WASP-52B. the Effect of Starspot Correction on Atmospheric Retrievals
Total Page：16
File Type：pdf, Size：1020Kb
MNRAS 000,1{12 (2019) Preprint 13 January 2020 Compiled using MNRAS LATEX style file v3.0 WASP-52b. The effect of starspot correction on atmospheric retrievals Giovanni Bruno,1;3? Nikole K. Lewis,2;3 Munazza K. Alam,4 Mercedes L´opez-Morales,4 Joanna K. Barstow,5 Hannah R. Wakeford,3 David Sing,6 Gregory W. Henry,7 Gilda E. Ballester,8 Vincent Bourrier,9 Lars A. Buchhave,10 Ofer Cohen,11 Thomas Mikal-Evans,12 Antonio Garc´ıa Mu~noz,13 Panayotis Lavvas,14 Jorge Sanz-Forcada15 1INAF - Catania Astrophysical Observatory, Via Santa Sofia, 78, 95123, Catania, Italy 2Department of Astronomy and Carl Sagan Institute, Cornell University, 122 Sciences Drive, Ithaca, NY 14853, USA 3Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 4Center for Astrophysics j Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA 5University College London, Department of Physics and Astronomy, United Kingdom 6Physics and Astronomy, Jonhs Hopkins University, Baltimore, MD, United States 7 Center of Excellence in Information Systems, Tennessee State University, Nashville, TN 37209, USA 8Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA 9Observatoire de l'Universit´ede Gen`eve, Chemin des Maillettes 51, Versoix, CH-1290, Switzerland 10 DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 328, DK-2800 Kgs. Lyngby, Denmark 11 Lowell Center for Space & Technology, 600 Suffolk St., Lowell MA 01854, USA 12 Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 37-241, Cambridge, MA 02139, USA 13 Zentrum fur¨ Astronomie und Astrophysik, Technische Universit¨at Berlin, Berlin, Germany 14 GSMA, Reims Champagne-Ardenne, F-51687 Reims, France 15 Centro de Astrobiolog´ıa(CSIC-INTA), ESAC Campus, Camino bajo del Castillo s/n, E-28692 Villanueva de la Ca~nada, Madrid, Spain Accepted XXX. Received YYY; in original form ZZZ ABSTRACT We perform atmospheric retrievals on the full optical to infrared (0:3−5 µm) transmis- sion spectrum of the inflated hot Jupiter WASP-52b by combining HST /STIS, WFC3 IR, and Spitzer/IRAC observations. As WASP-52 is an active star which shows both out-of-transit photometric variability and starspot crossings during transits, we ac- count for the contribution of non-occulted active regions in the retrieval. We recover a 0:1 − 10× solar atmospheric composition, in agreement with core accretion predictions for giant planets, and a weak contribution of aerosols. We also obtain a < 3000 K temperature for the starspots, a measure which is likely affected by the models used to fit instrumental effects in the transits, and a 5% starspot fractional coverage, com- patible with expectations for the host star's spectral type. Such constraints on the planetary atmosphere and on the activity of its host star will inform future JWST GTO observations of this target. Key words: planets and satellites: atmospheres { stars: starspots { techniques: photometric { techniques: spectroscopic arXiv:1911.05179v2 [astro-ph.EP] 10 Jan 2020 1 INTRODUCTION Morales) because it occupies an important place in the con- tinuum of mass, gravity and equilibrium temperature for Probing physical processes in exoplanet atmospheres that hot Jupiters (M ∼ 0:5M , log g ∼ 2:9, T ∼ 1300 K). Given are thought to depend strongly on planetary mass, grav- J eq trends noted in the amplitude of water features (e.g. Steven- ity and temperature requires a sample of targets spanning son 2016; Fu et al. 2017), WASP-52b is predicted to be on this phase space. WASP-52b was selected as a prime tar- the boundary of hot Jupiters whose transmission spectra are get for two large Hubble Space Telescope (HST ) programs strongly affected by the presence of clouds in the planet's (GO-14260, PI Deming and GO-14767, PIs Sing and L´opez- atmosphere, and where large scale heights produce strong atmospheric signals. WASP-52b's mass lies in between that of Jupiter and Saturn's, which also suggests that it might be ? E-mail: [email protected] © 2019 The Authors 2 G. Bruno et al. enriched in elements heavier than hydrogen and helium (e.g. tion, image processing and transit analysis were discussed in Thorngren et al. 2016). The mass of this planet also falls in a Alam et al.(2018) for the STIS and IRAC data, and Bruno range where atmospheric metallicity measurements are not et al.(2018) for the WFC3 data. available yet (Wakeford et al. 2018, and references therein). In Alam et al.(2018), each spectroscopic transit was Moreover, it is hosted by a relatively bright, magH = 10:1 corrected for stellar activity by taking advantage of a semi- star. In many ways WASP-52b is an ideal target for ex- continuous, ground-based photometric monitoring of the oplanet atmospheric characterization efforts, but one must host star. WASP-52 was observed with the All-Sky Au- carefully consider the influence of its moderately active K2V tomated Survey for Supernovae (ASAS-SN ) and with the host star on observed spectra (Kirk et al. 2016; Louden et al. Tennessee State University Celestron 14 (C14) Automated 2017; Mancini et al. 2017; Chen et al. 2017; Alam et al. 2018; Imaging Telescope (AIT) at Fairborn Observatory (see Ap- Bruno et al. 2018; May et al. 2018). pendix A for details). The flux dimming, assumed to be Stellar activity is indeed one of the main challenges to caused by non-occulted dark starspots, was used to correct correctly interpreting exoplanet observations. Starspots and the time-series observations with Sing et al.(2011) and Huit- faculae, in particular, are known to affect measured scatter- son et al.(2013)'s prescriptions. In order to perform the cor- ing slopes and molecular abundances (e.g. Pont et al. 2007; rection, an effective temperature of 4750 K was assumed for Lecavelier Des Etangs et al. 2008; Sing et al. 2011; Bal- the activity features. lerini et al. 2012; McCullough et al. 2014; Rackham et al. Bruno et al.(2018), on the other hand, observed a pos- 2017; Pinhas et al. 2018; Wakeford et al. 2019). Such het- sible starspot crossing in the WFC3 transit. By using an erogeneities on the stellar photosphere have different effects analytic model for starspot occultations in transits (Mon- whether they are occulted or not during a transit: non- talto et al. 2014), they fitted a starspot effective tempera- occulted dark starspots and faculae produce an apparent, ture of ∼ 4000 K, which follows expectations for the host wavelength-dependent increase and decrease of the transit spectral type (Berdyugina 2005). However, no monitoring of depth, respectively, and the apparent variation is opposite if the host star was available to the authors, so that no correc- the features are occulted (e.g. Czesla et al. 2009; Silva-Valio tion for non-occulted starspots was performed on the data & Lanza 2011; D´esert et al. 2011; Bruno et al. 2016). In set. In the hypothesis that dark starspots were dominant specific spectroscopic channels, active regions can imprint in the WFC3 observations as well, the baseline of the cor- stellar features on a transmission spectrum, especially in responding transmission spectrum was likely overestimated the case of cool, low-mass stars (e.g., Rackham et al. 2017, (e.g. Czesla et al. 2009; Sing et al. 2011). In this work, 2018). Stellar contamination was shown to be less problem- we adopted the transmission spectrum resulting from their atic for weakly-active FGK stars, but a case-by-case study modeling of the starspot crossing. is still warranted (Rackham et al. 2019). When observations In Figure1, the STIS-WFC3- Spitzer data sets are pre- from different epochs are available, the problem of taking sented, both with and without the activity correction carried the changing stellar photosphere into account also arises out by Alam et al.(2018). With such a wavelength coverage, (Barstow et al. 2015), as measurements obtained in differ- an atmospheric retrieval exercise can in principle constrain ent epochs could otherwise be offset one from each other. the scattering properties of the atmosphere, its metallicity When observations in different wavelengths and from differ- and cloud top pressure (Benneke & Seager 2012; Sing et al. ent epochs are combined, both effects leave their trace on 2016), as well as allow inferences on the carbon-to-oxygen the planetary spectrum. ratio thanks to the IRAC points in the infrared (IR). In or- The transmission spectra of WASP-52b have been ob- der to combine the data sets into a consistent transmission served separately with WFC3-IR (Bruno et al. 2018) and spectrum, however, it is necessary to take the contribution of with STIS and Spitzer/IRAC (Alam et al. 2018). In each stellar activity into account: if the offsets between the obser- case, stellar activity was taken into account with a different vations in different epochs and wavelength windows are not method. Here, we present a combined fit of the spectra with corrected, the retrieval will try to compensate for the differ- atmospheric retrievals, which we used to explore different ent baselines by changing parameters such as the molecular ways of taking the time-varying impact of stellar activity abundances or scattering factors, thereby biasing the result into account. We describe the analysis carried out and the (Barstow et al. 2015; Rackham et al. 2018). constraints obtained for both the planetary atmosphere and the activity features. In section2, we describe the previously secured observations, and in section3 detail how we imple- mented activity features in our retrieval scheme. In section 3 RETRIEVAL SCHEME 4 and5 we present and discuss our results, respectively, and Retrieving the atmospheric properties of an exoplanet orbit- we conclude in section6. ing an active star requires several factors to be taken into account.