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Register a new userEXPLORE/OC: A Search for Planetary Transits in the Field of the Southern Open Cluster NGC 6208
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Brian L. Lee
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The EXPLORE Project expanded in 2003 to include a campaign to monitor rich southern open clusters for transits of extrasolar planets (EXPLORE/OC). In May and June 2003, we acquired precise, high-cadence photometry of the second open cluster in our campaign, NGC 6208. Here, we present preliminary results from our I-band survey of over 60000 stars in the field of NGC 6208, around 5000 of which were monitored with photometric precision better than 1%.
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We present preliminary photometric results of a monitoring study of the open cluster NGC 2660 as part of the EXPLORE/OC project to find planetary transits in Galactic open clusters. Analyzing a total of 21000 stars (3000 stars with photometry to 1% or better) yielded three light curves with low-amplitude signals like those typically expected for transiting hot Jupiters. Although their eclipses are most likely caused by non-planetary companions, our methods and photometric precision illustrate the potential to detect planetary transits around stars in nearby open clusters.
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 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.
Open clusters are ideal targets for searching for transiting Hot Jupiters. They provide a relatively large concentration of stars on the sky and cluster members have similar metallicities, ages and distances. Fainter cluster members are likely to show deeper transit signatures, helping to offset sky noise contributions. A survey of open clusters will then help to characterise the Hot Jupiter fraction of main sequence stars, and how this may depend on primordial metallicity and stellar age. We present results from 11 nights of observations of the open cluster NGC 7789 with the WFC camera on the INT telescope in La Palma. From 684 epochs, we obtained lightcurves and B-V colours for ~25600 stars, with ~2400 stars with better than 1% precision. We expect to detect ~1 transiting Hot Jupiter in our sample assuming that 1% of stars host a Hot Jupiter companion.
We present results from 30 nights of observations of the open cluster NGC 7789 with the WFC camera on the INT telescope in La Palma. From ~900 epochs, we obtained lightcurves and Sloan r-i colours for ~33000 stars, with ~2400 stars with better than 1% precision. We expected to detect ~2 transiting hot Jupiter planets if 1% of stars host such a companion and that a typical hot Jupiter radius is ~1.2RJ. We find 24 transit candidates, 14 of which we can assign a period. We rule out the transiting planet model for 21 of these candidates using various robust arguments. For 2 candidates we are unable to decide on their nature, although it seems most likely that they are eclipsing binaries as well. We have one candidate exhibiting a single eclipse for which we derive a radius of 1.81+0.09-0.00RJ. Three candidates remain that require follow-up observations in order to determine their nature.
This work presents the first high-precision variability survey in the field of the intermediate-age, metal--rich open cluster NGC 6253. Clusters of this type are benchmarks for stellar evolution models. Continuous photometric monitoring of the cluster and its surrounding field was performed over a time span of ten nights using the Wide Field Imager mounted at the ESO-MPI 2.2m telescope. High-quality timeseries, each composed of about 800 datapoints, were obtained for 250,000 stars using ISIS and DAOPHOT packages. Candidate members were selected by using the colour-magnitude diagrams and period-luminosity-colour relations. Membership probabilities based on the proper motions were also used. The membership of all the variables discovered within a radius of 8 arcmin from the centre is discussed by comparing the incidence of the classes in the cluster direction and in the surrounding field. We discovered 595 variables and we also characterized most of them providing their variability classes, periods, and amplitudes. The sample is complete for short periods: we classified 20 pulsating variables, 225 contact systems, 99 eclipsing systems (22 Beta Lyr type, 59 Beta Per type, 18 RS CVn type), and 77 rotational variables. The time-baseline hampered the precise characterization of 173 variables with periods longer than 4-5 days. Moreover, we found a cataclysmic system undergoing an outburst of about 2.5 mag. We propose a list of 35 variable stars (8 contact systems, 2 eclipsing systems, 15 rotational variables, 9 long-period variables and the cataclysmic variable) as probable members of NGC 6253.
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