SCIENCE CHINA Physics, Mechanics & Astronomy

• Article • March 2015 Vol. 58 No. 3: 039503 doi: 10.1007/s11433-014-5593-z

Planet host stars in open clusters

YANG XiaoLing1,2, CHEN YuQin1* & ZHAO Gang1

1Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China; 2University of Chinese Academy of Sciences, Beijing 100049, China

Received June 3, 2014; accepted August 5, 2014; published online September 28, 2014

We have compiled a list of all planet host star candidates reported in the literature, which are likely to be cluster members, and we checked their memberships by the spatial location, radial velocity, proper motion and photometric criteria. We found that only six stars, BD-13 2130, HD 28305, Kepler-66, Kepler-67, Pr0201 and Pr0211, are planet orbiting stars in open clusters to date. Two stars, HD 70573 and HD 89744, belong to moving groups and one star, TYC 8975-2606-1, may not be a planet host star, while three stars, HD 16175, HD 46375 and HD 108874 are not members of open clusters. We note that all these six planetary systems in the stellar cluster environment are younger than ~1 Gyr, which might indicate that the planetary system in open cluster can not survive for a long time, and we speculate that close stellar encounters between member stars in open clus- ter can potentially destroy, or at least strongly affect, the presence of planetary systems.

extrasolar planets, planetary survival, open cluster

PACS number(s): 97.82.-j, 97.82.Fs, 98.20.-d, 98.20.Di

Citation: Yang X L, Chen Y Q, Zhao G. Planet host stars in open clusters. Sci China-Phys Mech Astron, 2015, 58: 039503, doi: 10.1007/s11433-014-5593-z

1 Introduction tems for different categories of stellar masses. However, the PMC is not found for giant stars with planets, and the effect of stellar mass is not well studied [3] since most extrasolar The search of planets surrounding stars and the study of planets detected to date have host stars with one solar-mass, planet formation environment are hot topics in astro- and thus little is known about planet formation around more physcics. To date, more than 1700 extrasolar planets have massive stars. Due to the high metallicity or solar metallici- been detected. With an increasing number of detected plan- ty of most open clusters and the difficulty of deriving the ets, many efforts are being made to explain a number of mass for a field giant star, the ideal solution to the metallic- characteristics from the observations and unveil the funda- ity and the mass effect on the planet frequency is to search mental mechanisms of planet formation and survival. It has for planets around intermediate-mass stars in open clusters, been suggested that the metallicity and mass of stars are which naturally comprise homogeneous star members with closely tied to the formation and evolution of planets. The well-constrained basic properties such as age, distance and planet-metallicity correlation (PMC) of planet host stars metallicity. Meanwhile, most planet host stars detected to (mainly dwarf stars) indicates that planets form more easily date are in the field and only a few stars are probably related in high-metallicity environment [1] and Lovis and Mayor [2] to open clusters. Thus it is speculated that stars in the open suggested that more massive stars do form more massive cluster evironment is somewhat different from that in the planetary systems than lower-mass stars based on the study field, which may affect the formation or the survival of of giant planet frequency and typical mass of planetary sys- planets. There have been several planet surveys in open clusters *Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2014 phys.scichina.com link.springer.com Yang X L, et al. Sci China-Phys Mech Astron March (2015) Vol. 58 No. 3 039503-2

[2,4]. In particular, the Kepler Mission has been successful- clusters. Among the 12 candidates, HD 70573 and HD ly launched on March 6, 2009, and observed cluster mem- 89744 belong to the moving groups while the other ten stars bers in four open clusters [5]. Due to limitations of search- are suggested to be members of open clusters. ing for planets in open clusters, such as the faintness of the targets and stellar activity for cluster younger than 1 Gyr [6], the detection efficiency in open clusters is significantly 3 The membership check lower than that in field. It is interesting to know whether the cluster environment would affect the planet formation and For the two stars belonging to moving groups, López-San- their survival time, and how significant these effects are. tiago et al. [14] provided their membership information Studies on the impact of cluster environment on planet for- based on kinematic (space motion), spectroscopic (lithium mation gave some different views. Armitage [7] argued that abundance) and photometric (isochrones fitting) criteria. the fraction of stars with giant planets in rich clusters is ex- They identified HD 70573 as a member of Hercules-Lyra pected to be substantially suppressed as compared to less Association and HD 89744 as a member of AB Dor Moving clustered star formation environments. It is suggested that Group. small clusters have relatively little effect on star and planet For the other ten stars in the open clusters, we can check formation, whereas large clusters can have a substantial their membership based on the comparison of stellar spatial impact [8]. However, Van Saders et al. [9] do not suggest a location, radial velocity, proper motion and the position in significant difference between the frequency of planets in the colour-magnitude diagram (CMD) with those of the open clusters and the field. relevant clusters. Table 2 [1,2,4,16,19–33] lists the distance, In this paper, we present the characteristics of stars with radial velocity, proper motion and metallicity information of planetary companion which are probably related to open the ten stars and the related clusters. By comparing the dis- clusters in the Extrasolar Planets Encyclopaedia catalogue. tance(taken from Extrasolar Planets Encyclopaedia) of the We aim to investigate how many planet host stars presently stars to the center of their probably related clusters, we detected in the literature can be related to cluster environ- found that HD 16175, HD 46375 and HD 108874 are far ment and what kind of clusters can have planet host stars, beyond the radius of the clusters. Thus, the three stars are which will provide important constrains on the planet for- most likely to be non-members of their corresponding clus- mation theory. ters. In addition, Kepler-66 and Kepler-67 could be mem- bers since they are slightly out of the cluster′s radius but 2 Sample still within the measurement uncertainties. Further check on the membership was carried out on the The initial sample of planet host stars probably related to basis of the radial velocities and proper motions. Although open clusters was taken from the Extrasolar Planets Ency- different clusters have somewhat different dispersions in clopaedia (http://exoplanet.eu/) which provides a list of radial velocity and proper motion due to different distances planet candidates detected to date and some information on and measurement uncertainties, most clusters have a disper- their host stars. From this website, we chose all the host sion in radial velocity less than 2 km/s, and a deviation of stars marked with relevant cluster names as our candidates. proper motion among cluster stars is less than 8 mas/yr [24, Table 1 [10–18] gives the list of 12 selected candidates as 32,34]. In view of this, we choose stars with |Vr|2 km/s, well as their locations, related clusters and possible IDs in |pmRA|8 mas/yr and |pmDE|8 mas/yr as most probably

Table 1 The initial sample of planet host stars probably related to open clusters or moving groups

Identifier number Ref. for IDs in Star RA Dec Related cluster in cluster clusters BD-13 2130 07 37 09.238 13 54 23.97 NGC 2423 NGC 2423 SN 4 [10] HD 16175 02 37 01.911 +42 03 45.48 NGC 1039 NGC 1039 W 410 [11] HD 28305 04 28 36.999 +19 10 49.54 Hyades Melotte 25 S 3 [12] HD 46375 06 33 12.622 +05 27 46.53 NGC 2244 NGC 2244 VS 126 [13] HD 70573a) 08 22 49.951 +01 51 33.55 HERCULES-LYRA SSOCIATION  [14] HD 108874 12 30 26.882 +22 52 47.38 Melotte 111 Melotte 111 AV 1942 [15] HD 89744a) 10 22 10.563 +41 13 46.31 AB DOR MOVING GROUP  [15] Kepler-66 19 35 55.576 +46 41 15.97 NGC 6811  [16] Kepler-67 19 36 16.801 +46 59 59.19 NGC 6811  [16] Pr 0201 08 41 43.823 +20 13 36.81 NGC 2632 NGC 2632 KW 418 [17] Pr 0211 08 42 11.492 +19 16 37.36 NGC 2632 NGC 2632 KW 448 [17] TYC 8975-2606-1 12 24 35.474 61 49 11.68 NGC 4349 NGC 4349 127 [18] a) stars related to moving groups Yang X L, et al. Sci China-Phys Mech Astron March (2015) Vol. 58 No. 3 039503-3

cluster members (Here Vr=VrstarVrcluster, pmRA=pmR- sample.

AstarpmRAcluster and pmDE=pmDEstarpmDEcluster). Based For the remaining six stars, BD-13 2130, HD 28305, on these criteria, five stars, BD-13 2130, HD 28305, Kep- Kepler-66, Kepler-67, Pr0201 and Pr0211, we finally use ler-66, Kepler-67, and TYC 8975-2606-1 are probably photometric criteria to check whether they are the cluster cluster members, while three stars, HD 16175, HD 46375 members. Figures 1(a)–(d) display the (V, B-V) col- and HD 108874, are obviously non-members of their corre- our-magnitude diagram for NGC 2423, Hyades, NGC 6811, sponding clusters. Pr0201 and Pr0211 have no radial veloc- and NGC 2632 based on photometric data from WEBDA ity available in the literature, but their proper motions are with the position of the six planet host stars; they are shown consistent with those of the open cluster, leading to a high as red squares. We can see that BD-13 2130 and HD 28305 fell on the stage of red clumps, Kepler-66, Kepler-67 and probability that they are indeed cluster members. The radial Pr0211 are situated in or very close to the main sequences velocity and proper motion values of HD 16175, HD 46375 except for Pr0201, which slightly deviates from the fiducial and HD 108874 show very large differences from their re- sequence. Almost all of the stars fell on the fiducial se- lated clusters with deviations of 25–40 km/s in radial veloc- quences and were confirmed as cluster members. ity and 30–150 mas/yr in proper motion values. Therefore, Base on the analysis above, six stars turned out to be the three stars are confirmed as non-members of clusters planet host stars in open clusters and two stars belong to and they were excluded from our sample. moving groups. We list their basic parameters in Table 3. Cluster members have the same metallicity since they are formed in the same molecular cloud. For the other seven stars, the [Fe/H] data given in Table 2 shows that none of 4 Discussion them has a large difference in metallicity from their related cluster metallicity value. We note that Lovis and Mayor [2] 4.1 Planets in moving groups gathered 20 radial velocity data points for TYC 8975-2606- 1 within a time span of 784 d and fitted a Keplerian orbit to HD 70573 and HD 89744 are the two planet host stars be- the radial velocity data. They suggested the companion longing to moving groups, Hercules-Lyra Association and around this star has a minimum mass m2sini=19.8MJup. AB Dor Moving Group. Dehnen [36] pointed out that most Since the companion around TYC 8975-2606-1 has a mass moving groups observed in the solar vicinity could be above the D-burning threshold [35], it should be referred to formed by orbital resonances which are related to the Ga- as a brown dwarf. Therefore, we also excluded it from our lactic spiral structure. Pakhomov et al. [37] demonstrated

Table 2 Position, radial velocity, proper motion and metallicity information of the ten stars except for two stars in moving groups

Related D (pc) RV (km/s) pmRA (mas/yr) Star R (pc) cluster cl star cluster star cluster star cluster BD-13 2130 NGC 2423 6.15 766 766 18.20±0.14 18.47±0.11 1.90±1.50 0.55±0.28 HD 16175a),b),c),d) NGC 1039 3.22 59.8 499 21.40±0.30 18.20±1.40 38.90±0.63 0.28±0.15 HD 28305 Hyades 2.16 45 45 38.36±0.13 39.29±0.25 106.19±0.38  HD 46375a),b),c) NGC 2244 9.27 33.4 1660 0.97±0.09 26.16±3.37 111.96±0.88 0.65±0.47 HD 108874a),b),c),d) Melotte 111 6.71 68.5 96 29.60±0.50 0.01±0.08 127.11±0.96 10.84±1.20 Kepler-66e) NGC 6811 6.36 1107±90 1215 7.80±0.20 7.70±0.80 3.80±7.80 5.47±0.27 Kepler-67e) NGC 6811 6.36 1107±90 1215 8.10±0.20 7.70±0.80 2.90±7.20 5.47±0.27 Pr 0201 NGC 2632 9.80  187  34.76±0.07 35.30±1.30 35.58±0.19 Pr 0211 NGC 2632 9.80  187  34.76±0.07 35.1±1.60 35.58±0.19 TYC 8975-2606-1f) NGC 4349 8.36 2176 2176 12.12±0.25 -11.87±0.24 10.0±4.90 5.77±2.22 Related pmDE (mas/yr) [Fe/H] Reference Star R (pc) cluster cl star cluster star cluster star cluster BD-13 2130 NGC 2423 6.15 5.40±1.60 2.50±0.28 0.14±0.09 0.14 9 4 6 2 2 2 HD 16175a),b),c),d) NGC 1039 3.22 -40.37±0.53 6.91±0.15 0.23±0.07 0.07 11 14 5 2 2 2 HD 28305 Hyades 2.16 37.84±0.30  0.17±0.04 0.13 7 14 13 2  2 HD 46375a),b),c) NGC 2244 9.27 97.17±0.67 0.67±0.39 0.24  10 14 3 2 2  HD 108874a),b),c),d) Melotte 111 6.71 89.47±0.84 3.20±1.20 0.14 0.07 17 14 1 2 2 2 Kepler-66e) NGC 6811 6.36 6.80±7.40 7.53±0.27 0.012±0.003  8 16 8 8 2  Kepler-67e) NGC 6811 6.36 14.40±6.70 7.53±0.27 0.012±0.003  8 16 8 8 2  Pr 0201 NGC 2632 9.80 17.30±1.20 12.90±0.19 0.18±0.08 0.27 15 12 2 2 2 Pr 0211 NGC 2632 9.80 16.4±1.60 12.90±0.19 0.19±0.08 0.27 15 12 2 2 2 TYC 8975-2606-1f) NGC 4349 8.36 10.90±4.30 1.44±2.17 0.12±0.04 0.12 9 4 6 2 2 2 a)–d) non-members based on spatial location, radial velocity, proper motion and metallicity analysis; e) possible member based on spatial location analysis; f) brown dwarf Yang X L, et al. Sci China-Phys Mech Astron March (2015) Vol. 58 No. 3 039503-4

Table 3 Basic parameters of six planet host stars in the open clusters and two stars in the moving groups. The data information is taken from Extrasolar Planets Encyclopaedia and SIMBAD

2 Mpsini Object N (1/pc ) r/Rcl Age (Gyr) Mass Msun V (mag) B-V (mag) Teff (K) MK type MJup BD-13 2130 4.44 0.109 0.750 2.4±0.2 9.45 1.27  GIV/Va) 10.6 HD 28305   0.625±0.05 2.7±0.1 3.53 1.02 4901±20 K0III 7.6±0.2 Kepler-66 0.12 ~1b) 1.0±0.170 1.038±0.044 15.30 0.71 5962±79 G0V 0.31±0.07 Kepler-67 0.12 ~1b) 1.0±0.170 0.865±0.034 16.40 0.93 5331±63 G9V 0.31±0.06 Pr0201 3.80 0.2 0.578±0.049 1.234±0.034 10.33 0.77 6174±50 F7 0.54±0.039 Pr0211 3.80 0.2 0.578±0.049 0.952±0.04 12.15 0.87 5326±50  1.844±0.064 HD 70573   0.101±0.024 1.0±0.1 8.70 0.63 5737±70 G1-1.5V 6.1±0.4 HD 89744   2.040 1.4±0.09 5.74 0.49 6196±43 F7V 7.2 a) spectral type should be GIII from the view of CMD in this paper; b) assuming that the stars are located at the edge of the cluster

Figure 1 (Color online) Colour-magnitude diagrams for four open clusters: (a) NGC 2423 (BD-13 2130); (b) Hyades (HD 28305); (c) NGC 6811 (Kep- ler-66 and Kepler-67); (d) NGC 2632 (Pr0201 and Pr0211). The planet host stars in our list are shown as the squares. that the Hercules stream is a heterogeneous group of objects nances, induced by the Galactic spiral structure, does not from the thin and thick disks and Barenfeld et al. [38] con- play significant role on the survival of the planetary system. cluded that the AB Dor stream contains stars from many different birth sites. Since the two moving groups are both 4.2 Planets in open clusters formed by orbital resonances and their members are not from the disruption of stellar clusters, it makes little sense to Although the number of open clusters being searched for study the impact of cluster environment on planet formation extrasolar planets is still limited, we still can try to derive based on the two stars. But the presence of planet host stars some characteristics of planetary systems in some stellar in moving groups indicates that the effect of orbital reso- clusters. As shown in sect. 3, BD-13 2130, HD 28305, Yang X L, et al. Sci China-Phys Mech Astron March (2015) Vol. 58 No. 3 039503-5

Kepler-66, Kepler-67, Pr0201, and Pr0211 are the only six planet host stars belonging to open clusters discovered so far. As can be seen from Table 3, the relative cluster for Kepler-66 and Kepler-67 has an age of 1 Gyr and the four open clusters for the remaining four planet host stars are all younger than 1 Gyr. Figure 2 shows the age distribution of all the planetary systems detected to date in a histogram. We can see that all of the planet host stars in the field have a large range in age from 0 to 14 Gyr. It seems that there is a lack of planet host stars in old open clusters, while many planet host stars in the field are quite old. Moreover, all the related clusters have low star densities and thus these plan- etary systems are not located in a very dense stellar envi- ronment, which might indicate that the planetary system with giant planets in open cluster will not stay for a long time, and we speculate that close stellar encounters can po- tentially destroy, or at least strongly affect, the presence of planetary systems. In addition, we also found an interesting coincidence that the two planet host stars (Pr0201 and Figure 2 The age distribution of all the planetary systems detected in the Pr0211) in NGC 2632 are located at the same distance that field. are very close to the radius of the core, but it is premature to draw the conclusion that the planet formation and survival are associated with the distance to the cluster center based easily in high-metallicity environment, it is more likely to on such a small number of open clusters. find planets in NGC 6791. However, none is detected [40]. We speculate that its high stellar density and old age (9.6 Gyr) give rise to a small opportunity for planetary systems 4.3 Planets in stellar clusters extensive search to form and survive due to collisions of member stars which The search for transiting planets in open clusters have tar- may destroy planetary systems. geted a number of open clusters: NGC 188, NGC 1245, NGC 2099, NGC 2158, NGC 6791, etc. Although the 5 Conclusion searching project has been undertaken for a long time, no convincing planet has been reported except for a possible transiting planet candidate (NGC 2158 TR1) in the open In this paper, we compiled 12 planetary systems probably in cluster NGC 2158 [39]. NGC 2158 has an intermediate age open clusters or moving groups from the literature. The four (t=2–3 Gyr), and it is a populous and rather metal-poor open criteria, the spatial location, radial velocity, proper motion cluster, which does not favor the formation of planetary and photometric color-magnitude diagram are used to check system in view of the high frequency of planet formation in their membership in the related cluster. Except for two metal rich and low collision environment in the field. We planet host stars, HD 70573 and HD 89744, which belong to note that this star is not included in the Extrasolar Planets moving groups, we found that only six planet host Encyclopaedia for some reasons. In particular, it is suspect- stars—BD-13 2130, HD 28305, Kepler-66, Kepler-67, ed that the candidate is a stellar binary with grazing eclipses Pr0201, Pr0211—are the members of open clusters. Among [39], rather than a planet. Furthermore, we have no radial these six planet host stars in open clusters, Kepler-66 and velocity, proper motion and metallicity data of this star to Kepler-67 have an age ~1 Gyr and the other four stars in the check its membership of NGC 2158. We exclude this star cluster environment orbited by giant planets are all younger from our list of planet host stars in open clusters. For other than 1 Gyr. This might indicate that the planetary systems in clusters being searched for extrasolar planets, their [Fe/H] open clusters will not stay for a long time, and we speculate values range from 0.14 to 0.32 dex and ages range from that close stellar encounters can potentially destroy, or at 0.25 to 9.65 Gyr, which cover the same ranges as those of least strongly affect, planetary systems. Specifical search planet host stars in the field. Howerer, no planet has been for planet host stars in open clusters has targeted a number detected in these open culsters. The nearly zero detection of open clusters and no convincing planet candidate has rate of planet host stars in these open clusters may indicate a been detected so far. In particular, NGC 6791 has a very low frequency of planet formation in cluster environment high metallicity which is favorable to form planets, but no than that in the field. In particular, NGC 6791 is a metal rich planet has been detected in this open cluster. The failure to cluster with [Fe/H]=0.32 dex. According to the mostly ac- detect any planet candidates in NGC6791 is probably due to cepted planet-metallicity correlation that planets form more its high stellar density and old age (9.6 Gyr). We proposed Yang X L, et al. Sci China-Phys Mech Astron March (2015) Vol. 58 No. 3 039503-6 that there is a lower frequency of planet host stars in open biting G and K dwarfs. Astrophys J, 2003, 582: 455–466 clusters than that in the field, and the cluster environment 20 Dias W S, Alessi B S, Moitinho A, et al. 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