Astronomy & Astrophysics manuscript no. RM-VARIATION-ARXIV c ESO 2018 September 5, 2018 Activity induced variation in spin-orbit angles as derived from Rossiter-McLaughlin measurements? M. Oshagh1;2, A. H. M. J. Triaud3, A. Burdanov4, P. Figueira2;5, A. Reiners1, N. C. Santos2;6, J. Faria2, G. Boue7, R. F. D´ıaz8;9, S. Dreizler1, S. Boldt1, L. Delrez10, E. Ducrot4, M. Gillon4, A. Guzman Mesa1, E. Jehin4, S. Khalafinejad11;12, S. Kohl11, L. Serrano2, S. Udry13 1 Institut f¨urAstrophysik, Georg-August Universit¨atG¨ottingen,Friedrich-Hund-Platz 1, 37077 G¨ottingen,Germany 2 Instituto de Astrof´ısicae Ci^enciasdo Espa¸co,Universidade do Porto, CAUP, Rua das Estrelas, PT4150-762 Porto, Portugal 3 University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK 4 Space sciences, Technologies and Astrophysics Research (STAR) Institute, Universit´ede Li`ege, All´eedu 6 Ao^ut17, Bat. B5C, 4000 Li`ege,Belgium 5 European Southern Observatory, Alonso de Cordova 3107, Vitacura Casilla 19001, Santiago 19, Chile 6 Departamento de F´ısicae Astronomia, Faculdade de Ci^encias,Universidade do Porto,Rua do Campo Alegre, 4169- 007 Porto, Portugal 7 IMCCE, Observatoire de Paris, UPMC Univ. Paris 6, PSL Research University, Paris, France 8 Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Buenos Aires, 1428, Argentina 9 CONICET - Universidad de Buenos Aires, Instituto de Astronoma y F´ısicadel Espacio (IAFE), Buenos Aires, 1428, Argentina 10 Astrophysics Group, Cavendish Laboratory, J. J. Thomson Avenue, Cambridge, CB3 0HE, UK 11 Hamburg Observatory, Hamburg University, Gojenbergsweg 112, 21029, Hamburg, Germany 12 Max Planck Institute for Astronomy, K¨onigstuhl17, 69117 Heidelberg, Germany 13 Observatoire de Gen`eve, Universit´ede Gen`eve, 51 chemin des Maillettes, 1290 Versoix, Switzerland Received XXX; accepted XXX ABSTRACT One of the most powerful methods used to estimate sky-projected spin-orbit angles of exoplanetary systems is through a spectroscopic transit observation known as the RossiterMcLaughlin (RM) effect. So far mostly single RM observations have been used to estimate the spin-orbit angle, and thus there have been no studies regarding the variation of estimated spin-orbit angle from transit to transit. Stellar activity can alter the shape of photometric transit light curves and in a similar way they can deform the RM signal. In this paper we discuss several RM observations, obtained using the HARPS spectrograph, of known transiting planets that all transit extremely active stars, and by analyzing them individually we assess the variation in the estimated spin-orbit angle. Our results reveal that the estimated spin-orbit angle can vary significantly (up to ∼ 42◦) from transit to transit, due to variation in the configuration of stellar active regions over different nights. This finding is almost two times larger than the expected variation predicted from simulations. We could not identify any meaningful correlation between the variation of estimated spin-orbit angles and the stellar magnetic activity indicators. We also investigated two possible approaches to mitigate the stellar activity influence on RM observations. The first strategy was based on obtaining several RM observations and folding them to reduce the stellar activity noise. Our results demonstrated that this is a feasible and robust way to overcome this issue. The second approach is based on acquiring simultaneous high-precision short-cadence photometric transit light curves using TRAPPIST/SPECULOOS telescopes, which provide more information about the stellar active region's properties and allow a better RM modeling. Key words. methods: observational, numerical- planetary system- techniques: photometry, spectroscopy 1 arXiv:1809.01027v1 [astro-ph.EP] 4 Sep 2018 1. Introduction gle λ) is one of the most important parameters of the or- bital architecture of planetary systems. According to the Understanding the orbital architecture of exoplanetary sys- core-accretion paradigm of planet formation, planets should tems is one of the main objectives of exoplanetary surveys, form on aligned orbits λ 0 (Pollack et al. 1996, and see in order to provide new strong constraints on the physi- a comprehensive review∼ inDawson & Johnson 2018, and cal mechanisms behind planetary formation and evolution. references therein ), as observed in the case of the solar The sky-projected spin-orbit angle (hereafter spin-orbit an- system. Planetary evolution mechanisms such as disk mi- gration predict that planets should maintain their primor- dial spin-orbit angle, but the dynamical interactions with a ? RV measurements obtained from the HARPS pipeline are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u- 1 The angle between the stellar spin axis and normal to the strasbg.fr/viz-bin/qcat?J/A+A/606/A107 planetary orbital plane. 1 M. Oshagh et al.: Activity induced variation in measured spin-orbit angle third body and tidal migration could alter the spin-orbit an- 2014), and that will be provided by ESPRESSO (Pepe et al. gle (see a comprehensive review in Baruteau et al. 2014, and 2014). references therein). Moreover, several studies have shown In almost all (all could be an overstated or exagger- that the misalignment of a planet could be primordial con- ated statement) studies of RM observations, only one RM sidering the disks being naturally and initially misaligned observation during a single transit was acquired, and single- (e.g., Batygin 2012; Crida & Batygin 2014). In this context, epoch RM observations were used to estimate the spin-orbit the measurement of spin-orbit angles under a wide range angle λ of the systems. The main reason for this is the high of host and planet conditions (including multiple transiting telescope pressure and oversubscription on high-precision planets) is extremely important in order to obtain feedback spectrographs that makes requesting for several RM obser- on modeling. vations of same targets inaccessible. Therefore, the lack of As a star rotates, the part of its surface that rotates multiple RM observations has not permitted us to explore toward the observer is blueshifted and the part that ro- possible contamination on the estimated λ from the stellar tates away is redshifted. During the transit of a planet, the activity. corresponding rotational velocity of the portion of the stel- In this paper we assess the impact of stellar activity lar disk that is blocked by the planet is removed from the on the spin-orbit angle estimation by observing several RM integration of the velocity over the entire star, creating a ra- observations of known transiting planets which all transit dial velocity (RV) signal known as the Rossiter{McLaughlin extremely active stars. We organize our paper as follows. (RM) effect (Holt 1893; Rossiter 1924; McLaughlin 1924). We describe our targets' selection and their observations in The RM observation is the most powerful and efficient tech- Sect. 2. In Sect. 3 we define our fitting procedure to estimate nique for estimating the spin-orbit angle λ of exoplanetary the spin-orbit angles, and also present its result. We inves- systems (or eclipsing binary systems) (e.g., Winn et al. tigate the possibility of mitigating the influence of stellar 2005; H´ebrardet al. 2008; Triaud et al. 2010; Albrecht et al. activity on RM observations by combining several RM ob- 2012, and a comprehensive review in Triaud 2017, and ref- servations in Sect. 4. In Sect. 5 we present the simultaneous erences therein). The number of spin-orbit angle λ mea- photometric transit observation with our RM observation surements has dramatically increased over the past decade and we discuss the advantages of having simultaneous pho- (200 systems to date)2, and have revealed a large num- tometric transit observation in eliminating stellar activity ber of unexpected systems, which include planets on highly noise in RM observations. In Sect. 6 we discuss possible misaligned (e.g., Triaud et al. 2010; Albrecht et al. 2012; contamination from other sources of stellar noise that af- Santerne et al. 2014), polar (e.g., Addison et al. 2013), and fect our observations, and conclude in Sect 7. even retrograde orbits (e.g., Bayliss et al. 2010; H´ebrard et al. 2011). The contrast of active regions (i.e., stellar spots, plages, 2. Observations and data reduction and faculae) present on the stellar surface during the transit of an exoplanet alter the high-precision photometric tran- 2.1. Target selection sit light curve (both outside and inside transit) and lead to Our stars were selected following four main criteria: an inaccurate estimate of the planetary parameters (e.g., Czesla et al. 2009; Sanchis-Ojeda & Winn 2011; Sanchis- { I) the star should have a known transiting planet; Ojeda et al. 2013; Oshagh et al. 2013b; Barros et al. 2013; { II) the planet host star should be very active (e.g., the Oshagh et al. 2015a,b; Ioannidis et al. 2016). Since the published photometric transit light curves clearly show physics and geometry behind the transit light curve and large stellar spots' occultation anomalies); the RM measurement are the same, the RM observations { III) at least one RM measurement of the planet has been are expected to be affected by the presence of stellar active observed (preferably with the HARPS spectrograph as regions in a similar way. Furthermore, the inhibition of the we use HARPS observations in our study) to ensure the convective blueshift inside the active regions can induce ex- feasibility of detecting the RM signal; tra RV noise on the RM observations, which is absent in { IV) the transiting planet should have several observable the photometric transit observation. transits from the southern hemisphere during the period Boldt et al. (in prep. 2018) demonstrate, using simu- from March 2017 to October 2017 (this criterion is to lations, that unocculted stellar active regions during the ensure observability).