arXiv:astro-ph/0409653v2 7 Dec 2004 1 lnt i h ailmtoso hi aetsa caused detecting parent on their based of motions method radial This the via planets method. radial-velocity successful “wobble” very or the for from search the planets in space extrasolar parameter Transit different of a . durations explore main-sequence transit studies Galactic and around days, hours) few few a Jupiter-radius, a ‘hot to order one or of of type’ period radius ‘51-Peg orbital with as planets giant to i.e., extrasolar surveys referred Jupiters’, close-in ongoing also transiting 20 (CEGPs; detect about planets to currently aim of the one with is 2003) al. et rpittpstuigL using typeset Preprint 2018 8, September version Draft e http://star-www.st-and.ac.uk/ See h XLR rjc Ml´nOnlse l 03 Yee 2003; (Mall´en-Ornelas al. et Project EXPLORE The ujc headings: o Subject continuously targets. and between o observing alternating EXPLORE/OC high-cadence tar than the includes present cluster we which mee open addition, can detection of In age selection transits. and planet careful distance, observability, detect a richness, how cluster as discuss such We obtained. be h XLR/Cdt rmtooe lsesNC26 n G 620 NGC se and including 2660 surveys th NGC cluster ( findin in clusters open stars facing result of open issues astrophysical chance two unrecognized valuable from the largely statistically data maximizing a EXPLORE/OC of for the goals providing are the (EXPLORE of open survey kn clusters with and for monitoring same designed open challenges photometric was the dozen and Our clusters motivations of a planets. the over stars giant review motivation, of period We this number groups. With large different relatively distance. a same feature they since ERHN O LNTR RNISI AATCOE CLUSTERS OPEN GALACTIC IN TRANSITS PLANETARY FOR SEARCHING pncutr oetal rvd nielevrnetfrtesear the for environment ideal an provide potentially clusters Open eto srnm n srpyis nvriyo Toronto, of University Astrophysics, and Astronomy of Dept eto srnm n srpyis nvriyo Toronto, of University Astrophysics, and Astronomy of Dept angeIsiuin eto ersra ants,5241 Magnetism, Terrestrial of Dept Institution, Carnegie angeIsiuin eto ersra ants,5241 Magnetism, Terrestrial of Dept Institution, Carnegie > 0)adrltvl o ubro lse ebr o hc ihprec high which for members cluster of number low relatively and 80%) A avr-mtsna etrfrAtohsc,6 adnSt Garden 60 Astrophysics, for Center Harvard-Smithsonian 1. T E tl mltajv 11/12/01 v. emulateapj style X introduction eea,idvda NC26,NC6208) NGC 2660, (NGC individual general, ehius htmti,sres lntr ytm,oe clu open systems, planetary surveys, photometric, techniques: ∼ angeOsraois 1 at abr t,Psdn,C Pasadena, St., Barbara Santa 813 Observatories, Carnegie d1tast/al.tl anandb .Horne. K. by maintained kdh1/transits/table.html, areaMall Gabriela rf eso etme ,2018 8, September version Draft ihe .Gladders D. Michael [email protected] aprvnBraun von Kaspar [email protected] [email protected] [email protected] [email protected] [email protected] ra .Lee L. Brian .K .Yee C. K. H. ABSTRACT .Seager S. 1 and 1 en-Ornelas ´ urw ta.20;Gilt&Somn20;Baraffe 2002; Showman therein). references & and Guillot 2003, example 2000; al. for et al. (see systems et in- planetary Burrows evolu- of modeling formation, migration the in and hence role tion, and important planets of an structure plays ternal planet’s the mass of and Knowledge radius radius. stellar planets. and depth extrasolar transit of existence Fainter the for environments 2002). probed photometrically distant be al. monitored more can Thus, et be mass can Butler of spectroscopically. stars in than center more) 3 common thus table the (and example about for motion (see star’s the by ra rnhR.N,Wsigo,D 20015 DC Washington, NW, Rd. Branch Broad ra rnhR.N,Wsigo,D 20015 DC Washington, NW, Rd. Branch Broad rniigpaesaecretyteol lnt whose planets only the currently are planets Transiting on based radius, measured a have planets transiting All 0S.Gog t,Trno aaaMS3H8 M5S Canada Toronto, St., George St. 60 0S.Gog t,Trno aaaMS3H8 M5S Canada Toronto, St., George St. 60 et S1,Cmrde A02138 MA Cambridge, MS-15, reet, h hlegs aiiigcacsto chances maximizing challenges, the t eecnaiainb aatcfield Galactic by contamination vere srigsrtg ootmz planet optimize to strategy bserving esudrawd ag fcriteria of range wide a under gets srigidvda lsesrather clusters individual bserving hfrtastn xrslrplanets extrasolar transiting for ch lse rni uvy o short- for surveys transit cluster aeo odtcin.W use We detections. no of case e O)o aatcsuhr open southern Galactic of /OC) w g n ealct tthe at and age own o en oioe yfour by monitored being now n hrceiigplanets, characterizing and g 91101 A oilsrt oeo the of some illustrate to 8 tr n associations: and sters so htmtycan ision XLR O/C EXPLORE : 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 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 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 (or due to The Survey For Transiting Extrasolar Planets In both effects). The less-crowded OCs of the of- • Stellar Systems (STEPSS6), described in Burke fer a range of 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. Metallicity, age, distance, and foreground reddening semi-major axis a 0.05 (Marcy et al. 2004; Naef et al. are either known or may be determined for clus- 2004). Of those CEGP∼ systems, approximately 10–20% ter members (more easily and accurately than for (probability R∗/a) would, by chance, have a favorable random field stars; see 5 and, e.g., Burke et al. orientation such∼ that a transit is visible from Earth. We 2004). Thus, planets detected§ around open cluster assume that planets can only be detected around single stars will immediately represent data points for any stars and conservatively adopt a binary fraction of 50%; statistic correlating planet frequency with age, stel- although there are known planets orbiting binary stars and lar environment, or metallicity of the parent star. multiple star systems (Zucker & Mazeh 2002; Eggenberger et al. 2004), their detection by transits would be difficult 2. The planet-formation and planet-migration pro- due to a reduced photometric signature in the presence cesses, and hence planet frequencies, may differ be- of the additional star. Combining the above estimates, tween the OC, globular cluster, and Galactic field we arrive at the at the value of 1 star in 3000 (a 0.05 populations. Planet transit searches in OCs, to- AU) having a hot Jupiter planetary transit around a∼ main gether with many ongoing transit field searches and sequence star. GC surveys (e.g., the ground-based work on 47 Tuc The probability of detecting an existing transiting hot by Weldrake et al. 2003, 2004), enable comparison Jupiter (1/3000) applies only to stars for which it is possi- between these different environments. ble to detect a planetary transit given the observational setup of the survey. This number of “suitable” stars 3. Specific masses and radii for cluster stars may be is frequently equated to the number of stars with high targeted (within certain limits of other survey de- enough relative photometric precision to detect the tran- sign choices, see 7.2) in the planet-search by the § siting planet (see Fig. 1). Planet transit surveys in gen- choice of cluster distance and by adjusting expo- eral reach photometric precision sufficiently high to de- sure times for the target. tect Jupiter-sized transiting planets around main sequence stars (see Fig. 1) for up to 40% of stars in their survey, de- 3. main challenges for transit surveys pending on crowdedness and other factors. For example, Open cluster planet transit surveys are a subset of planet the EXPLORE search reached relative photometric preci- sion better than 1% on 37,000 stars from 14.5 I 18.2 transit surveys and therefore have some important chal- ≤ ≤ lenges in common. Articulating these challenges is crucial out of 350,000 stars down to I = 21 (M03); OGLE-III in light of the fact that over 20 planet transit surveys have reached better than 1.5% relative photometric precision been operating for a few years (Horne 2003), with only six on 52,000 stars out of a total of 5 million monitored stars known transiting planets of which five were discovered by (Udalski et al. 2002b); the Sleuth survey (O’Donovan et al. transits. 2004), reaches better than 1.5% relative photometry over The most basic goals of any transit survey are to (1) the entire dataset on the brightest 4000 stars out of 10,000 in their 6 6 degree2 field, using an automated 10 cm tele- detect planets and provide their characteristics, and (2) × to provide (even in the case of zero detections) statistics scope with a 6 degree square field of view. concerning planet frequencies as a function of the astro- The real number of stars suitable for planet transit de- physical properties of the surveyed environment. Although tection, however, is not equivalent to the number of stars the most important considerations for designing a success- with 1% relative photometry. One may see from Fig. 1, ful transit survey were presented in Mall´en-Ornelas et al. for instance, that a Jupiter-sized planet would cause a 2% (2003, M03 hereafter) for the EXPLORE Project, we sum- eclipse around a parent M0-star. Furthermore, Pepper & marize and provide updates to the three key issues: num- Gaudi (in preparation) find that, if a planet with given ber of stars, detection probability, and blending. For OC properties around a cluster member star on the main se- surveys, these issues (with the exception of blending) can quence produces a detectable transit signature, a planet be optimized or overcome by a careful survey strategy, par- with identical properties orbiting any other main-sequence ticularly by the selection of the target OC (see 5 through cluster member will produce a detection of approximately 7). § the same signal-to-noise-ratio (SNR) unless the sky flux within a seeing disk exceeds the flux of the star. Since 3.1. Maximizing Number of Stars with High Photometric most transit surveys aim to find planets of approximately Precision Jupiter-size around stars whose radii are close to, or less than, a solar radius, the number of stars with 1% relative Any survey’s goal should be to maximize the number of photometric precision can therefore be regarded as a lower stars for which it is possible to detect a transiting planet. limit to the number of stars suitable for transit detection. We discuss the three most important aspects below: the For the rest of this publication, we thus use this number astrophysical frequency of detectable transiting planets, as a proxy for the number of stars around which we (or the probability with which existing planetary transits are other transit surveys) can detect planets. observed, and the number of stars for which the relative photometric precision is sufficiently high. The frequency of detectable transiting planets is calcu- 3.2. Probability to Detect an Existing Transit lated by considering the astrophysical factors: frequency The actual observed hot-Jupiter transit frequency will of CEGPs around the surveyed stars; likelihood of the ge- be lower than 1/3000 due to the probability with which ometrical alignment between star and planet necessary to an existing transit would be observed two or more times detect transits; and binary fraction. We assume a planet during an observing run. This probability, which we call frequency around isolated stars of 0.7% for planets with Pvis, is equivalent to the window function of the obser- 4 von Braun et al. vations. Although the probability function has been de- high enough for 20 data points during transit to scribed in detail before (Borucki & Summers 1984; Gaudi constitute a detection, then Pvisphased is high for 2000, and M03), we extend the discussion to include the all transit periods between one and five days. If, recently discovered class of 1-day period planets, as well in contrast, the data SNR is lower, and 40 or 60 as considering different metrics for probability to detect points per transit are required for detection, then an existing planet. In all of our Pvis simulations we as- Pvisphased is low for P > 2 days. sume the simplified case of a solar mass, solar radius star with the planet crossing the star center, thus focusing on Alternatively, one could imagine a strategy of alter- stars we are most interested in. The transit duration is • nating between cluster fields (to increase the num- then related to the planet’s period by tduration = P R⊙/πa ber of monitored stars), in which case the observing (typically a few hours for a period of a few days). cadence is reduced. Panel b shows Pvisphased when In panel a of Figure 2 we show the Pvis for detecting observing for 21 nights with a 15-minute cadence ( existing transiting planets with different orbital periods 900 measurements in the light curve). The proba-∼ under the requirement that two or more full transits must bility to detect transits with N = 20 is very low for be observed. We consider different runs (7, 14, 21 nights) P > 2 days, and it is zero for N = 40 or 60. of consecutive nights with 10.8 hours of uninterrupted observing each night with 5-min time-sampling. This is Panel c shows Pvisphased for 40 nights of contin- equivalent to approximately 125 observations of a given • uous observing (10.8h) with an observing cadence cluster per night. The P for 1- to 2-day period planets of 5 minutes ( 5200 data points). For N = 20 vis ∼ is basically complete for the 14- and 21-night runs, while and N = 40, Pvisphased is close to complete for all the Pvis is markedly lower for a 14-night run compared to shown periods. a 21-night run for planets with periods between 2 and 4 days. Panel d shows Pvisphased for 40 nights of observing • We now turn to Pvis for a transit detection strategy with a 15-minute cadence (again simulating a strat- where it is not necessary to detect a transit in its entirety egy of alternating between cluster fields; 1700 ∼ during a single observing night. Instead, the strategy re- data points). Pvisphased is very low for N > 20, in- quires a transit to be detected in phased data (from at dicating that, in order to be able to observe with a least two individual transit events). Such a Pvis (which we 15-cadence, many more than 40 nights are needed if call Pvisphased) is relevant for transit detections based on more than 20 data points are required for a transit period-folding transit-searching algorithms (for instance detection. with data covering partial nights or a strategy of alter- nating targets throughout the course of a night). With We conclude that the ability to particularly detect a period-folding algorithm, each individual transit need longer-period (P > 2 days) planets depends on observ- not be fully sampled. In order to quantify Pvisphased, we ing strategy. For the rest of this paper we adopt the Pvis specify that the phased transit must be sampled by at least criterion of seeing two full transits which is a good strategy N points. Since a typical duty cycle of a transit is on the for a limited number ( 20) of observing nights. order of a few percent, we choose N = 20, 40, or 60 to rep- ∼ resent light curves with a total of a few hundred to a thou- 3.3. Blending and False Positives sand data points (the phased OGLE planets’ light curves typically have a few tens of data points obtained during Blending in a planet transit light curve due to the pres- transit). Pvisphased is then calculated to be likelihood (as ence of an additional star is a serious challenge inherent a function of period) that at least N in-transit points are in planet transit surveys, one that has only recently been accumulated for observing runs of different lengths and gaining recognition. If the light of multiple stars are in- different observing cadences. terpreted as being due to one individual star, then the Note that in reality a detection of a planet transit de- relative depth of any eclipse will be decreased. This “light pends on the number of photons observed during the tran- pollution” may either cause (1) an eclipsing binary sys- siting phase. This number of photons is contained in the tems to mimic a more shallow transiting planet signal, or combination of the SNR per individual data point and the (2) a true planet’s transit signal’s depth to be decreased to number of data points (during any transit). A back-of-the- a fraction of its already very small amplitude, rendering it envelope calculation would give a transit SNR for a ∆m harder or even impossible to detect. 2%-depth transit with M = 20 data points and a relative∼ Blending can be caused either by optical projection in photometry precision of rms 1%: crowded fields, or by physically associated stellar systems. ∼ The crowdedness can be considered in the choice of target ∆m 0.02 and observational setup. Blending in spatially unresolved, SNR √M = √20 9. (1) ≃ × rms × 0.01 ∼ physically associated systems generally consists of a wide For comparison with the two-full-transit Pvis, we show binary of which one component hosts an additional close- in Fig. 3 Pvisphased for N = 20, 40, 60 by a solid, dotted, by stellar companion or a transiting planet. The compo- and dashed line, respectively. The four panels represent nent without the close-by stellar or planetary companion different observing strategies: would produce the polluting light. Although we have ac- counted for binary stars in our probability estimate ( 3.1), § Panel a shows Pvisphased for 21 nights (10.8h) with the contribution due to this kind of “false positive” may • 5-minute time-sampling, resulting in a light curve be larger due to the unknown wide-binary component dis- with around 2700 data points. If the data SNR is tances. EXPLORE/OC 5

Recently the effect of blending on the probability of de- in the “coronae” of the clusters. Furthermore, if the tecting planets has been addressed by several authors (e.g., target is located such that the line of sight includes M03; Brown 2003; O’Donovan et al. 2004; Konacki et al. a long path through the (e.g., low Galactic 2003b) in the context of causing false positives. Several so- latitude and longitude towards the Galactic bulge), lutions have been proposed to avoid false positive transit background giants may start polluting the sample candidates that are actually blended star light curves. Sea- of stars with apparent magnitudes monitored with ger & Mall´en-Ornelas (2003) show that one may eliminate high relative photometric precision (see for exam- some false positives due to blending with photometric data ple the discussion in Street et al. 2003). Getting alone if the light curve is of sufficient relative photomet- a handle on the issue of contamination is vital for ric precision and the observing cadence is high enough to OC surveys since any statistical statements about clearly resolve the individual temporal components of the the result will need to be based on estimates of sur- transit. Using spectroscopic data, other solutions include veyed cluster members. a careful modeling of the additional star properties to de- tect a second cross correlation peak caused by a physically 3. Differential Reddening Differential reddening associated star (M03; Konacki et al. 2003b; Kotredes et al. across the cluster field and along the line of sight 2003; Torres et al. 2004b). Finally, estimates for blending can make isochrone fitting (and subsequent deter- (associated either with chance alignment of foreground or mination of age, distance, and metallicity) difficult. background stars, or physical triplets) effects on the prob- Ranges of ∆EB−V 0.2 or higher across fields-of- ability of detecting existing transits can be quantified for view of 10–20 arcmin∼ on the side are not uncommon individual surveys, as done by Brown (2003) for shallow (see for example the studies by Munari & Carraro wide-field transit surveys. 1996; Raboud et al. 1997; Rosvick & Balam 2002; Carraro & Munari 2004; Villanova et al. 2004; Pris- 4. main challenges for open cluster transit inzano et al. 2004). For an RV =3.1 reddening-law, surveys a differential ∆EB−V 0.2 corresponds to a differ- ential ∆V 0.6 and ∆∼I 0.3 (Cardelli et al. 1989). The difficulties and challenges involved in searching for ∼ ∼ planetary transits specifically in OCs are: the fixed and The calculated effective temperature of a solar- metallicity main-sequence star with V I 0.8 somewhat low number of stars in an open cluster, deter- − ∼ mining OC cluster membership in the presence of signif- would vary by about 500 K for a differential red- dening effect of ∆EB−V 0.2 (Houdashelt et al. icant contamination, and differential reddening along the ∼ cluster field and along the line of sight. We outline these 2000). It should, however, also be noted that some aspects individually below. OCs do not seem to suffer from differential redden- ing, such as NGC 1245 as examined in Burke et al. 1. The Number of Monitored Stars The num- (2004) and NGC 2660 in our preliminary analysis ber of monitored stars is typically lower than in of its CMD. rich Galactic fields (in part due to the smaller field size of the used detectors), reducing the statisti- 5. open cluster selection cal chance of detecting planets. Open clusters can have up to 10,000 member stars (Friel 1995), de- Open cluster target selection can help overcome or re- pending on∼ the magnitude range taken into con- duce some of the main challenges of OC planet transit surveys described in 3 and 4. More specifically, careful sideration. Only a subset of these stars, perhaps § § 10–20%, however, will be observed with sufficient cluster selection can help maximize the number of stars, relative photometric precision to detect transits (cf. maximize the probability of detecting existing transits, 3.1). The number of these stars in rich OCs is com- and reduce line-of-sight and differential reddening. Most parable§ to the number in wide-field, shallow transit importantly, cluster selection allows for targeting specific surveys, e.g., Sleuth: 4000 stars in 6 6 square spectral type for a given telescope and observing cadence. degrees of 9

In spite of the lack of OC data, a very useful place to of sight (Schlegel et al. 1998). Differential reddening will start is the WEBDA8 database (Mermilliod 1996). From cause the main sequence of the cluster to appear broad- the long list of potential OC monitoring targets, one can ened (e.g., von Braun & Mateo 2001). This, in turn, will then start eliminating cluster candidates by applying the cause large errors in the determination of cluster parame- criteria we describe below. ters such as age, metallicity, etc, by means of isochrone fit- ting. Furthermore, a broad main sequence will make any 5.1. Cluster Richness and Observability attempts to estimate contamination (based on isochrone Apart of its observability for a given observing run9, the fitting; see for example Mighell et al. 1998; von Hippel most important selection criterion for a cluster is its rich- et al. 2002) more challenging. ness, simply to increase the statistical chance of detecting planets. The richness of the cluster field can be estimated 5.3. Cluster Age 10 by looking at sky-survey plots of the appropriate region. The consideration of cluster age in the OC selection Estimating the richness of the cluster itself is a much more is not as crucial as richness, observability, and distance. difficult process since field star contamination is usually However, choosing both younger and older OCs may im- significant due to the typically low Galactic latitude of the pose different challenges with respect to the transit-finding Pop. I OCs (see below and Bramich et al. 2003; Street process. et al. 2003; Burke et al. 2004). One may use published Stellar surface activity, which would introduce noise into cluster richness classifications, such as in Cox (2000), taken the light curve of a given star, decreases with age. As they from Janes & Adler (1982). The WEBDA database also age, stars lose angular momentum and thus magnetic ac- contains information for clusters in the 1987 Lynga catalog tivity on their surfaces (see for example Donahue 1998; (online data published in Lynga 1995). The data published Wright 2004, and references therein). The photometric there, however, only present lower estimates for richness variability for a sample of Hyades OC stars was found to classes. The depths of the studies from which the rich- be on the order 0.5–1% (Paulson et al. 2004) with periods ness classes were derived may differ significantly from one in the 8–10 day range (the Hyades cluster has an age of study to the next. Thus, these classes are rough estimates 650 Myr; see Perryman et al. 1998; Lebreton et al. 2001). only, and the best way to judge the richness of a cluster While∼ these photometric variations do not necessarily rep- field is to rely on one’s own test data obtained with the resent a source of contamination in the sense of creating same setup as the one used for the monitoring study (see false positives, they nevertheless will introduce noise into 7.5). § the stellar light curves and thus render existing transits more difficult to detect. 5.2. Cluster Distance Stellar surface activity is potentially an issue not just The distance to the target cluster is an important crite- for host star photometric variability, but more so for ra- rion for cluster selection for four reasons: (1) to ensure the dial velocity follow-up. Paulson et al. (2004) found that cluster is sufficiently distant to fit into the field of view, (2) radial velocity rms due to rotational modulation of stellar to allow RV follow-up of potential candidates, (3) to tar- surface features can be as high as 50 m/s and is on aver- get the desired range of spectral types for given observing age 16 m/s for the same sample of Hyades stars. Further conditions, and (4) to minimize reddening. such correlation between radial velocity rms and photo- Since all stars in an OC are at approximately the same metric variability was found by Queloz et al. (2001) who distance, one may, with appropriate adjustment of the ex- observed a vr amplitude of 180 m/s for HD 166435 (age posure time for given telescope parameters, cluster dis- 200 Myr), a star without a∼ planet. The associated pho- tance, and foreground reddening, target certain spectral tometric variability with a period of around 3.8 days is of types of stars for high-precision photometry. We are in- order 5%. terested in G0 or later spectral type/smaller stars, since This variability issue favors older OCs as targets, partic- early-type stars have larger radii which would make tran- ularly since most of the decrease in surface activity occurs sit detections more challenging. The transit depth in the between stellar ages between 0.6 Gyrs and 1.5 Gyrs (Pace light curve is, for small ∆m, simply given by & Pasquini 2004). A 16 m/s rms may not be a problem 2 for deep OC surveys; for short-period Jupiter-mass planets ∆F R ∆m = planet , (2) relatively large radial velocity signatures are expected and ≃ F0  Rstar  the faint stellar magnitudes limit radial velocity precision where ∆m and ∆F are the changes in magnitude and flux, to 50–100 m/s (Konacki et al. 2003a, 2004; Bouchy et al. ∼ respectively, and F0 is the out-of-transit flux of the parent 2004) using currently available telescopes and instrumen- star (see Fig. 1). In addition to featuring larger radii, early tation. spectral types are fast rotators which exhibit broad spec- Older star clusters offer an additional advantage: in tral lines, making mass-determination of planetary com- general older OCs are richer and more concentrated, and panions more difficult. therefore offer a larger number of member stars to be sur- Foreground reddening, increasing with cluster distance, veyed (Friel 1995). On the other hand, some old OCs ap- usually represents a proxy for the amount of differential pear to be dynamically relaxed and mass-segregated (such reddening across the field of view as well as along the line as NGC 1245; Burke et al. 2004), and in the case of NGC 8 http://obswww.unige.ch/webda/ 9 To optimize this observability for our potential cluster targets, one may use SKYCALC, written by J. Thorstensen and available at ftp://iraf.noao.edu/iraf/contrib/skycal.tar.Z 10 Available, for instance, at http://cadcwww.hia.nrc.ca/cadcbin/getdss EXPLORE/OC 7

3680, for instance, evidence seems to point toward some to target selection, specifically designed to maximize the resulting evaporation of low-mass stars over time (Nord- number of target stars of appropriate spectral type. stroem et al. 1997). Since low-mass stars are the primary monitoring targets, some dynamically evolved OCs may 6.1. Overall Potential Targets actually be less favorable for observing campaigns. Our potential OC targets listed in Table 1 were cho- 5.4. Other Criteria sen with the basic goal that we observe as many cluster member stars as possible at a sufficiently high photomet- Given a sufficiently high remaining number of suitable ric precision and high-cadence of observations to detect open clusters after consideration of the previous four selec- CEGPs around them. Richness classes are given when- tion criteria, cluster metallicity and galactic location are ever they were available. We note that we used these pub- additional relevant selection criteria. lished richness classes only as a guideline (i.e., we gave extra considerations to OCs classified as rich, but did not Range of metallicities. In order to be able to make • necessarily discard any OCs classified as poor) and relied quantitative statements about planet frequency as more on visual inspection and photometric analysis of sky a function of metallicity of the parent star ( 2), § survey images of the cluster regions. one needs to have a sample of clusters with varying Targets in Table 1 were further selected based on the metallicities. Surveys based on the radial-velocity published estimates for distance and foreground redden- method indicate that solar-neighborhood stars with ing12. To select a cluster with a suitable distance we con- higher metallicities are more likely to harbor plan- sider the preferred range of spectral types (G to M), our ets than metal-poor ones (Fischer & Valenti 2003). adopted relatively short exposure times (see 7.2), and the It may therefore be advantageous to favor higher- size of the LCO Swope telescope. § metallicity clusters for monitoring studies to find As an example of how distance, exposure time, target planets. spectral type, and reddening are related we use our OC The target’s Galactic coordinates. On average, the NGC 6208. In Fig. 5, we show our photometric precision • closer the OC is to the Galactic disk, the higher the as a function of I magnitude of our NGC 6208 data (night contamination due to Galactic field stars (see 3). 15), obtained during May and June 2003 at the LCO 1m Moreover, if the target is located close to, or even§ in Swope Telescope. We conservatively estimate that, with front of, the Galactic bulge, contamination may be our exposure time of 300s per frame, we attain 1% preci- severe. It should be pointed out that background sion for a range of about 2.5 magnitudes (14.5