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

Abstract

The ultraviolet-visible wavelength range holds critical spectral diagnostics for the chemistry and physics at work in planetary atmospheres. To date, exoplanet time-series atmospheric characterization studies have relied on several combinations of modes on Hubble's STIS/COS instruments to access this wavelength regime. Here for the first time, we apply the Hubble WFC3/UVIS G280 grism mode to obtain exoplanet spectroscopy from 200-800 nm in a single observation. We test the G280 grism mode on the hot Jupiter HAT-P-41b over two consecutive transits to determine its viability for exoplanet atmospheric characterization. We obtain a broadband transit depth precision of 29-33 ppm and a precision of on average 200 ppm in 10 nm spectroscopic bins. Spectral information from the G280 grism can be extracted from both the positive and negative first order spectra, resulting in a 60% increase in the measurable flux. Additionally, the first HST orbit can be fully utilized in the time-series analysis. We present detailed extraction and reduction methods for use by future investigations with this mode, testing multiple techniques. We find the results fully consistent with STIS measurements of HAT-P-41b from 310-800 nm, with the G280 results representing a more observationally efficient and precise spectrum. HAT-P-41b's transmission spectrum is best fit with a model with T eq =2091 K, high metallicity, and significant scattering and cloud opacity. With these first of their kind observations, we demonstrate that WFC3/UVIS G280 is a powerful new tool to obtain UV-optical spectra of exoplanet atmospheres, adding to the UV legacy of Hubble and complementing future observations with the James Webb Space Telescope.

Indications for very high metallicity and absence of methane in the eccentric exo-Saturn WASP-117b

“…We argue that additional transit measurements of WASP-117b with HST/UVIS and the G280 grism could yield the necessary accuracy (200-100 ppm, Wakeford et al 2020) to constrain a haze and cloud layer in WASP-117b. A low-resolution transmission spectrum in that range would reveal cloud properties due to the different scattering slopes between 0.3 and 0.5 µm for a clear atmosphere with high metallicity, a gray cloudy and hazy atmosphere with low metallicity (Fig.…”

“…The authors reported an accuracy of 29-33 ppm for the broadband depth between 0.3 and 0.8 µm and 200 ppm in 10 nm spectroscopic bins. Wakeford et al (2020) observed two transits of the hot Jupiter HAT-P-41b around a quiet F star with magnitude m V = 11.08. Our target WASP-117b likewise orbits a quiet F star that is even brighter than the host star of HAT-P-42b.…”

“…19, top panels). Wakeford et al (2020) recently showed that it is in principle possible to accurately measure a transmission spectrum of a transiting exoplanet in this wavelength range using the WFC3/UVIS G280 grism. The authors reported an accuracy of 29-33 ppm for the broadband depth between 0.3 and 0.8 µm and 200 ppm in 10 nm spectroscopic bins.…”

Aerosols in Exoplanet Atmospheres

“…Transmission spectra of a variety of exoplanets ranging from hot Jupiters to cool rocky worlds have been observed from the near-UV to the mid-infrared by ground-and space-based instruments (e.g. Charbonneau et al, 2002;Bean et al, 2010;Wood et al, 2011;Gibson et al, 2012;Deming et al, 2013;Swain et al, 2013;Crossfield et al, 2013;Knutson et al, 2014;Sing et al, 2015;de Wit et al, 2016;Wakeford, Stevenson, et al, 2017;Kreidberg, Line, Thorngren, et al, 2018;Bruno et al, 2018;Kilpatrick et al, 2018;Benneke, Wong, et al, 2019;Chachan et al, 2019;Libby-Roberts et al, 2020;Thao et al, 2020;Wakeford et al, 2020), all of which have been shown to be impacted by aerosols.…”

Transmission Spectroscopy for the Warm Sub-Neptune HD 3167c: Evidence for Molecular Absorption and a Possible High-metallicity Atmosphere

“…The Hubble Space Telescope (HST) has proven a productive facility for characterizing the atmospheres of transiting exoplanets. Observational studies released since 2019 alone include Arcangeli et al (2019), Benneke et al (2019aBenneke et al ( , 2019b, Chachan et al (2019), dos Santos et al (2019), Mikal-Evans et al (2019, Sing et al (2019), Spake et al (2019), Guo et al (2020), Wong et al (2020), Alam et al (2020), Fu et al (2020), Wakeford et al (2020), Carter et al (2020), Bruno et al (2020), Sotzen et al (2020), Carone et al (2020), Kreidberg et al (2020), and Colón et al (2020). Transmission spectroscopy measurements, made during primary transit, allow the composition of the day-night terminator atmosphere to be probed, while at other phases in the orbit when the irradiated dayside hemisphere is visible, the planetary emission can be constrained (for overviews, see Deming & Seager 2017;Deming et al 2019).…”