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Register a new userThe HST PanCET Program: Exospheric Mg II and Fe II in the Near-UV transmission spectrum of WASP-121b using Jitter Decorrelation
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We present HST near-ultraviolet (NUV) transits of the hot Jupiter WASP-121b, acquired as part of the PanCET program. Time series spectra during two transit events were used to measure the transmission spectra between 2280 and 3070 Angstroms at a resolution of 30,000. Using HST data from 61 STIS visits, we show that data from HSTs Pointing Control System can be used to decorrelate the instrument systematic errors (Jitter Decorrelation), which we used to fit the WASP-121b light curves. The NUV spectrum show very strong absorption features, with the NUV white light curve found to be larger than the average optical and near-infrared value at 6-$sigma$ confidence. We identify and spectrally resolve absorption from the Mg ii doublet in the planetary exosphere at a 5.9-$sigma$ confidence level. The Mg ii doublet is observed to reach altitudes of $R_{pl}/R_{star}=0.284pm0.037$ for the 2796 Angstrom line and $0.242pm0.0431$ in the 2804 Angstrom line, which exceeds the Roche lobe size as viewed in transit geometry ($R_{rm eqRL}/R_{star}$ = 0.158). We also detect and resolve strong features of the Fe ii UV1 and UV2 multiplets, and observe the lines reaching altitudes of $R_{pl}/R_{star}approx0.3$. At these high altitudes, the atmospheric Mg ii and Fe ii gas is not gravitationally bound to the planet, and these ionized species may be hydrodynamically escaping or could be magnetically confined. Refractory Mg and Fe atoms at high altitudes also indicates that these species are not trapped into condensate clouds at depth, which places constraints on the deep interior temperature.
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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.
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We present an atmospheric transmission spectrum for the ultra-hot Jupiter WASP-121b, measured using the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST). Across the 0.47-1 micron 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 micron). Wavelength-dependent variations in the opacity rule out a gray cloud deck at a confidence level of 3.8-sigma and may instead be explained by VO spectral bands. We find a cloud-free model assuming chemical equilibrium for a temperature of 1500K and metal enrichment of 10-30x 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 3-sigma 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 micron in wavelength. If this feature is caused by Rayleigh scattering due to uniformly-distributed aerosols, it would imply an unphysically high temperature of 6810+/-1530K. 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.
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