Draft version April 23, 2021 Typeset using LATEX twocolumn style in AASTeX63 HST PanCET Program: A Complete Near-UV to Infrared Transmission Spectrum for the Hot Jupiter WASP-79b Alexander D. Rathcke ,1 Ryan J. MacDonald ,2 Joanna K. Barstow ,3, 4 Jayesh M. Goyal ,2 Mercedes Lopez-Morales ,5 Joao~ M. Mendonc¸a ,1 Jorge Sanz-Forcada ,6 Gregory W. Henry ,7 David K. Sing ,8 Munazza K. Alam ,5 Nikole K. Lewis ,2 Katy L. Chubb ,9 Jake Taylor ,10 Nikolay Nikolov ,11 and Lars A. Buchhave 1 1DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 328, DK-2800 Kgs. Lyngby, Denmark 2Department of Astronomy and Carl Sagan Institute, Cornell University, 122 Sciences Drive, Ithaca, NY 14853, USA 3School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK 4Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK 5Center for Astrophysics j Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA 6Centro de Astrobiolog´ıa(CSIC-INTA), ESAC Campus, Villanueva de la Ca~nada,Madrid, Spain 7Center of Excellence in Information Systems, Tennessee State University, Nashville, TN 30209 USA 8Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, USA 9SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, Netherlands 10Department of Physics (Atmospheric, Oceanic and Planetary Physics), University of Oxford, Parks Rd, Oxford, OX1 3PU, UK 11Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA Submitted to AJ ABSTRACT 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 µm. Combining these transits with previous observations, we construct a complete 0:3−5:0 µm 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 H2O (at 4:0σ confidence), while providing moderate evidence of H− bound-free opacity (3:3σ) and strong evidence of stellar contamination from unocculted faculae (4:7σ). The retrieved H2O abundance (∼ 1%) suggests a super-stellar atmospheric metallicity, though stellar or sub-stellar abundances remain consistent with present observations (O/H = 0:3−34× stellar). All three retrieval codes obtain a precise H− abundance − constraint: log(XH− ) ≈ −8:0 ± 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 ∼ 500 K hotter than the stellar photosphere, covering ∼ 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. arXiv:2104.10688v1 [astro-ph.EP] 21 Apr 2021 Keywords: methods: observational { planets and satellites: atmospheres { planets and satellites: gaseous planets { methods: data analysis 1. INTRODUCTION Transmission spectroscopy has proven a powerful method to study the atmospheres of transiting exo- planets. This technique takes advantage of the differ- Corresponding author: Alexander D. Rathcke ing wavelength-dependence of absorption and scatter- [email protected] ing processes in planetary atmospheres, resulting in a wavelength-dependent planetary radius during transit 2 Rathcke et al. (Seager & Sasselov 2000; Brown 2001). Transmission (also known as CD-30 1812) with a period of P = 3.662 spectra are sensitive to molecular, atomic, and ionic days. WASP-79 is of spectral type F5 (Smalley et al. species, temperature structures, clouds, and hazes at the 2012) and is located in the constellation Eridanus 248 day-night terminator region (see Madhusudhan 2019, for pc from Earth (Gaia Collaboration et al. 2018), mak- a recent review). If the transit chord exhibits different ing it relatively bright with V = 10.1 mag. WASP- stellar properties from the average stellar disk, trans- 79b exhibits spin-orbit misalignment between the spin mission spectra are also sensitive to unocculted spots or axis of the host star and the planetary orbital plane, faculae (e.g. Rackham et al. 2018; Pinhas et al. 2018). revealing that this planet follows a nearly polar orbit The last two decades have shown Hubble Space Tele- (Addison et al. 2013). Recently, Sotzen et al.(2020) re- scope (HST) transmission spectroscopy observations to ported evidence for H2O and FeH absorption in WASP- be very successful in probing the atmospheres of giant 79b's atmosphere. They used near-infrared HST Wide planets, yielding detection of several species. A non- Field Camera 3 (WFC3) transmission spectra observa- exhaustive list of HST highlights include: detections of tions, combined with ground-based Magellan/Low Dis- the alkali metals Na and K (e.g., Charbonneau et al. persion Survey Spectrograph 3 (LDSS3) optical trans- 2002; Nikolov et al. 2014; Alam et al. 2018), escap- mission spectra. Similar findings were reported by Skaf ing atomic species from large exospheres (e.g., Vidal- et al.(2020) for a different WFC3 data reduction. Madjar et al. 2003; Ehrenreich et al. 2015), H2O detec- Here, we expand upon previous studies of WASP-79b's tions and abundance measurements (e.g., Deming et al. transmission spectrum by presenting new HST/STIS ob- 2013; Pinhas et al. 2019), thermal inversions (e.g., Evans servations. Our paper is structured as follows. In Sec- et al. 2017; Baxter et al. 2020), and a diverse range of tion2 we present our observations and reduction pro- cloud and haze properties (e.g., Sing et al. 2016; Gao cedure. We present the analysis of the light curves in et al. 2020). Transmission spectra of Neptune-sized and Section3, and assess the likelihood of stellar activity sub-Neptune-sized planets (e.g., Crossfield & Kreidberg contaminating our transmission spectrum in Section4. 2017; Benneke et al. 2019; Libby-Roberts et al. 2020) We then go on to describe our retrieval procedures, and have also been reported. Ground-based observations present the results from these in Section5, discuss the have also reported several detections, including Na (e.g., results in Section6, and summarize in Section7. Sing et al. 2012; Nikolov et al. 2018), K (e.g., Nikolov et al. 2016; Sedaghati et al. 2016), Li (e.g., Tabernero 2. OBSERVATIONS AND DATA REDUCTION et al. 2020), He (e.g., Nortmann et al. 2018; Allart et al. 2.1. Observations 2018, and clouds/hazes (e.g., Huitson et al. 2017). These WASP-79b was observed during three primary tran- results illustrate a dynamic movement from character- sit events with HST STIS, two with the G430L grating ization of individual exoplanet atmospheres to a sta- and one with the G750L grating. The specific observing tistically significant sample. High-quality transmission dates and instrument settings are summarized in Ta- spectra spanning a wide wavelength range enable preci- ble1. Combined, the two gratings cover the wavelength sion retrievals of atmospheric properties, allowing com- regime from 2900 A˚ to 10270 A,˚ with an overlapping parative studies across the exoplanet population (e.g. region from ∼5260-5700 A.˚ The two gratings have a Barstow et al. 2017; Welbanks et al. 2019). resolving power of ∼2.7 and ∼4.9 A˚ per pixel for the Here we present a new optical transmission spec- G430L and G750L gratings, respectively. Thus, they trum of the hot Jupiter WASP-79b, part of the HST offer a resolution of R = λ/∆λ = 500 − 1000. Panchromatic Comparative Exoplanetary Treasury Pro- Each transit event consists of 57-77 spectra, spanning gram (PanCET)(PIs: Sing & L´opez-Morales, Cycle 24, five HST orbits, where each HST orbit takes about ∼95 GO 14767). PanCET targeted 20 planets, allowing a minutes. Because HST is in a low-Earth orbit, the data simultaneous ultra-violet, optical, and infrared compar- collection is truncated for ∼45 minutes in each orbit ative study of exoplanetary atmospheres. This program when HST is occulted by the Earth. The observations also offers valuable observations in the UV and blue- were scheduled such that the transit event occurs in the optical (λ < 0:6 µm) that will be inaccessible to the third and fourth HST orbit, while the remaining or- James Webb Space Telescope (JWST). bits provide an out-of-transit baseline before and after WASP-79b was discovered in 2012 by the ground- each transit. All observations were made with the 52x2 based, wide-angle transit search WASP-South (Smal- arcsec2 slit to minimize slit losses. Readout times were ley et al. 2012). WASP-79b is an inflated hot Jupiter reduced by only reading out a 1024x128 pixel subarray with R = 1.7 R , M = 0.9 M , and a mean density p J p J of the CCD. This strategy has previously been found to of ρ ∼ 0.23 g cm−3. It orbits its host star WASP-79 deliver high signal-to-noise ratios (SNR) near the Pois- HST transmission spectroscopy of WASP-79b 3 60000 STIS G430L STIS G750L 50000 40000 30000 Counts 20000 10000 0 3000 4000 5000 6000 7000 8000 9000 10000 Wavelength (Å) Figure 1. Sample stellar spectra of WASP-79, obtained from the STIS G430L grating (blue) and the G750L grating (red). son limit during the transit events (e.g., Huitson et al. This was done by comparing every pixel with the corre- 2012; Sing et al. 2013). We show an example G430L and sponding value in all of the other spectra and flagging G750L spectra of WASP-79 in Figure1. values more than 5σ above the median of that pixel.