A&A 445, 341–346 (2006) Astronomy DOI: 10.1051/0004-6361:20054308 & c ESO 2005 Astrophysics A posteriori detection of the planetary transit of HD 189733 b in the Hipparcos photometry G. Hébrard and A. Lecavelier des Etangs Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France e-mail: [email protected] Received 5 October 2005 / Accepted 10 October 2005 ABSTRACT Aims. Using observations performed at the Haute-Provence Observatory, the detection of a 2.2-day orbital period extra-solar planet that transits the disk of its parent star, HD 189733, has been recently reported. We searched in the Hipparcos photometry Catalogue for possible detections of those transits. Methods. Statistic studies were performed on the Hipparcos data in order to detect transits of HD 189733 b and to quantify the significance of their detection. Results. With a high level of confidence, we find that Hipparcos likely observed one transit of HD 189733 b in October 1991, and possibly two others in February 1991 and February 1993. Using the range of possible periods for HD 189733 b, we find that the probability that none of those events are due to planetary transits but are instead all due to artifacts is lower than 0.15%. Using the 15-year temporal baseline available, . +0.000006 we can measure the orbital period of the planet HD 189733 b with particularly high accuracy. We obtain a period of 2 218574−0.000010 days, corresponding to an accuracy of ∼1 s. Such accurate measurements might provide clues for the presence of companions. Key words. stars: individual: HD 189733 – stars: planetary systems 1. Introduction the first known transiting extra-solar planet (Charbonneau et al. 2000; Henry et al. 2000; Mazeh et al. 2000), were a posteriori Bouchy et al. (2005) recently announced the detection of a detected in Hipparcos data by Robichon & Arenou (2000), 2.2-day orbital period extra-solar planet that transits the disk Castellano et al. (2000), and Söderhjelm (1999). The tran- of its parent star, the dwarf HD 189733, which is located sits of HD 149026 b (Sato et al. 2005) are not deep enough only 10 arcmin from the famous Dumb-Bell Nebula. This (0.003 mag) to be detectable with Hipparcos (Hébrard et al. detection was performed with spectroscopic and photomet- 2005). Here we show that transits of HD 189733 b were de- ric data collected at the Haute-Provence Observatory, France, tected by Hipparcos, and we quantify the signification of this as part of the ELODIE metallicity-biased search for transit- a posteriori detection. The long temporal baseline available al- ing hot Jupiters (Da Silva et al. 2005). Together with radial lows us to obtain an accurate orbiting period of this hot Jupiter. velocity measurements, observations of transits allow the ac- tual mass and radius of an extra-solar planet to be measured. 2. Folding the Hipparcos photometric Transiting planets also allow follow-up observations to be per- measurements formed during transits (Charbonneau et al. 2002; Vidal-Madjar et al. 2003, 2004) or anti-transits (Charbonneau et al. 2005; 2.1. Hipparcos photometric data of HD 189733 Deming et al. 2005), yielding physical constraints on the atmo- The Hipparcos Catalogue includes HD 189733 photometry spheres of these planets. measurements at 185 different epochs. We only used in the To date, HD 189733 b is only the ninth known transiting present study the 176 measurements that are “accepted” in the extra-solar planet (Bouchy et al. 2005), and the third transit- Catalogue; the 9 remaining ones are flagged in the Catalogue ing a star bright enough to be in the Hipparcos Catalogue as perturbed and not reliable. These 176 values are plotted in (Perryman et al. 1997). The Epoch Photometry Annex of the Fig. 1 over the 3-year observation baseline. The epochs of the Hipparcos Catalogue contains between ∼40 and ∼300 mea- measurements are given in Terrestrial Time corresponding to surements performed during the 1990–1993 mission for each the Solar System Barycentric Julian Date (BJD). The difference of the 118 204 stars of the Catalog. Transits of HD 209458 b, between BJD and Heliocentric Julian Date (HJD) is negligible Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20054308 342 G. Hébrard and A. Lecavelier des Etangs: A posteriori detection of the planetary transit of HD 189733 b with Hipparcos Fig. 1. The 176 reliable Hipparcos photometric measurements of Fig. 2. Top: χ2 of the fit of the Hipparcos magnitudes with a transit HD 189733. The four measurements performed during planetary tran- light curve, as a function of the trial period. The period interval al- sits of HD 189733 are surrounded by circles. The typical error bar lowed by Bouchy et al. (2005) is represented by vertical dotted lines. on H is represented on the bottom, left. p The ticks show the positions of the possible periods assuming that a transit occurred on 1991, Oct. 17th (see Sect. 5.1). Bottom: zoom around the minimum χ2, found for a 2.218574-day period. for our study. The Hipparcos measurements were performed in the specific Hp band, which is centered near 4500 Å and has a width of ∼2400 Å. The estimated standard errors of each in- ones possibly detected by Hipparcos, the accuracy of the phase dividual Hp magnitude are around 0.012 mag; this makes the is exactly the accuracy of T0. Bouchy et al. (2005) reported ∼ 3% deep HD 189733 planetary transit in principle detectable. T0 = 2 453 629.3890 ± 0.0004 (HJD) so the uncertainty on Two sets of numerous, dispersed measurements performed the phase in the Hipparcos data is 0.000018, corresponding to at two neighboring epochs are apparent in Fig. 1. Owing to 0.0004 day. This assumes that the period is constant, or at least the Hipparcos scanning law, there are actually four time inter- that if any, the variations of the period are small, with a constant vals of about 1.5 day each (BJD – 2 440 000 = 8308.5, 8314.3, average value. 9039.0, and 9044.7) during which numerous photometric mea- For each of the 8000 periods tested, we computed the χ2, surements were performed. The dispersion of these measure- i.e. the quadratic sum of the weighted difference between the ments shows the stellar variability of HD 189733, which is observed magnitudes and a transit model. The transit model classed as microvariable in the Hipparcos Catalogue. As we is an approximation of the light curve presented by Bouchy see below, the microvariability of HD 189733 does not prohibit et al. (2005). It assumes a 2.7% deep transit, and durations from transits detection (see also Sect. 5.3). the 1st to the 4th contacts and from the 2nd to the 3rd contacts of 1.60 h and 0.66 h, respectively. χ2 2.2. Period search Figure 2 shows the as a function of the trial period, which is the only free parameter. A clear minimum is seen 2 HD 189733 b orbits its parent star every ∼2.2 days with a tran- for the period PHipp = 2.218574 days. The minimum is χ = sit duration of ∼1.6 h, so about 3% of randomly chosen obser- 251.0. We attribute this high χ2, considering the 175 degrees vations would be expected to fall during the transit. This corre- of freedom, to the microvariability of HD 189733 (Sect. 5.3). sponds to about 5 measurements in the case of the 176 available The value of the orbital period which we found, PHipp,isin Hipparcos values, which are however not regularly sampled in agreement with the one reported by Bouchy et al. (2005), time as we described above. Nevertheless, it is likely that a few 2.2190 ± 0.0005 days. The Hipparcos data folded with the pe- planetary transits were sampled in these 176 measurements. riod PHipp are plotted in Fig. 3. 2 We performed a χ analysis to detect transits. We scanned The median value for Hp is 7.827. Assuming that PHipp the possible periods around the period of 2.2190 days given is the orbital period of HD 189733 b, the weighted average −7 by Bouchy et al. (2005) with steps of 5 × 10 day (or about value of the four Hp measurements obtained during the plan- 0.04 s), in the range [2.217−2.221] days, that is four times the etary transit is 7.859 ± 0.006 whereas the weighted average uncertainties given by Bouchy et al. (2005). A broader search of the 172 remaining points is 7.8261 ± 0.0009. Thus, the was also performed (see Sect. 5.2). The phase of the transit depth of the transit light curve as measured with Hipparcos is within the Hipparcos data is a function of the assumed period. (0.033 ± 0.006) mag or (3.1 ± 0.6)% in flux. This agrees with Indeed, for a given period, the phase is strongly constrained the light curve reported by Bouchy et al. (2005) from accu- by the mean transit epoch, T0, as determined by the ground- rate and well sampled ground-based observations of the transit. based discovery and follow-up observations. As, for a given The Hipparcos data of HD 209458, the host of the first known period, there are integer numbers of HD 189733 b orbits be- transiting planet (Charbonneau et al. 2000; Henry et al. 2000), tween the transits observed by Bouchy et al. (2005) and the yield a transit marginally deeper than the actual one, which G.