Submitted to ApJ May 18, 2001 Candidate Kinematic Groups Among Stars with Planets Jean-Marc Deltorn1 Space Telescope Science Institute, Baltimore, MD 21218 and Paul Kalas2,3 University of California, Berkeley, CA 94720 [email protected] ABSTRACT We use the Hipparcos catalog to search for stars that share the space velocities of 57 stars with planets. We find 27 stars with planets have at least one “kine- matic companion” in a U, V, W box with size 4, 2 and 6 km s−1, respectively. Thirteen stars with planets have space motions that match those of possible kine- matic groups. A Kolmogorov-Smirnov test on the metallicity distribution of F, G, and K stars shows that the stars with planets and their kinematic companions are likely members of the same, higher metallicity population, and separate from a control sample of stars without planets and their companions. Six stars with planets share similar space velocities with the Hyades moving group. Four stars share the space motions of the Pleiades moving group. HD 19994, HD 160691, and HD 186427 match the space velocities of the IC 2391, Centaurus-Crux, and Wolf 630 moving groups, respectively. HD 27442 and HD 92788 have no com- panions in velocity space except for each other, kinematically appearing as sister stars with planets. Future observational and theoretical tests are required to understand the nature of these candidate kinematic groups. Subject headings: planetary systems—Hipparcos—stellar kinematic groups 1Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 2Astronomy Dept., 601 Campbell Hall, University of California, Berkeley, CA 94720 3NSF Center for Adaptive Optics, University of California –2– 1. Introduction One of the most significant findings related to planet formation is that 5% of nearby F, G, and K dwarfs are orbited by extrasolar giant planets (Marcy et al. 2000). Given the 58 known stars with planets (SWPs), we can begin to test how and why the properties of these systems are unique relative to the entire population of nearby stars. Particularly helpful information would be to identify other stars that formed in the same physical environment as each SWP. A study of such a group could reveal the circumstances and physical conditions that are linked to the formation of giant planets. One distinguishing characteristic of stars with planets is that they have significantly higher metallicities than comparable stars of the same spectral type (Butler et al. 2000; Santos et al. 2001). This correlation of extrasolar planets with high metallicity stars could be due to the accretion of rocky material or planets onto the stellar photosphere after the protoplanetary phase (Laughlin 2000; Sandquist et al. 1998). On the other hand, the high metallicities may originate from the time of star formation due to conditions unique to the parental cloud (Santos et al. 2001; Pinsonneault et al. 2001). Objects that are members of the same star forming environment will share similar locations and kinematics when they are young. Their velocity dispersion will initially trace the velocity dispersion of the parental cloud. As the parental cloud disperses in 107-108 years, the velocity dispersion of some members may increase significantly due to interactions with other stars, spiral arms, and massive molecular clouds (Asiain et al. 1999b). Without any significant disruption, the random dispersion in V4 will stretch the group into a tube during its Galactic orbit (Skuljan et al. 1997). The U and W velocities will also evolve from their initial values and eventually oscillate out of phase. However, even though the spatial cohesiveness disappears on timescales of one galactic orbit, some fraction of the initial group will share a similar V velocity component on timescales of several galactic orbits (Skuljan et al. 1999). Numerous stellar kinematic groups (Table 1) have been identified based on the above arguments. Most kinematic group identified so far are relatively young, <1 Gyr associations of stars in the process of being scattered. Older kinematic associations could, in principle, retain enough cohesiveness in velocity space to be classified as stellar kinematic associations even after 1 Gyr (Wielen 1971; Terlevich 1987), as in the case of the HR 1614 kinematic group (Feltzing & Holmberg 2000). The density in (U,V,W) space of such groups shall 4Unless otherwise noted, space velocities are given relative to the Sun with positive U directed toward the galactic center, positive V in the direction of galactic rotation, and positive W in the direction of the north galactic pole. –3– nonetheless decrease with time, impairing the probability of identifying a large fraction of the same initial kinematic association. The present sample of SWPs are chromospherically quiet and generally have estimated ages between 1 and 10 Gyr. The likelihood that sister stars from the time of star formation still share their kinematics is low. Nevertheless, we find that two SWPs, HD 75732 (55 Cnc) and HD 134987, are already listed as possible members of the Hyades Group (Eggen 1985, 1996). These pre-Hipparcos studies relied on proper motion and parallax information only. In the post-Hipparcos era the study of stellar kinematic groups is pushing forward with more precise position, proper motion, and parallax information that when combined with radial velocity data gives improved knowledge of the 3-dimensional spatial kinematics (Chereul et al. 1999; Skuljan et al. 1999; Asiain et al. 1999a). The primary goal of this paper is to identify stars in the Hipparcos catalog that share the U, V, W space motions of SWPs. We will call stars that have matching U, V, W motions “kinematic companions” without implying that this test establishes a real physical connection in a group’s dynamical or chemical history. We present our list of kinematic companions purely as candidate moving groups that should be investigated more thoroughly. In addition to the kinematic test, we will study the metallicity correlation between the SWPs and their companions, and a control group and their companions. 2. Method We cross-correlate the 118,218 stars in the Hipparcos catalog with the 36,145 stars in the Barbier-Brossat & Figon (2000) catalog of stellar radial velocities (hereafter BBF). For the 21,497 stars shared between the two catalogs, we derive the U, V, W velocity components and their standard deviations (Johnson & Soderblom 1987) using the Hipparcos data for positions, proper motions, and parallaxes.5 For the SWPs we use the mean radial velocities and standard deviations that have been determined by the planet search programs. The SWP BD -10 3166 was not detected by Hipparcos and is excluded from our study, leaving a total SWP sample of 57 stars. We define a box with dimensions (∆U, ∆V, ∆W) = (4, 2, 6) km s−1 centered on the average U, V, W values for each SWP. The space velocities of a given co-eval group will evolve out of this box, and in fact the U, V, W distribution resembles a tilted spheroid rather than 5Relative to the Local Standard of Rest, the Sun has U = 10.0 km s−1,V=5.25kms−1, and W = 7.17 km s−1 (Dehnen & Binney 1998). –4– a box (Skuljan et al. 1997). By making the box small we are selecting only a core group of co-moving stars. Thus our results give a lower limit to the number of members in each possible stellar kinematic group. The benefit of this approach is that we are excluding a large number of interlopers. We select the stars that have average U, V, W values within −1 −1 −1 the box. We then reject those that have σU >2kms ,σV >1kms and σW >3kms . We therefore limit the sample to only stars with relatively well-determined space velocities. Even though the average U, V, W values will place stars within the U, V, W box of a given SWP, the standard deviation in the space motions may give a significant probability that the companion lies outside of the box. We treat the uncertainties using a Monte-Carlo draw, assuming a Gaussian distribution for the errors, centered on the average U, V, W values, and with dispersions equal to σU , σV and σW . We produce 1000 random draws for each kinematic companion. A probability of kinematic membership, PKM, is computed by incrementing a counter, Ci, by 1 each time the U, V, W of a companion falls within the kinematic box of the SWP. We then have PKM =Ci / 1000. Finally, we select only those stars that have PKM > 0.75 as the most secure members of a possible kinematic association. 3. Results: Candidate kinematic companions and the metallicity distribution In Table 2 we give our derived velocity information for the SWPs and any Hipparcos stars that are kinematic companions based on the search parameters defined in the previous section. We find that 27 SWPs have at least one kinematic companion, 14 have three or more kinematic companions, and that two pairs of SWPs are kinematic companions with each other. If we compare Table 2 to Table 1 with the goal of finding matches in U, V and W 6, we find that 13 SWPs and their companions appear kinematically associated with known moving groups. Each of these possible matches will be examined on a case-by-case basis below. However, to estimate the probability that the SWP and SWP companion populations are physically linked, we compare their stellar abundances, which are unusually high for the SWP population. We take take only the F, G, and K stars and use only the Schuster & Nissen (1989) method for determining stellar abundances. As a control group, we take the 37 F, G, and K stars without known planets from Santos et al.