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Register a new userAn HST Optical to Near-IR Transmission Spectrum of the Hot Jupiter WASP-19b: Detection of Atmospheric Water and Likely Absence of TiO
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Catherine Huitson Mrs
Publication date
2013
fields
Physics
and research's language is
English
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We measure the transmission spectrum of WASP-19b from 3 transits using low-resolution optical spectroscopy from the HST Space Telescope Imaging Spectrograph (STIS). The STIS spectra cover a wavelength range of 0.29-1.03 microns with resolving power R=500. The optical data are combined with archival near-IR data from the HST Wide Field Camera 3 (WFC3) G141 grism, covering the wavelength range 1.087-1.687 micron, with resolving power R=130. We obtain the transmission spectrum from 0.53-1.687 microns with S/N levels between 3000 and 11,000 in 0.1 micron bins. WASP-19 is a very active star, with optical stellar flux varying by a few per cent over time. We correct the transit light curves for the effects of stellar activity using ground-based activity monitoring with the Cerro Tololo Inter-American Observatory (CTIO). While we were not able to construct a transmission spectrum using the blue optical data due to the presence of large occulted star spots, we were able to use the spot crossings to help constrain the mean stellar spot temperature. To search for predicted features in the hot-Jupiter atmosphere, we also define spectral indices for differential radius measurements to specifically search for the presence of TiO and alkali line features. Our measurements rule out TiO features predicted for a planet of WASP-19bs equilibrium temperature (2050 K) in the transmission spectrum at the 2.7-2.9 sigma confidence level, depending on atmospheric model formalism. The WFC3 transmission spectrum shows strong absorption features due to the presence of H2O, which is detected at the 4 sigma confidence level between 1.1 and 1.4 microns. The results indicate that WASP-19b is a planet with no or low levels of TiO and without a high C/O ratio. The lack of observable TiO features are possibly due to rainout, breakdown from stellar activity or the presence of other absorbers in the optical.
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Detailed characterization of exoplanets has begun to yield measurements of their atmospheric properties that constrain the planets origins and evolution. For example, past observations of the dayside emission spectrum of the hot Jupiter WASP-12b indicated that its atmosphere has a high carbon-to-oxygen ratio (C/O $>$ 1), suggesting it had a different formation pathway than is commonly assumed for giant planets. Here we report a precise near-infrared transmission spectrum for WASP-12b based on six transit observations with the Hubble Space Telescope/Wide Field Camera 3. We bin the data in 13 spectrophotometric light curves from 0.84 - 1.67 $mu$m and measure the transit depths to a median precision of 51 ppm. We retrieve the atmospheric properties using the transmission spectrum and find strong evidence for water absorption (7$sigma$ confidence). This detection marks the first high-confidence, spectroscopic identification of a molecule in the atmosphere of WASP-12b. The retrieved 1$sigma$ water volume mixing ratio is between $10^{-5}-10^{-2}$, which is consistent with C/O $>$ 1 to within 2$sigma$. However, we also introduce a new retrieval parameterization that fits for C/O and metallicity under the assumption of chemical equilibrium. With this approach, we constrain C/O to $0.5^{+0.2}_{-0.3}$ at $1,sigma$ and rule out a carbon-rich atmosphere composition (C/O$>1$) at $>3sigma$ confidence. Further observations and modeling of the planets global thermal structure and dynamics would aid in resolving the tension between our inferred C/O and previous constraints. Our findings highlight the importance of obtaining high-precision data with multiple observing techniques in order to obtain robust constraints on the chemistry and physics of exoplanet atmospheres.
We present a new optical transmission spectrum of the hot Jupiter WASP-79b. We observed three transits with the STIS instrument mounted on HST, spanning 0.3 - 1.0 um. Combining these transits with previous observations, we construct a complete 0.3 - 5.0 um transmission spectrum of WASP-79b. Both HST and ground-based observations show decreasing transit depths towards blue wavelengths, contrary to expectations from Rayleigh scattering or hazes. We infer atmospheric and stellar properties from the full near-UV to infrared transmission spectrum of WASP-79b using three independent retrieval codes, all of which yield consistent results. Our retrievals confirm previous detections of H$_{2}$O (at 4.0$sigma$ confidence), while providing moderate evidence of H$^{-}$ bound-free opacity (3.3$sigma$) and strong evidence of stellar contamination from unocculted faculae (4.7$sigma$). The retrieved H$_{2}$O abundance ($sim$ 1$%$) suggests a super-stellar atmospheric metallicity, though stellar or sub-stellar abundances remain consistent with present observations (O/H = 0.3 - 34$times$ stellar). All three retrieval codes obtain a precise H$^{-}$ abundance constraint: log(X$_{rm{H^{-}}}$) $approx$ -8.0 $pm$ 0.7. The potential presence of H$^{-}$ suggests that JWST observations may be sensitive to ionic chemistry in the atmosphere of WASP-79b. The inferred faculae are $sim$ 500 K hotter than the stellar photosphere, covering $sim$ 15$%$ of the stellar surface. Our analysis underscores the importance of observing UV - optical transmission spectra in order to disentangle the influence of unocculted stellar heterogeneities from planetary transmission spectra.
We present the optical transmission spectrum of the hot Jupiter WASP-104b based on one transit observed by the blue and red channels of the DBSP spectrograph at the Palomar 200-inch telescope and 14 transits observed by the MuSCAT2 four-channel imager at the 1.52 m Telescopio Carlos Sanchez. We also analyse 45 additional K2 transits, after correcting for the flux contamination from a companion star. Together with the transit light curves acquired by DBSP and MuSCAT2, we are able to revise the system parameters and orbital ephemeris, confirming that no transit timing variations exist. Our DBSP and MuSCAT2 combined transmission spectrum reveals an enhanced slope at wavelengths shorter than 630 nm and suggests the presence of a cloud deck at longer wavelengths. While the Bayesian spectral retrieval analyses favour a hazy atmosphere, stellar spot contamination cannot be completely ruled out. Further evidence, from transmission spectroscopy and detailed characterisation of the host stars activity, is required to distinguish the physical origin of the enhanced slope.
There has been increasing progress toward detailed characterization of exoplanetary atmospheres, in both observations and theoretical methods. Improvements in observational facilities and data reduction and analysis techniques are enabling increasingly higher quality spectra, especially from ground-based facilities. The high data quality also necessitates concomitant improvements in models required to interpret such data. In particular, the detection of trace species such as metal oxides has been challenging. Extremely irradiated exoplanets (~3000 K) are expected to show oxides with strong absorption signals in the optical. However, there are only a few hot Jupiters where such signatures have been reported. Here we aim to characterize the atmosphere of the ultra-hot Jupiter WASP-33b using two primary transits taken 18 orbits apart. Our atmospheric retrieval, performed on the combined data sets, provides initial constraints on the atmospheric composition of WASP-33b. We report a possible indication of aluminum oxide (AlO) at 3.3-sigma significance. The data were obtained with the long slit OSIRIS spectrograph mounted at the 10-meter Gran Telescopio Canarias. We cleaned the brightness variations from the light curves produced by stellar pulsations, and we determined the wavelength-dependent variability of the planetary radius caused by the atmospheric absorption of stellar light. A simultaneous fit to the two transit light curves allowed us to refine the transit parameters, and the common wavelength coverage between the two transits served to contrast our results. Future observations with HST as well as other large ground-based facilities will be able to further constrain the atmospheric chemical composition of the planet.
The hot Jupiter WASP-79b is a prime target for exoplanet atmospheric characterization both now and in the future. Here we present a thermal emission spectrum of WASP-79b, obtained via Hubble Space Telescope Wide Field Camera 3 G141 observations as part of the PanCET program. Given the temporal coverage of WASP-79bs secondary eclipse, we consider two scenarios: a fixed mid-eclipse time based on the expected occurrence time and a mid-eclipse time as a free parameter. In both scenarios, we can measure thermal emission from WASP-79b from 1.1-1.7 $mu$m at 2.4$sigma$ confidence consistent with a 1900 K brightness temperature for the planet. We combine our observations with Spitzer dayside photometry (3.6 and 4.5 $mu$m) and compare these observations to a grid of atmospheric forward models. Given the precision of our measurements, WASP-79bs infrared emission spectrum is consistent with theoretical spectra assuming equilibrium chemistry, enhanced abundances of H-, VO, or FeH, as well as clouds. The best match equilibrium model suggests WASP-79bs dayside has a solar metallicity and carbon-to-oxygen ratio, alongside a recirculation factor of 0.75. Models including significant H- opacity provide the best match to WASP-79bs emission spectrum near 1.58 $mu$m. However, models featuring high-temperature cloud species - formed via vigorous vertical mixing and low sedimentation efficiencies - with little day-to-night energy transport also match WASP-79bs emission spectrum. Given the broad range of equilibrium chemistry, disequilibrium chemistry, and cloudy atmospheric models consistent with our observations of WASP-79bs dayside emission, further observations will be necessary to constrain WASP-79bs dayside atmospheric properties.
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