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Comprehensive Time Series Analysis of the Transiting Extrasolar Planet WASP-33B,

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Comprehensive Time Series Analysis of the Transiting Extrasolar Planet WASP-33B�, A&A 553, A44 (2013) Astronomy DOI: 10.1051/0004-6361/201219642 & c ESO 2013 Astrophysics Comprehensive time series analysis of the transiting extrasolar planet WASP-33b, G. Kovács1,2,T.Kovács1,J.D.Hartman3,G.Á.Bakos3,, A. Bieryla4,D.Latham4,R.W.Noyes4, Zs. Regály1, and G. A. Esquerdo4 1 Konkoly Observatory, 1121 Budapest, Hungary e-mail: [email protected] 2 Department of Physics and Astrophysics, University of North Dakota, 58202-7129 Grand Forks, ND, USA 3 Department of Astrophysical Sciences, Princeton University, 08544 Princeton, NJ, USA 4 Harvard–Smithsonian Center for Astrophysics, 02138 Cambridge, MA, USA Received 22 May 2012 / Accepted 5 March 2013 ABSTRACT Context. HD 15082 (WASP-33) is the hottest and fastest rotating star known to harbor a transiting extrasolar planet (WASP-33b). The lack of high precision radial velocity (RV) data stresses the need for precise light curve analysis and gathering further RV data. Aims. By using available photometric and RV data, we perform a blend analysis, compute more accurate system parameters, confine the planetary mass, and, attempt to cast light on the observed transit anomalies. Methods. We combined the original HATNet observations and various followup data to jointly analyze the signal content and extract the transit component and used our RV data to aid the global parameter determination. Results. The blend analysis of the combination of multicolor light curves yields the first independent confirmation of the planetary nature of WASP-33b. We clearly identify three frequency components in the 15–21 d−1 regime with amplitudes 7–5 mmag. These frequencies correspond to the δ Scuti-type pulsation of the host star. None of these pulsation frequencies or their low-order linear combinations are in close resonance with the orbital frequency. We show that these pulsation components explain some but not all of the observed transit anomalies. The grand-averaged transit light curve shows that there is a ∼1.5 mmag brightening shortly after the planet passes the mid-transit phase. Although the duration and amplitude of this brightening varies, it is visible even through the direct inspections of the individual transit events (some 40–60% of the followup light curves show this phenomenon). We suggest that the most likely explanation of this feature is the presence of a well-populated spot belt which is highly inclined to the orbital plane. This geometry is consistent with the inference from the spectroscopic anomalies. Finally, we constrain the planetary mass to Mp = 3.27 ± 0.73 MJ by using our RV data collected by the TRES spectrograph. Key words. planets and satellites: individual: WASP-33b – stars: variables: delta Scuti – methods: data analysis 1. Introduction capability of the orbiting planet and showed very clearly that the orbital revolution was highly inclined (i.e., retrograde) in respect = The short-period (P 1.22 d) transiting extrasolar planet to the stellar rotational axis. WASP-33b was discovered by Christian et al. (2006)inthe The WASP-33 system stands out from the other transiting course of the transit survey conducted by the WASP project systems not only for its highest rotation rate and highest Teff of (Pollacco et al. 2006). The host star, HD 15082, is a bright, 7400 K (CC10) but also because of the planet’s highest substellar V = 8.3 mag A-type star with a high projected rotational ve- = ∼ −1 temperature (T0 3800 K, see Sect. 9). Due to the high incident locity of 90 km s . As a result, high precision radial veloc- flux and the early spectral type of the star we expect that a large ity (RV) measurements usually demanded by the planet verifi- ffi amount of UV radiation is deposited in the planet’s atmosphere. cation and planet mass estimation processes are very di cult to This, together with its high Roche-lobe filling factor (see Budaj take. Followup spectroscopic observations by Collier Cameron 2011) make this system perhaps the best candidate for studying et al. (2010, hereafter CC10) could only yield an upper limit effective planetary envelope evaporation and mass flow onto the of 4.1 MJ on the planetary mass but no bisector analysis could host star. be performed due to the several km s−1 scatter of the measured Due to its brightness and significant transit depth RV values. However, due to the large rotational velocity, the of ∼14 mmag, the WASP-33 system is highly suitable for fol- authors were able to utilize the surface velocity field mapping lowup photometric observations. Some 801 events have been ob- served over the years by professional and amateur astronomers. Appendix A is available in electronic form at http://www.aanda.org Most of these light curves (LCs) display various anomalies, such as transit depth variation, mid-transit humps, tilted full transit Photometric time series and lightcurves are only available at the / CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) phase, asymmetric ingress egress phases and small-amplitude or via oscillations. Although not all of these variations are necessarily http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/553/A44 Alfred P. Sloan Research Fellow. 1 http://var2.astro.cz/ETD/ Article published by EDP Sciences A44, page 1 of 17 A&A 553, A44 (2013) real, it is clear that the system is peculiar and very much worth 1.2 0 0.81978 -.0109 40.7 for further attention. 1 WASP-33 was also observed by HATNet (Bakos et al. 2002, 2004) in the course of the search for transiting extra- .8 solar planets (TEPs). After assigning the “candidate” status to .6 the target in January 2010, a followup reconnaissance spec- .4 troscopy conducted by the TRES spectrograph (F˝urész 2008) has led to the conclusion that the rotational velocity was very .2 high. Nevertheless, we continued the spectroscopic followup and 0 found that the low velocity amplitude may suggest the presence 0.511.52 of a planetary companion. After the SuperWASP announcement we stopped pursuing this target but here we utilize both the RV and the photometric (HATNet and our early followup) data. Fig. 1. BLS spectrum of the TFA-filtered/reconstructed time series In this paper we perform a comprehensive time series analy- based on the observations of WASP-33 by HATNet. The label in the sis by utilizing the HATNet data, the followup LCs deposited at upper right corner shows, from left to right, the pre-whitening order, peak frequency, transit depth (assuming trapezoidal transit shape) and the Exoplanet Transit Database (ETD, see Poddany et al. 2010), / other published photometric data and new LCs from the Fred the S N of the peak frequency. The BLS amplitude is normalized to 2 unity at the highest peak and refers to the signal without correction for Lawrence Whipple and Konkoly observatories . Our goal is to blending by the nearby companion (see text). verify the planetary nature of WASP-33b purely from photome- try (the “Kepler-way” of system validation – see, e.g., Muirhead -0.030 et al. 2012), to derive more accurate system parameters and ex- amine the possible causes of light curve anomalies. We also use -0.015 our RV archive, based on the observations obtained by the TRES spectrograph, to improve the mass estimate of the planet. 0.000 2. HATNet detection delta mag 0.015 WASP-33 is located in two HATNet fields, internally labeled as 0.030 #166 and 167. The observations were made through Bessel I fil- ter between November 06, 2003 and January 30, 2007 by the -0.50 -0.25 0.00 0.25 0.50 phase telescopes located at the Arizona (FLWO) and Hawaii (Mauna Kea) sites of HATNet. Altogether 12 149 data points have been Fig. 2. Folded light curve of the TFA-filtered/reconstructed time se- gathered in three seasons, spanning 1181 days. The data were ries based on the observations of WASP-33 by HATNet (dots). The collected with an integration time of ∼5 min. We detected a best-fitting simplified light curve model (with bin averaging in the robust transit signal by using the method of box-fitting least out-of-transit part) is shown by the thick line. squares (BLS, Kovács et al. 2002) after applying the trend filter- ing algorithm (TFA, Kovács et al. 2005) to remove systematics The total transit duration (from the first to the last contact) (note that the signal was detectable with high S/N without using / / is 2.7 h, the ingress egress (each) last for 14 min. These val- TFA; however, filtering out systematics helped to increase S N ues agree well with the ones derived in other, accurate followup by a factor of two and substantially clean the folded light curve). works (e.g., by CC10). However, the measured transit depth The BLS spectrum of the TFA-filtered data is shown (computed with the above transit model) is 13 mmag, which is in in Fig. 1. The detection is very strong with the standard the lower tail of the distribution of the transit depths derived from (sub)harmonics at integer frequency ratios. With the peak fre- −1 the nine followup observations in the same color (see Table 1 and quency of 0.819759 d the signal was reconstructed (i.e., fil- Appendix). The average value of the depth from these data is tered out from systematics) by using a modified trapezoidal tran- 14 mmag, with a standard deviation of 1.5 mmag, much higher sit model with rounded bottom (to simulate limb darkening, with than the formal errors. If we consider only the most accurate 15% maximum deviation from the trapezoidal flat bottom) and data gathered by larger telescopes (i.e., Konkoly, CC10), we get an arbitrary out of transit variation (represented by 40 bin aver- a range of 13–17 mmag, indicating that the transit depth might ages of the light curve through the full orbital phase).
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