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SCIENCE CHINA Planet Host Stars in Open Clusters 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=VrstarVrcluster, pmRA=pmR- sample. AstarpmRAcluster and pmDE=pmDEstarpmDEcluster). 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.
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