THE ASTROPHYSICAL JOURNAL, 553:410È420, 2001 May 20 V ( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A. STELLAR ENCOUNTERS WITH THE b PICTORIS PLANETESIMAL SYSTEM PAUL KALAS1 AND JEAN-MARC DELTORN2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 AND JOHN LARWOOD Astronomy Unit, Queen Mary and WestÐeld College, London, E1 4NS, UK Received 2000 May 8; accepted 2001 January 24 ABSTRACT We use data from the Hipparcos catalog and the Barbier-Brossat & Figon catalog of stellar radial velocities to test the hypothesis that the b Pic planetesimal disk was disrupted by a close stellar encoun- ter. We trace the space motions of 21,497 stars and discover 18 that have passed within 5 pc of b Pic in the past 1 Myr. b PicÏs closest encounter is with the K2 III star HIP 27628 (D0.6 pc), but dynamically the most important encounter is with the F7 V star HIP 23693 (D0.9 pc). We calculate the velocity and eccentricity changes induced by the 18 perturbations and conclude that they are dynamically signiÐcant if planetesimals exist in a b Pic Oort cloud. We provide a Ðrst-order estimate for the evolutionary state of a b Pic Oort cloud and conclude that the primary role of these stellar perturbations would be to help build a comet cloud rather than destroy a preexisting structure. The stellar sample is D20% complete and motivates future work to identify less common close interactions that would signiÐcantly modify the observed circumstellar disk. For future radial velocity study, we identify six stars in the Hipparcos catalog that may have approached b Pic to within 0.1 pc and therefore remain as candidate disk perturbers. Subject headings: circumstellar matter È planetary systems È stars: individual (b Pictoris) On-line material: color Ðgure 1. INTRODUCTION The dust disk around the A5 V star b Pic displays evi- dence for both types of dynamical mechanism. A planet The dynamical mechanisms that dominate the formation may be responsible for clearing dust within a D20 AU and subsequent evolution of planetary systems can be radius of the star (Smith & Terrile 1984; Lagage & Pantin broadly described as either endogenic or exogenic in origin. 1994), perturbing families of comets toward the stellar The formation of giant planets and their apparent orbital photosphere (Beust & Morbidelli 2000), vertically dis- migration (Goldreich & Tremaine 1980; Malhotra, rupting the disk at 50 AU radius (Burrows et al. 1995; Duncan, & Levison 2000; Marcy, Cochran, & Mayor Mouillet et al. 1997; Heap et al. 2000), and creating a few 2000b) is probably the most signiÐcant endogenic source for hour duration, 0.06 mag, achromatic drop in b PicÏs light modifying a planetary system over time. In our solar curve (Lecavelier des Etangs et al. 1997). A recent and close system, the giant planets have displaced a signiÐcant frac- stellar Ñyby perturbation may be responsible for producing tion of small bodies from their formation sites to either the the radially and vertically asymmetric disk structure at Oort cloud or interstellar space. For circumstellar dust large radii (Kalas & Jewitt 1995), as well as substructure in disks observed around nearby main-sequence stars, unseen one disk midplane between 500 and 800 AU projected giant planets are believed to produce the ubiquitous central radius (Kalas et al. 2000; Larwood & Kalas 2001). Since the depletions in the dust distributions (Roques et al. 1994; age of b Pic is between 8 Myr (Crifo et al. 1997) and 20 Myr Pantin, Lagage, & Artymowicz 1997; Greaves et al. 1998; (Barrado y Navascues et al. 1999), understanding the Liou & Zook 1999; Wyatt et al. 1999). dynamics of objects surrounding b Pic could elucidate con- However, young systems, such as preÈmain-sequence ditions and events that determined the early evolution of stars in clusters and the proplyd objects in Orion, experi- our solar system. ence signiÐcant exogenic forces (Larwood 1997). Close Here we examine possible exogenic perturbations on b stellar encounters with other cluster members (Laughlin & Pic. A key piece of observational evidence needed to Adams 1998) and radiation pressure from nearby massive conÐrm the stellar Ñyby hypothesis is to identify a per- stars (Henney & OÏDell 1999) remove disk material and can turbing star. Given improved trigonometric parallaxes and disrupt the vertical settling of circumstellar dust and gas. proper motions from Hipparcos (European Space Agency During the evolution of our solar system, the Galactic tide 1997), we are motivated to test the stellar Ñyby hypothesis and encounters with molecular clouds and passing stars empirically. Section 2 presents our method for using Hip- decoupled comets from the planetary region, preserving parcos and radial velocity data to trace the Galactocentric them in the Oort cloud (Oort 1950; Fernandez 1997). motions of stars relative to b Pic. Section 3 identiÐes stars that have passed within 5 pc of b during the past 1 Myr. In 1 Current Address: Department of Astronomy, 601 Campbell Hall, University of California, Berkeley, CA 94720; kalas=stsci.edu. ° 4 we assess the signiÐcance of the stellar encounters for the 2 Laboratoire dÏAstrophysique de Marseille, Traverse du Siphon, Les evolution of a possible Oort cloud around b Pic. Section 5 trois-Lucs, 13376, Marseille Cedex 12, France. demonstrates the plausibility of very close stellar encoun- 410 TABLE 1 NPUT ARAMETERS FOR THE TARS WITH I P S Dca \ 5 pc NAME np l b R p pm(a) p pm(d) p SPECTRAL n 1950 1950 V RV pm(a) pm(d) ~1 ~1 YPE OTES Hipparcos HD Other (mas) (mas) a1950 d1950 (deg) (deg) (km s ) (km s ) (mas) (mas) (mas) (mas) T N 27321 ....... 39060 b Pic 51.87 0.51 05 46 05.93 [51 05 01.8 258.36 [30.61 19.83 0.67 4.65 0.53 81.96 0.61 A5 V 10798 ....... 14412 HR 683 78.88 0.72 02 16 44.06 [26 10 53.2 214.45 [70.41 7.00 0.50 [218.07 0.60 444.51 0.48 G5 V High proper motion star 17378 ....... 23249 d Eri 110.58 0.88 03 40 51.03 [09 55 52.9 198.09 [46.00 [6.00 0.70 [91.71 0.98 742.23 0.92 K0 IV Variable of RS CVn type 19893 ....... 27290 c Dor 49.26 0.50 04 14 42.85 [51 36 43.2 259.83 [44.78 25.20 1.20 100.57 0.46 184.23 0.50 F4 III Pulsating variable Star 19921 ....... 27442 v Ret 54.84 0.50 04 15 37.02 [59 25 18.7 270.20 [42.56 29.30 0.50 [47.99 0.52 [167.81 0.50 K2 IVa Star in double system 22122 ....... 30501 G J176.3 48.90 0.64 04 44 22.88 [50 09 32.5 256.99 [40.37 17.00 2.50 [445.81 0.59 [337.31 0.66 K 1V High proper motion star 23693 ....... 33262 f Dor 85.83 0.46 05 04 39.05 [57 32 25.6 266.03 [36.72 [2.10 0.40 [31.87 0.46 117.42 0.48 F7 V 25544 ....... 36435 G J204.1 51.10 0.52 05 27 03.23 [60 27 15.3 269.31 [33.62 12.90 1.20 [148.89 0.56 [92.83 0.64 G6/G8 V 27628 ....... 39425 b Col 37.94 0.57 05 49 11.74 [35 47 09.6 241.34 [27.06 89.10 0.80 55.74 0.50 404.68 0.47 K2 III High proper motion star 29568 ....... 43162 HR 2225 59.90 0.75 06 11 41.23 [23 50 53.0 230.85 [18.52 22.30 0.30 [47.06 0.45 110.90 0.62 G5 V 29958 ....... ... G J9210 32.30 2.10 06 16 04.60 [13 51 09.6 221.79 [13.51 103.80 2.00 66.60 1.91 341.18 1.64 M0 IV High proper motion star 31711 ....... 48189 HR 2468 46.15 0.64 06 37 28.46 [61 29 19.5 271.20 [25.20 32.30 0.30 [50.08 0.73 72.69 0.66 G1.5 V Double or multiple star 37504 ....... 63295 f Vol 24.36 0.48 07 42 26.97 [72 29 10.6 284.57 [22.16 48.10 0.50 33.38 0.44 15.29 0.40 K0 III Star in double system 38908 ....... 65907 G J294A 61.76 0.51 07 56 51.58 [60 10 06.1 273.38 [15.68 14.10 1.20 518.38 0.63 118.86 0.51 G0 V High proper motion star 83990 ....... 154577 G J656 73.07 0.91 17 05 40.03 [60 40 27.1 329.82 [12.22 8.80 1.20 71.14 0.76 589.82 0.62 K2 V High proper motion star 89042 ....... 165499 • Pav 56.32 0.68 18 05 47.26 [62 00 55.1 332.21 [19.16 29.70 1.20 [77.60 0.59 234.68 0.44 G0 V High proper motion star 93506 ....... 176687 f Sag 36.61 1.37 18 59 25.89 [29 57 12.3 6.84 [15.35 24.10 3.30 [14.10 2.62 3.66 2.00 A2 III Double or multiple star 114996 ...... 219571 c Tuc 45.40 0.61 23 14 31.51 [58 30 36.7 324.30 [54.82 18.40 0.50 [34.93 0.44 79.59 0.50 F1 III 116250 .....