Transmission spectroscopy of the hot Jupiter TrES-3 b: Disproof of an overly large Rayleigh-like feature Mackebrandt, F., Mallonn, M., Ohlert, J. M., Granzer, T., Lalitha, S., Munoz, A. G., Gibson, N. P., Lee, J. W., Sozzetti, A., Turner, J. D., Vanko, M., & Strassmeier, K. G. (2017). Transmission spectroscopy of the hot Jupiter TrES-3 b: Disproof of an overly large Rayleigh-like feature. Astronomy & Astrophysics, 608, 1-13. [A26]. https://doi.org/10.1051/0004-6361/201730512 Published in: Astronomy & Astrophysics Document Version: Publisher's PDF, also known as Version of record Queen's University Belfast - Research Portal: Link to publication record in Queen's University Belfast Research Portal Publisher rights Copyright ESO 2017. This work is made available online in accordance with the publisher’s policies. Please refer to any applicable terms of use of the publisher. General rights Copyright for the publications made accessible via the Queen's University Belfast Research Portal is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The Research Portal is Queen's institutional repository that provides access to Queen's research output. Every effort has been made to ensure that content in the Research Portal does not infringe any person's rights, or applicable UK laws. If you discover content in the Research Portal that you believe breaches copyright or violates any law, please contact [email protected]. Download date:27. Sep. 2021 A&A 608, A26 (2017) DOI: 10.1051/0004-6361/201730512 Astronomy c ESO 2017 & Astrophysics Transmission spectroscopy of the hot Jupiter TrES-3 b: Disproof of an overly large Rayleigh-like feature?,?? F. Mackebrandt1; 2, M. Mallonn1, J. M. Ohlert3; 4, T. Granzer1, S. Lalitha5, A. García Muñoz6, N. P.Gibson7, J. W. Lee8; 9, A. Sozzetti10, J. D. Turner11, M. Vankoˇ 12, and K. G. Strassmeier1 1 Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany e-mail: [email protected] 2 Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany 3 Michael Adrian Observatorium, Astronomie Stiftung Trebur, 65468 Trebur, Germany 4 University of Applied Sciences, Technische Hochschule Mittelhessen, 61169 Friedberg, Germany 5 Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India 6 Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany 7 Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast, Belfast BT7 1NN, UK 8 Korea Astronomy and Space Science Institute, 34055 Daejeon, Korea 9 Astronomy and Space Science Major, Korea University of Science and Technology, 34113 Daejeon, Korea 10 INAF–Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy 11 Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA 12 Astronomical Institute, Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia Received 27 January 2017 / Accepted 2 September 2017 ABSTRACT Context. Transit events of extrasolar planets offer the opportunity to study the composition of their atmospheres. Previous work on transmission spectroscopy of the close-in gas giant (TrES)-3 b revealed an increase in absorption towards blue wavelengths of very large amplitude in terms of atmospheric pressure scale heights, too large to be explained by Rayleigh-scattering in the planetary atmosphere. Aims. We present a follow-up study of the optical transmission spectrum of the hot Jupiter TrES-3 b to investigate the strong increase in opacity towards short wavelengths found by a previous study. Furthermore, we aim to estimate the effect of stellar spots on the transmission spectrum. Methods. This work uses previously published long slit spectroscopy transit data of the Gran Telescopio Canarias (GTC) and pub- lished broad band observations as well as new observations in different bands from the near-UV to the near-IR, for a homogeneous transit light curve analysis. Additionally, a long-term photometric monitoring of the TrES-3 host star was performed. Results. Our newly analysed GTC spectroscopic transit observations show a slope of much lower amplitude than previous studies. We conclude from our results the previously reported increasing signal towards short wavelengths is not intrinsic to the TrES-3 system. Furthermore, the broad band spectrum favours a flat spectrum. Long-term photometric monitoring rules out a significant modification of the transmission spectrum by unocculted star spots. Key words. planets and satellites: atmospheres – stars: individual: TrES-3 – methods: data analysis – techniques: spectroscopic – techniques: photometric 1. Introduction is absorbed until an optical depth of about unity. Wavelength- dependent variations in the atmospheres optical depth cause vari- Transiting exoplanets present a unique opportunity to probe the ations in that transit depth. This yields information about atoms, upper regions of their atmospheres. During transit, the stellar molecules, and condensates in the outer (optically thin) region light passes through the annulus of the planet’s atmosphere and of the planetary atmosphere. ? Based on (1) data obtained with the STELLA robotic telescopes in The magnitude of the wavelength dependence of the tran- Tenerife, an AIP facility jointly operated by AIP and IAC, (2) observa- sit depth can be approximated by the area of the planetary at- tions collected at the German-Spanish Astronomical Center, Calar Alto, mosphere compared to the area of the star. Therefore, the ob- jointly operated by the Max-Planck-Institut für Astronomie Heidelberg servability of the transmission signal is maximised for planets and the Instituto de Astrofísica de Andalucía (CSIC) and (3) observa- with a large planet-to-star radius ratio and a large atmospheric tions made with the Italian Telescopio Nazionale Galileo (TNG) oper- pressure scale height. The majority of atmospheric character- ated on the island of La Palma by the Fundación Galileo Galilei of the isations have been accomplished for hot Jupiter exoplanets INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del (Sing et al. 2016). However, observations of transmission spec- Roque de los Muchachos of the Instituto de Astrofisica de Canarias. ?? Newly observed photometric data from Sects. 2.2 and 2.3 and tables tra of Neptune-sized planets (e.g. Fraine et al. 2014) and super- of the lightcurves are only available at the CDS via anonymous ftp to Earths (e.g. Kreidberg et al. 2014) have also been published. cdsarc.u-strasbg.fr (130.79.128.5) or via The vertical extent of the atmospheric annulus probed by http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/608/A26 transmission spectroscopy at optical wavelengths is about five Article published by EDP Sciences A26, page 1 of 13 A&A 608, A26 (2017) to ten scale heights for gas giants (Seager et al. 2009; Burrows in units of scale height among all hot Jupiter transmission spec- 2014; Madhusudhan et al. 2014). This signal size is predicted tra and so far cannot be theoretically explained (P16). However, for atomic and molecular absorption in cloud/haze-free atmo- P16 analysed only a single transit measurement. sphere models as well as for Rayleigh-scattering in hazy at- In this work we target the reliability and repeatability of mospheres (Fortney et al. 2010; Miller-Ricci & Fortney 2010; this outstanding transmission signal. Therefore, we reanalyse Wakeford & Sing 2015). The vast majority of detected trans- the Gran Telescopio Canarias (GTC) transit observation used by mission signals is in agreement with this amplitude or lower P16 and homogeneously analyse 36 published and 15 newly ob- (e.g. Sing et al. 2016; Sedaghati et al. 2016; Kreidberg et al. served broad band transit observations in nine band passes from 2015; Fraine et al. 2014; Nikolov et al. 2016; Gibson et al. the near-UV until the near-IR to build a broad band spectropho- 2017). Stronger amplitudes measured in the UV can be un- tometry transmission spectrum for comparison. Our aim is to derstood as Lyα absorption in the planetary escaping exo- verify whether the signal is intrinsic to the TrES-3 system or sphere (Ehrenreich et al. 2015; Vidal-Madjar et al. 2003). How- rather caused by artefacts in the data or aspects of the data anal- ever, measured transmission signals of more than ten scale ysis, rather than a detailed atmospheric characterisation. heights amplitude at optical wavelengths have been proposed We performed a photometric long-term monitoring of the (Southworth et al. 2015; Mancini et al. 2014; Turner et al. 2016; host star TrES-3 to search for rotational modulation potentially Parviainen et al. 2016). The physical interpretation of these re- caused by star spots (Strassmeier 2009). Unocculted star spots sults is challenging, because according to our current under- on the visible hemisphere of the star influence the transit pho- standing these signals cannot entirely be caused by the plan- tometry and therefore the derived transit parameters (Pont et al. etary atmosphere. Does stellar activity influence the measure- 2013). This effect is wavelength dependent and can potentially ments (Oshagh et al. 2013), or does contamination from a nearby introduce a slope in the transmission spectrum similar to a spec- star play a role (Southworth & Evans 2016)? Are the values of tral scattering signature produced by Rayleigh or Mie scatter- the planetary scale height underestimated, which would point to ing by condensate particles (Oshagh et al. 2014). The monitoring a severely wrong estimation of planetary parameters, or are the program allows us to estimate an upper limit of the spot filling uncertainties in the transmission spectroscopy measurements un- factor and the modification of the measured transit depth. derestimated? Could there be missing physics in the modelling This paper is structured as follows: Sects. 2 and 3 describe of exoplanet transmission spectra? the observations and the data reduction.