EXPLORE/OC: Photometry Results for the Open Cluster NGC 2660

1. g h b c a e f l h i j k d g j k l • NGC 2660 Properties • RA / Dec (2000) = 8:42:38 / - 47:12:02 • l / b = 265.9 / - 3.01 • distance = 2.8 kpc • E(B-V) = 0.313 • age = 1.1 Gyr • [Fe/H] ~ 0 • (WEBDA Database & Sandrelli et al. 1999) EXPLORE/OC: Photometry Results for the Open Cluster NGC 2660 K. von Braun (Carnegie/DTM), B. L. Lee (Toronto), S. Seager (Carnegie/DTM), H. K. C. Yee (Toronto), G. Mallén-Ornelas (CfA), and M. D. Gladders (OCIW) • 1. Introduction / Strategies • The EXPLORE/OC Project is a photometric monitoring survey of Galactic southern open clusters (OCs) with the aim to find transiting close-in extrasolar giant planets. A by-product of this search is the discovery of a wide range of different variable stars. Our search for planets in the open cluster NGC 2660 has produced several tens of variables in the field of the OC. In this presentation, we give some background information on our survey and present some selected photometry results on NGC 2660. • Target selection is difficult due to very limited availability of data on Galactic open clusters. To maximize the number of stars observed for long periods of time at high relative photometric precision, we base our target selection on: • observability of the cluster per night. • distance to the cluster, which determines what relative photometric precision we attain for a given telescope, exposure time, and foreground reddening. • number of stars we can monitor at high relative photometric precision, cluster-members or not. • Observing is performed in the I band (to reduce the effects of limb darkening and the amplitude of variability due to stellar surface activity). We observe one cluster only for ~ 20 nights to maximize our detection efficiency. 3. Variable Stars Our observing program of NGC 2660 resulted in approximately 1000 epochs (300s exposure time at the Las Campanas Obs. 1-m Telescope) in the I filter. Variable stars with amplitudes > 0.05 mag are easily detectable by a chi-squared algorithm. In a plot of magnitude versus time, we “expect” a straight line for any non-variable stars. Stars whose chi-squared > 3.0 with respect to fitting a straight line are flagged and inspected by eye (after removing outliers). In the field of NGC 2660, we detected several tens of such high-amplitude variables, examples of which we show below (Fig. 3 a – i). Note that the magnitude scale is different for every figure. Variable stars with amplitudes < 0.05 mag cannot be detected by the chi-squared algorithm and thus present a much greater challenge. Planetary transits in particular would have amplitudes of ~0.01 mag for a Jupiter-sized planet and a Sun-sized star. We detect low-amplitude systems using the “box-fitting algorithm” (Kovacs et al. 2002, A&A 391, 369). Examples are shown in Fig. 3 j – l. a b d c 2. The Open Cluster NGC 2660 e f c d i • Fig 3 a – l: Example light curves of variable stars in the field of NGC 2660. • Panels a – h represent eclipsing binary systems of different brightnesses with an increasing amount of contact (Panels a & b: detached; Panels c – f: semi-detached, Panels g & h: contact binary). • Panels i& j are pulsating variables. Panel i is most likely an RR Lyrae star. Note the very small amplitude of the light curve in Panel j. • Panels k & l are the kinds of signals we are looking for in a planetary transit: a 1 – few percent flat-bottomed dip in the otherwise flat light curve. Unfortunately, neither of them is likely be caused by a transiting planet. Fig. 1: Color image of NGC 2660. N is up and E is to the left. The field of view is 15’ x 24’. Fig 4: CMD of the field of NGC 2660 with the locations of the variable stars shown in Fig. 3. The letter in each orange circle corresponds to the light curve in the Panel with the same letter. Fig 2a: This figure compares the stellar density (measured in stars per 10000 pixel2 for 13 < I < 17) as a function of radial distance from the cluster center, versus two control fields taken in identical observing conditions at the same Galactic latitude (one degree toward either direction in longitude). The cluster field is indicated by the solid line, the control fields by the dotted and dashed lines. The total number of stars under the curves is 3500 for the cluster field and 2800 average for the two control fields, corresponding to a contamination of ~80% over the entire field and ~30% to a radial distance of 500 pix / 4 arcmin (red box). Fig 2b: Color-magnitude diagram (CMD) of the field of NGC 2660. The black crosses are all stars in the field of view, the red circles are the stars located within 4 arcmin of the cluster center (red box in Fig 2a). For additional information: http://www.ciw.edu/seager/EXPLORE/open_clusters_survey.htm (EXPLORE/OC web site) http://www.ciw.edu/seager/EXPLORE/noao_survey.htm (EXPLORE web site) Kaspar von Braun: phone: +1 (202) 478-8859 e-mail: kaspar@dtm.ciw.edu Fig 5: Examples of a real-time (unphased) light curve in the field of NGC 2660. Every panel represents data taken during one night, starting on the bottom left with night 1. Night 2’s data are directly above it, night 3 above that and so on. We did not obtain any data during nights 13-15. This light curve is caused by a rapidly pulsating Delta Scuti star. The pulsations have multiple overtones and can thus not be phased with a single period, which is why neither of methods described above can detect them. Note the very small amplitude of the photometric variability.