The Hubble Space Telescope PanCET Program: Exospheric Mg ii and Fe ii in the Near-ultraviolet Transmission Spectrum of WASP-121b Using Jitter Decorrelation

Abstract

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 3070Å at a resolution of 30,000. Using HST data from 61 STIS visits, we show that data from HST's 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-σ confidence. We identify and spectrally resolve absorption from the Mg ii doublet in the planetary exosphere at a 5.9-σ confidence level. The Mg ii doublet is observed to reach altitudes of R pl /R star = 0.284 ± 0.037 for the 2796Å line and 0.242 ± 0.0431 in the 2804Å line, which exceeds the Roche lobe size as viewed in transit geometry (R 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 ≈ 0.3. 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 2 Sing et al.high altitudes also indicates that these species are not trapped into condensate clouds at depth, which places constraints on the deep interior temperature.

Detection of neutral atomic species in the ultra-hot Jupiter WASP-121b

“…The Hα line is detected with contrast D = 0.0187 ± 0.0011, centroid λ 0 = 6562.93 ± 0.02Å, and FWHM = 0.75±0.05Å ( Figure 5). We confirm the previous detection of the Sodium doublet (Sindel 2018). We measure D = 0.0069±0.0012, λ 0 = 5890.01±0.06Å, and FWHM = 0.73 ± 0.09Å for the D2 line, and D = 0.0025 ± 0.0009, λ 0 = 5896.09±0.09Å, and FWHM = 0.9±0.1Å for the D1 line ( Figure 6).…”

“…We measure D = 0.0069±0.0012, λ 0 = 5890.01±0.06Å, and FWHM = 0.73 ± 0.09Å for the D2 line, and D = 0.0025 ± 0.0009, λ 0 = 5896.09±0.09Å, and FWHM = 0.9±0.1Å for the D1 line ( Figure 6). These measurements are comparable with those reported in (Sindel 2018). The transmission spectra shown in Figures 5 and 6 are in the rest-frame of the planet.…”

Detection of Fe i in the atmosphere of the ultra-hot Jupiter WASP-121b, and a new likelihood-based approach for Doppler-resolved spectroscopy

“…It is also possible that an escaping atmosphere does not neatly follow the same orbit as the planet, which could complicate detection at high-resolution, yet not be obvious from the UV observations. This potential for smearing out the lines high in the exosphere could in principle also hide some remnant neutral Fe present in the upper atmosphere, which makes it difficult to completely rule out its presence in the exosphere, although Fe i lines were not detected in the UV by Sing et al (2019).…”

Into the UV: A Precise Transmission Spectrum of HAT-P-41b Using Hubble’s WFC3/UVIS G280 Grism

“…lengths not accessible to JWST. This wavelength range is important to understand and measure cloud opacity sources and their scattering profiles that are defined by the particle sizes (e.g., Lecavelier des Etangs et al 2008;Wakeford et al 2017), escaping atmospheres (e.g., Ehrenreich et al 2014;Sing et al 2019), and absorption from Na.…”

“…Jitter decorrelation -uses HST's Pointing Control System to detrend photometric time-series data. Based on the results of(Sing et al 2019), we include optical state vectors traditionally used for STIS(Sing et al 2011) as …”