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Draft version September 8, 2018 A Preprint typeset using L TEX style emulateapj v. 11/12/01 SEARCHING FOR PLANETARY TRANSITS IN GALACTIC OPEN CLUSTERS: EXPLORE O/C Kaspar von Braun Carnegie Institution, Dept of Terrestrial Magnetism, 5241 Broad Branch Rd. NW, Washington, DC 20015 [email protected] Brian L. Lee Dept of Astronomy and Astrophysics, University of Toronto, 60 St. George St., Toronto, Canada M5S 3H8 [email protected] S. Seager Carnegie Institution, Dept of Terrestrial Magnetism, 5241 Broad Branch Rd. NW, Washington, DC 20015 [email protected] H. K. C. Yee Dept of Astronomy and Astrophysics, University of Toronto, 60 St. George St., Toronto, Canada M5S 3H8 [email protected] Gabriela Mallen-Ornelas´ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-15, Cambridge, MA 02138 [email protected] and Michael D. Gladders Carnegie Observatories, 813 Santa Barbara St., Pasadena, CA 91101 [email protected] Draft version September 8, 2018 ABSTRACT Open clusters potentially provide an ideal environment for the search for transiting extrasolar planets since they feature a relatively large number of stars of the same known age and metallicity at the same distance. With this motivation, over a dozen open clusters are now being monitored by four different groups. We review the motivations and challenges for open cluster transit surveys for short- period giant planets. Our photometric monitoring survey (EXPLORE/OC) of Galactic southern open clusters was designed with the goals of maximizing the chance of finding and characterizing planets, and of providing for a statistically valuable astrophysical result in the case of no detections. We use the EXPLORE/OC data from two open clusters NGC 2660 and NGC 6208 to illustrate some of the largely unrecognized issues facing open cluster surveys including severe contamination by Galactic field stars (> 80%) and relatively low number of cluster members for which high precision photometry can be obtained. We discuss how a careful selection of open cluster targets under a wide range of criteria arXiv:astro-ph/0409653v2 7 Dec 2004 such as cluster richness, observability, distance, and age can meet the challenges, maximizing chances to detect planet transits. In addition, we present the EXPLORE/OC observing strategy to optimize planet detection which includes high-cadence observing and continuously observing individual clusters rather than alternating between targets. Subject headings: techniques: photometric, surveys, planetary systems, open clusters and associations: general, individual (NGC 2660, NGC 6208) 1. introduction by the star’s motion about the common center of mass (see for example table 3 in Butler et al. 2002). Fainter The EXPLORE Project (Mall´en-Ornelas et al. 2003; Yee (and thus more) stars can be monitored photometrically et al. 2003) is one of currently about 20 ongoing surveys1 with the aim to detect transiting close-in extrasolar giant than spectroscopically. Thus, more distant environments can be probed for the existence of extrasolar planets. planets (CEGPs; also referred to as ‘51-Peg type’ or ‘hot All transiting planets have a measured radius, based on Jupiters’, i.e., planets with radius of order Jupiter-radius, orbital period of one to a few days, and transit durations of transit depth and stellar radius. Knowledge of the planet’s radius and mass plays an important role in modeling in- a few hours) around Galactic main-sequence stars. Transit studies explore a different parameter space in the search for ternal structure of planets and hence the formation, evolu- extrasolar planets from the very successful radial-velocity tion, and migration of planetary systems (see for example Burrows et al. 2000; Guillot & Showman 2002; Baraffe or “wobble” method. This method based on detecting planets via the radial motions of their parent star caused et al. 2003, and references therein). Transiting planets are currently the only planets whose 1 See http://star-www.st-and.ac.uk/∼kdh1/transits/table.html, maintained by K. Horne. 1 2 von Braun et al. physical characteristics can be measured. In addition to The OC NGC 6791 was furthermore monitored for plan- mass and radius, several parameters can be constrained etary transits by Bruntt et al. (2003). As all these sur- from follow-up measurements. For example, the fact that veyed OCs are located the northern hemisphere, EX- a transiting planet will be superimposed on its parent star PLORE/OC7 is currently the only OC survey operating can be used to determine constituents of the planet’s at- in the south where most of the Galactic OCs are located. mosphere by means of transmission spectroscopy (Char- With the growing number of open star cluster sur- bonneau et al. 2002; Vidal-Madjar et al. 2003), or put veys this publication describes incentives, difficulties, and constraints on the existence of planetary moons or rings strategies for open cluster planet transit surveys, thereby (e.g., Brown et al. 2001). In addition, the secondary eclipse including a discussion on transit surveys in general. We can provide information about the planetary temperature use data from the first two targets (NGC 2660 and NGC or its emission spectrum (Richardson et al. 2003a,b). 6208) from our program to illustrate the major issues for At the time of writing, six transiting planets are known. OC transit surveys. HD209458b (Charbonneau et al. 2000; Henry et al. 2000; The concept and advantages of monitoring OCs for the Brown et al. 2001) was discovered by radial-velocity mea- existence of transiting planets were originally described in surements (Henry et al. 2000; Mazeh et al. 2000) and the Janes (1996). Written before the hot Jupiter planets were transits were discovered by photometric follow-up. OGLE- discovered, Janes (1996) focused on 12-year period orbits TR-56 (Udalski et al. 2002a,b; Konacki et al. 2003a) was and long-term photometric precision required to determine the first planet discovered by the transit method, and the or put useful limits on the Jupiter-like planet frequency. first of currently four planets based on photometry of the This paper is intended to be a modern version of Janes OGLE-III survey (Udalski et al. 2002a,b, 2003): OGLE- (1996) based on the existence of short-period planets and TR-113 (Bouchy et al. 2004; Konacki et al. 2004), OGLE- practical experience we have gained from both the EX- TR-132 (Bouchy et al. 2004), and OGLE-TR-111 (Pont PLORE and the EXPLORE/OC planet transit surveys. et al. 2004). Very recently, Alonso et al. (2004) found Section 2 motivates OC transit surveys. Section 3 ad- a further transiting planet, TrES-1, using telescopes with dresses the challenges facing transit surveys in general, 10cm apertures. Over 20 transit searches are currently and 4 addresses challenges specifically facing OC tran- ongoing to find more planets. sit surveys.§ Section 5 focuses on strategies to select OCs As part of the EXPLORE2 Project, we have recently be- which are most suited for transit surveys and which min- gun a survey – EXPLORE/OC – of southern open clusters imize challenges described in the previous sections. The (OCs) with the aim of detecting planetary transits around EXPLORE/OC strategies concerning target selection, ob- cluster member stars. During the course of 3 years, we serving methods, photometric data reduction, and spec- hope to conduct searches of up to 10 OCs using∼ the Las troscopic follow-up observations are described in 6, 7, Campanas Observatory (LCO) 1m Swope Telescope. To and 8, respectively. These Sections contain relevant§ pre-§ date we have monitored five OCs (see 6). liminary§ results on EXPLORE/OC’s first two observed In addition to EXPLORE/OC, at the§ time of writing clusters NGC 2660 and NGC 6208. We summarize and there are currently three OC planet-transit surveys under- conclude in 9. way3: § The Planets In Stellar Clusters Extensive Search 2. motivation for open cluster planet transit • (PISCES4) reported the discovery of 47 and 57 low- searches amplitude variables in the open clusters NGC 6791 Open clusters present themselves as “laboratories” (Mochejska et al. 2002) and NGC 2158 (Mochejska within which the effects of age, environment, and espe- et al. 2004), respectively. cially metallicity on planet frequency may be examined. Evidence that planet formation and migration are corre- The University of St. Andrews Planet Search lated with metallicity comes from radial velocity planet • 5 (UStAPS ) has monitored the OCs NGC 6819 searches (Fischer & Valenti 2003). The fact that no plan- (Street et al. 2002) and NGC 7789 (Bramich et al. etary transits were discovered in the monitoring study of 2003) and published data on variable stars in NGC 47 Tuc by Gilliland et al. (2000) may be due to its low 6819 (Street et al. 2003). metallicity, or alternatively due to the high-density envi- ronment in a system such as a globular cluster (or due to The Survey For Transiting Extrasolar Planets In both effects). The less-crowded OCs of the Milky Way of- • Stellar Systems (STEPSS6), described in Burke fer a range of metallicities and may thus be further used et al. (2003) and Gaudi et al. (2002). They have an- to disentangle the effects of metallicity versus high-density alyzed monitoring data of the OC NGC 1245 (Burke environment upon planet frequency. et al., in preparation) and determined its fundamen- Monitoring OCs for the existence of planetary transits tal parameters (Burke et al. 2004). Analysis of their offers the following incentives (see also Janes 1996; Char- data on NGC 2099 and M67 is currently ongoing. bonneau 2003; Lee et al. 2004; von Braun et al. 2004): 2 http://www.ciw.edu/seager/EXPLORE/explore.htm 3 see also http://www.ciw.edu/kaspar/OC transits/OC transits.html 4 http://cfa-www.harvard.edu/∼bmochejska/PISCES 5 http://crux.st-and.ac.uk/∼kdh1/ustaps.html and http://star-www.st-and.ac.uk/∼dmb7 6 http://www.astronomy.ohio-state.edu/∼cjburke/STEPSS 7 http://www.ciw.edu/seager/EXPLORE/open clusters survey.html EXPLORE/OC 3 1.

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