out of chemical equilibrium. Directly imaged exoplanets usually have redder near-infrared colors than the ?eld-age population of brown dwarfs on a color–magnitude diagram. These objects along the L-to-T transition show reduced methane absorption and evidence of photospheric clouds. Compared to the H- and K-bands, L-band (3 μm–4 μm) spectroscopy provides stronger constraints on the methane abundances of brown dwarfs and directly imaged exoplanets that have similar effective temperatures to L-to-T transition objects. When combined with near-infrared spectra, the L-band extends our conventional wavelength coverage, increasing our understanding of atmospheric cloud structure. Our model comparisons show that relatively strong vertical mixing and photospheric clouds can explain the molecular absorption features and continua of VHS 1256 b and PSO 318.5. We also discuss the implications of this work for future exoplanet-focused instruments and observations with the James Webb Space Telescope.
关键词： stars: individual (VHS J125601.92–125723.9b, PSO J318.5338–22.8603),planets and satellites: gaseous planets,brown dwarfs,planets and satellites: atmospheres
更新于2025-09-04 15:30:14
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The LEECH Exoplanet Imaging Survey: Limits on Planet Occurrence Rates under Conservative Assumptions
摘要： We present the results of the largest L′ (3.8 μm) direct imaging survey for exoplanets to date, the Large Binocular Telescope Interferometer Exozodi Exoplanet Common Hunt (LEECH). We observed 98 stars with spectral types from B to M. Cool planets emit a larger share of their ?ux in L′ compared to shorter wavelengths, affording LEECH an advantage in detecting low-mass, old, and cold-start giant planets. We emphasize proximity over youth in our target selection, probing physical separations smaller than other direct imaging surveys. For FGK stars, LEECH outperforms many previous studies, placing tighter constraints on the hot-start planet occurrence frequency interior to ～20 au. For less luminous, cold-start planets, LEECH provides the best constraints on giant-planet frequency interior to ～20 au around FGK stars. Direct imaging survey results depend sensitively on both the choice of evolutionary model (e.g., hot- or cold-start) and assumptions (explicit or implicit) about the shape of the underlying planet distribution, in particular its radial extent. Arti?cially low limits on the planet occurrence frequency can be derived when the shape of the planet distribution is assumed to extend to very large separations, well beyond typical protoplanetary dust-disk radii (?50 au), and when hot-start models are used exclusively. We place a conservative upper limit on the planet occurrence frequency using cold-start models and planetary population distributions that do not extend beyond typical protoplanetary dust-disk radii. We ?nd that ?90% of FGK systems can host a 7–10 MJup planet from 5 to 50 au. This limit leaves open the possibility that planets in this range are common.
关键词： planetary systems,techniques: high angular resolution,planets and satellites: gaseous planets,stars: imaging
更新于2025-09-04 15:30:14